Abstract:
A force-sensing electronic pen comprises a user-replaceable cartridge, a retaining boot for securely retaining the cartridge, a force sensor coupled to the retaining boot, a spring for biasing the retaining boot towards engagement with the force sensor; and an end-stop for limiting travel of the retaining boot against the bias of the spring. The cartridge is extractable from the pen by pulling the cartridge against the bias of the spring until the retaining boot engages with the end-stop. When the retaining boot engages with the end-stop, further pulling of the cartridge releases the cartridge from the retaining boot.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 11/193,435 filed Aug. 1, 2005, all of which are herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the fields of interactive paper, printing systems, computer publishing, computer applications, information appliances, human-computer interfaces, and in particular electronic styli. 
     CO-PENDING REFERENCES 
     
         
         U.S. Ser. Nos. 11/193,481 11/193,482 11/193,479 
       
    
     CROSS-REFERENCES 
                                                 6,750,901   6,476,863   6,788,336   7,249,108   6,566,858       6,331,946   6,246,970   6,442,525   7,346,586   09/505,951       6,374,354   7,246,098   6,816,968   6,757,832   6,334,190       6,745,331   7,249,109   7,197,642   7,093,139   7,509,292       10/636,283   10/866,608   7,210,038   7,401,223   10/940,653       10/942,858   7,364,256   7,258,417   7,293,853   7,328,968       7,270,395   7,461,916   7,510,264   7,334,864   7,255,419       7,284,819   7,229,148   7,258,416   7,273,263   7,270,393       6,984,017   7,347,526   7,357,477   7,465,015   7,364,255       7,357,476   11/003,614   7,284,820   7,341,328   7,246,875       7,322,669   10/815,621   7,243,835   10/815,630   10/815,637       10/815,638   7,251,050   10/815,642   7,097,094   7,137,549       10/815,618   7,156,292   10/815,635   7,357,323   10/815,634       7,137,566   7,131,596   7,128,265   7,207,485   7,197,374       7,175,089   10/815,617   10/815,620   7,178,719   7,506,808       7,207,483   7,296,737   7,270,266   10/815,614   10/815,636       7,128,270   11/041,650   11/041,651   7,506,168   7,441,712       11/041,610   11/041,609   11/041,626   11/041,627   11/041,624       7,395,963   7,457,961   11/041,580   7,467,300   7,467,299       11/041,648   7,457,007   7,150,398   7,159,777   7,450,273       7,188,769   7,097,106   7,070,110   7,243,849   6,623,101       6,406,129   6,505,916   6,457,809   6,550,895   6,457,812       7,152,962   6,428,133   7,204,941   7,282,164   7,465,342       7,278,727   7,417,141   7,452,989   7,367,665   7,138,391       7,153,956   7,423,145   7,456,277   10/913,376   7,122,076       7,148,345   11/172,816   7,470,315   11/172,814   7,416,280       7,252,366   7,488,051   7,360,865   6,746,105   7,156,508       7,159,972   7,083,271   7,165,834   7,080,894   7,201,469       7,090,336   7,156,489   7,413,283   7,438,385   7,083,257       7,258,422   7,255,423   7,219,980   10/760,253   7,416,274       7,367,649   7,118,192   10/760,194   7,322,672   7,077,505       7,198,354   7,077,504   10/760,189   7,198,355   7,401,894       7,322,676   7,152,959   7,213,906   7,178,901   7,222,938       7,108,353   7,104,629   7,246,886   7,128,400   7,108,355       6,991,322   7,287,836   7,118,197   10/728,784   7,364,269       7,077,493   6,962,402   10/728,803   7,147,308   10/728,779       7,118,198   7,168,790   7,172,270   7,229,155   6,830,318       7,195,342   7,175,261   7,465,035   7,108,356   7,118,202       7,510,269   7,134,744   7,510,270   7,134,743   7,182,439       7,210,768   7,465,036   7,134,745   7,156,484   7,118,201       7,111,926   7,431,433   7,018,021   7,401,901   7,468,139       11/188,017   11/097,308   7,448,729   7,246,876   7,431,431       7,419,249   7,377,623   7,334,876   10/944,043   11/182,002       7,249,901   7,477,987   7,156,289   7,178,718   7,225,979       11/084,796   11/084,742   11/084,806   09/575,197   7,079,712       6,825,945   7,330,974   6,813,039   7,190,474   6,987,506       6,824,044   7,038,797   6,980,318   6,816,274   7,102,772       7,350,236   6,681,045   6,678,499   6,679,420   6,963,845       6,976,220   6,728,000   7,110,126   7,173,722   6,976,035       6,813,558   6,766,942   6,965,454   6,995,859   7,088,459       6,720,985   7,286,113   6,922,779   6,978,019   6,847,883       7,131,058   7,295,839   7,406,445   09/693,690   6,959,298       6,973,450   7,150,404   6,965,882   7,233,924   09/575,181       09/722,174   7,175,079   7,162,259   6,718,061   7,464,880       7,012,710   6,825,956   7,451,115   7,222,098   10/291,825       7,263,508   7,031,010   6,972,864   6,862,105   7,009,738       6,989,911   6,982,807   7,518,756   6,829,387   6,714,678       6,644,545   6,609,653   6,651,879   10/291,555   7,293,240       7,467,185   7,415,668   7,044,363   7,004,390   6,867,880       7,034,953   6,987,581   7,216,224   7,506,153   7,162,269       7,162,222   7,290,210   7,293,233   7,293,234   6,850,931       6,865,570   6,847,961   10/685,523   10/685,583   7,162,442       10/685,584   7,159,784   10/804,034   7,404,144   6,889,896       10/831,232   7,174,056   6,996,274   7,162,088   7,388,985       7,417,759   7,362,463   7,259,884   7,167,270   7,388,685       6,986,459   10/954,170   7,181,448   10/981,626   10/981,616       7,324,989   7,231,293   7,174,329   7,369,261   7,295,922       7,200,591   11/020,106   11/020,260   11/020,321   11/020,319       7,466,436   7,347,357   11/051,032   7,382,482   11/107,944       7,446,893   11/082,940   11/082,815   7,389,423   7,401,227       6,991,153   6,991,154   11/124,256   11/123,136   7,322,524       7,068,382   7,007,851   6,957,921   6,457,883   7,044,381       7,094,910   7,091,344   7,122,685   7,038,066   7,099,019       7,062,651   6,789,194   6,789,191   10/900,129   7,278,018       7,360,089   10/982,975   7,467,416   6,644,642   6,502,614       6,622,999   6,669,385   6,827,116   7,011,128   7,416,009       6,549,935   6,987,573   6,727,996   6,591,884   6,439,706       6,760,119   7,295,332   7,064,851   6,826,547   6,290,349       6,428,155   6,785,016   6,831,682   6,741,871   6,927,871       6,980,306   6,965,439   6,840,606   7,036,918   6,977,746       6,970,264   7,068,389   7,093,991   7,190,491   7,511,847       10/932,044   10/962,412   7,177,054   7,364,282   10/965,733       10/965,933   10/974,742   7,468,809   7,180,609   10/986,375       7,466,438   7,292,363   7,515,292   6,982,798   6,870,966       6,822,639   6,474,888   6,627,870   6,724,374   6,788,982       7,263,270   6,788,293   6,946,672   6,737,591   7,091,960       7,369,265   6,792,165   7,105,753   6,795,593   6,980,704       6,768,821   7,132,612   7,041,916   6,797,895   7,015,901       7,289,882   7,148,644   10/778,056   10/778,058   10/778,060       7,515,186   10/778,063   10/778,062   10/778,061   10/778,057       7,096,199   7,286,887   7,400,937   7,474,930   7,324,859       7,218,978   7,245,294   7,277,085   7,187,370   10/917,436       10/943,856   10/919,379   7,019,319   10/943,878   10/943,849       7,043,096   7,148,499   7,463,250   11/155,556   11/155,557       7,055,739   7,233,320   6,830,196   6,832,717   7,182,247       7,120,853   7,082,562   6,843,420   10/291,718   6,789,731       7,057,608   6,766,944   6,766,945   7,289,103   7,412,651       7,299,969   7,264,173   10/409,864   10/537,159   7,111,791       7,077,333   6,983,878   10/786,631   7,134,598   7,431,219       6,929,186   6,994,264   7,017,826   7,014,123   7,134,601       7,150,396   7,469,830   7,017,823   7,025,276   7,284,701       7,080,780   7,376,884   10/492,169   7,469,062   7,359,551       7,444,021   7,308,148   10/502,575   10/531,229   10/531,733       10/683,040   10/510,391   10/510,392   10/778,090   6,957,768       7,456,820   7,170,499   7,106,888   7,123,239   6,982,701       6,982,703   7,227,527   6,786,397   6,947,027   6,975,299       7,139,431   7,048,178   7,118,025   6,839,053   7,015,900       7,010,147   7,133,557   6,914,593   7,437,671   6,938,826       7,278,566   7,123,245   6,992,662   7,190,346   7,417,629       7,468,724   11/075,917   7,221,781   11/102,843   10/727,181       10/727,162   7,377,608   7,399,043   7,121,639   7,165,824       7,152,942   10/727,157   7,181,572   7,096,137   7,302,592       7,278,034   7,188,282   10/727,159   10/727,180   10/727,179       10/727,192   10/727,274   10/727,164   7,523,111   10/727,198       10/727,158   10/754,536   10/754,938   10/727,160   10/934,720       7,369,270   6,795,215   7,070,098   7,154,638   6,805,419       6,859,289   6,977,751   6,398,332   6,394,573   6,622,923       6,747,760   6,921,144   10/884,881   7,092,112   7,192,106       7,457,001   7,173,739   6,986,560   7,008,033   11/148,237       7,195,328   7,182,422   7,374,266   7,427,117   7,448,707       7,281,330   10/854,503   7,328,956   10/854,509   7,188,928       7,093,989   7,377,609   10/854,495   10/854,498   10/854,511       7,390,071   10/854,525   10/854,526   10/854,516   7,252,353       10/854,515   7,267,417   10/854,505   10/854,493   7,275,805       7,314,261   10/854,490   7,281,777   7,290,852   7,484,831       10/854,523   10/854,527   10/854,524   10/854,520   10/854,514       10/854,519   10/854,513   10/854,499   10/854,501   7,266,661       7,243,193   10/854,518   10/934,628   7,448,734   7,425,050       7,364,263   7,201,468   7,360,868   7,234,802   7,303,255       7,287,846   7,156,511   10/760,264   7,258,432   7,097,291       10/760,222   10/760,248   7,083,273   7,367,647   7,374,355       7,441,880   10/760,205   10/760,206   7,513,598   10/760,270       7,198,352   7,364,264   7,303,251   7,201,470   7,121,655       7,293,861   7,232,208   7,328,985   7,344,232   7,083,272       11/014,764   11/014,763   7,331,663   7,360,861   7,328,973       7,427,121   7,407,262   7,303,252   7,249,822   11/014,762       7,311,382   7,360,860   7,364,257   7,390,075   7,350,896       7,429,096   7,384,135   7,331,660   7,416,287   7,488,052       7,322,684   7,322,685   7,311,381   7,270,405   7,303,268       7,470,007   7,399,072   7,393,076   11/014,750   11/014,749       7,249,833   11/014,769   7,490,927   7,331,661   11/014,733       7,300,140   7,357,492   7,357,493   11/014,766   7,380,902       7,284,816   7,284,845   7,255,430   7,390,080   7,328,984       7,350,913   7,322,671   7,380,910   7,431,424   7,470,006       11/014,732   7,347,534   7,441,865   7,469,989   7,367,650       6,454,482   6,808,330   6,527,365   6,474,773   6,550,997       7,093,923   6,957,923   7,131,724   7,396,177   7,168,867       7,125,098   7,413,363                    
The disclosures of these co-pending applications are incorporated herein by cross-reference.
 
     BACKGROUND OF THE INVENTION 
     The Applicant has developed the Netpage system discussed in detail below and in many of the above cross reference documents. As the invention is particularly well suited to this system, it will be described in a Netpage context. However, it will be appreciated that hand-held optical sensors have broad ranging application in many different fields and the invention is not limited to its use within the Netpage system. 
     This Netpage system involves the interaction between a user and a computer network (or stand alone computer) via a pen and paper based interface. The ‘pen’ is an electronic stylus with a marking or non-marking nib and an optical sensor for reading a pattern of coded data on the paper (or other surface). 
     Netpage pens have a unique identity so that the owner of the pen can be recorded in the network. Registering the owner of each pen has a number of advantages such ‘walk-up’ printing (described in the co-pending application Ser. No. 11/193,479), signature recognition and so on. In light of this, pen owners will want to be able to quickly identify their own Netpage pen(s) from those of others. 
     Beyond the Netpage context, most people with quality pens consider them to be personal property and may engrave them to indicate ownership. However, this has little impact on the appearance of the pen and there is still a risk of confusion if several co-workers have the same brand of pen. 
     SUMMARY OF THE INVENTION 
     According to a first aspect, the present invention provides a stylus comprising: 
     an elongate chassis molding; 
     a nib at one end of the chassis molding; and, 
     an elongate cover molding for close-fitting engagement with the chassis molding; such that, 
     the cover molding is user replaceable. 
     By providing a cover molding that the user can easily remove and replace at will, each pen can be individually customized. The owner of each pen can quickly distinguish their pen from those of others. Regular replacement of the cover prevents the pen from looking worn and lets a user choose a new appearance if they tire of the old one, or if they discover a co-worker already has the same cover. 
     Optionally, the stylus is an electronic stylus wherein the chassis molding houses electronic components. Optionally, the cover molding is a tubular molding that snap locks onto the chassis molding exterior. Optionally, the tubular molding is slid into place on the chassis molding, wherein the chassis molding has a location detail to indicate that the tubular molding has been pushed home. Optionally, the tubular molding can be slid off the chassis molding by grasping the nib end of the stylus and pulling the tubular molding off the opposing end. Optionally, the cover molding is a suitable substrate for aquagraphic prints. 
     Optionally, the chassis has LEDs to indicate the operational status of the stylus and the tubular molding has one or more transparent windows for viewing the LEDs. 
     ADDITIONAL ASPECTS 
     Related aspects of the invention are set out below together with a discussion of their backgrounds to provide suitable context for the broad descriptions of these aspects. 
     Electronic Stylus with Substantially Triangular Cross-Section 
     Background 
     The Netpage system involves the interaction between a user and a computer network (or stand alone computer) via a pen and paper based interface. The ‘pen’ is an electronic stylus with a marking or non-marking nib and an optical sensor for reading a pattern of coded data on the paper (or other surface). 
     The pen is intended to be held in the same manner as a normal pen and therefore inclined relative to paper instead of normal to the plane of the paper. The optical sensor is adjacent the nib and so the distance between the lens and the surface of the page will differ depending on whether the lens is above, below or beside the nib as it is held inclined to the paper. As the focal length of the lens is generally fixed, the optics require a large depth of field and blur tolerance to accommodate every possible position of the lens relative to the paper. This imposes practical limits on the size of the coded data, the optics and the tilt of the pen during use. 
     Summary 
     Accordingly, this aspect provides a hand-held, electronic stylus for use with a surface having coded data disposed thereon, the stylus comprising: 
     an elongate casing with a grip having a substantially triangular cross section for pen-like manipulation of the stylus; 
     a nib at one end of the casing for contact with the surface, the nib having a longitudinal axis that is offset from the longitudinal axis of the casing; and, 
     a sensor positioned adjacent the nib for optically sensing the coded data; wherein during use, 
     the stylus is held such that the longitudinal axis of the nib is proximate the apex of the substantially triangular cross section. 
     A rounded triangular profile gives the pen an ergonomically comfortable shape to grip and use the pen in the correct functional orientation. It offers a natural conformity to a triangular shape between thumb, index finger and middle finger. The range of pitch angles over which the pen is able to image the pattern on the paper can be optimised for this asymmetric usage. The shape of the pen helps to orient the pen correctly in the user&#39;s hand and to discourage the user from using the pen “upside-down”. 
     It is also a practical shape for accommodating the internal components. The ballpoint pen cartridge fits naturally into the apex of the triangular cross section, placing it consistently with the user&#39;s grip. This in turn provides space for the main PCB in the centre of the pen and for the battery in the base of the pen. It also naturally places the tag-sensing optics unobtrusively below the nib (with respect to nominal pitch). 
     Optionally, the nib is a ball point nib mounted to an elongate ink cartridge such that the cartridge extends along the longitudinal axis of the nib, proximate the apex of the substantially triangular cross section. 
     Optionally, the stylus further comprises an elongate battery mounted along the base of the triangular cross section, opposite the apex. 
     Optionally, the stylus further comprises a printed circuit board mounted between the battery and the cartridge. 
     Optionally, the sensor has an image sensor and lens for capturing images of the coded data when the sensor is in an operative position relative to the surface; 
     the sensor further comprising a plurality of light sources for illuminating the coded data for the image sensor, the light sources each configured for illuminating an area of the surface such that there is a common region illuminated by all the light sources; wherein during use, 
     at least one of the light sources is selectively extinguishable while at least one of the light sources provides sufficient illumination for image capture. 
     Optionally, the plurality if light sources are two LEDs mounted on either side of the lens. 
     Optionally, the substantially triangular cross section extends the length of the elongate casing. 
     Optionally, the substantially triangular cross section has rounded corners. 
     Optionally, the substantially triangular cross section approximates an equilateral triangle. 
     Hand-Held Optical Sensor with Multiple Light Sources 
     Background 
     The Netpage system involves the interaction between a user and a computer network (or stand alone computer) via a pen and paper based interface. The ‘pen’ is an electronic stylus with a marking or non-marking nib and an optical sensor for reading a pattern of coded data on the paper (or other surface). A source of light in the optical sensor brightly illuminates the surface so that an image of the coded data on the paper is focused by a lens onto the active region of an image sensor. The spectral emission peak of the light source is matched to the spectral absorption peak of the ink used to print the coded data to maximise contrast in captured image. 
     Unfortunately, as the pen is hand-held, it may be held to the paper at an angle that causes reflections from the light source that are detrimental to the image sensor. Glossy paper is particularly prone to this and the user is not likely to realise that any failure of the optical sensor to read the coded data is caused by the angle at which they are holding the pen. 
     Summary 
     Accordingly, this aspect provides a hand-held optical sensor for sensing coded data disposed on a surface, the sensor comprising: 
     an image sensor and lens for capturing images of the coded data when the optical sensor is in an operative position relative to the surface; 
     a plurality of light sources for illuminating the coded data for the image sensor, the light sources each configured for illuminating an area of the surface such that there is a common region illuminated by all the light sources; wherein during use, 
     at least one of the light sources is selectively extinguishable while at least one of the light sources provides sufficient illumination for image capture. 
     The use of two light sources that can be individually selected allows dynamic avoidance of undesirable reflections when the pen is held at some angles, especially on glossy paper. It also ensures a more uniform illumination of the coded data. 
     Optionally, the plurality of light sources are two illumination sources mounted on opposite sides of the lens. 
     Optionally, the two illumination sources have intersecting axes of illumination. Optionally, the optical sensor further comprises a control unit connected to the image sensor and the two illumination sources such that the control unit extinguishes one of the two illumination sources upon detection of undesirable reflection from the surface. Optionally the control unit predicts undesirable reflection from the surface using past detection of the undesirable reflection. Optionally, the control unit uses one or more captured images to compute the position of the stylus relative to the surface in order to predict when undesirable reflection will occur. 
     End Cap Switch for Electronic Stylus 
     Background 
     The Netpage system involves the interaction between a user and a computer network (or stand alone computer) via a pen and paper based interface. The ‘pen’ is an electronic stylus with a marking or non-marking nib and an optical sensor for reading a pattern of coded data on the paper (or other surface). 
     For convenience the electronics within the pen are powered by a rechargeable battery. This affords the pen a high degree of portability is likely to be carried about by the user for much of the day. However there will be prolonged periods where the pen is not used and it is inconvenient to return it to a battery recharger. To preserve the battery the user should ideally switch the pen off after each use. Unfortunately, users often forget to turn off the pen after each and every use. The pen can automatically power down after a set period of being idle. However a significant amount of battery power is wasted during the idle period. 
     Summary 
     Accordingly, this aspect provides an electronic stylus and end cap assembly comprising: 
     a stylus with an elongate casing that houses battery powered electronic components; 
     electrical contacts exposed by an opening in the elongate casing; and, 
     a cap that fits over one end of the stylus, the cap having a conductive portion positioned such that fitting the cap over said one end of the stylus electrically connects the contacts to control power to the electronic components. 
     By linking the power switch to the removal and replacement of the end cap, the pen is only active when it is uncapped. Whenever it is capped (and therefore not in use) it switched to a low power state to conserve power and extend battery life. The contacts of the on/off switch can be proximate the nib, in which case fitting the cap over the nib and closing the switch deactivates the pen. Alternatively the contacts can be at the opposite end of the pen and the cap closes the contacts to activate the pen immediately before use. 
     Optionally, the electronic stylus further comprises a nib at said one end of the elongate casing and fitting the cap over the nib, and the electrical contacts are proximate the nib such that fitting the cap over the nib switches the stylus to a low power inactive state. 
     Optionally, the stylus is configured to use the electrical contacts to recharge the battery. 
     Optionally, the conductive portion in the cap is a conductive elastomeric molding. 
     Optionally, the nib is a ball point nib and the stylus further comprises an tubular ink cartridge and a structure defining a cavity for retaining the ink cartridge, the structure having an open end for axially receiving the ink cartridge as it is slid into the cavity; wherein, 
     the open end of the structure is at least partially formed by the electrical contacts. 
     Optionally, the structure has conductive sections connected to the electrical contacts for transmitting power to the battery. Optionally, the structure is tubular with an internal conductive layer insulated from an outer conductive layer by an insulating layer. 
     Ink Cartridge with Inbuilt Cartridge Removal Tool 
     Background 
     The Netpage system involves the interaction between a user and a computer network (or stand alone computer) via a pen and paper based interface. The ‘pen’ is an electronic stylus with a marking or non-marking nib and an optical sensor for reading a pattern of coded data on the paper (or other surface). 
     One of the primary features of the Netpage pen is its ability to ‘click’ on interactive elements on a Netpage in the same way a mouse can click on screen-based interactive elements (e.g. hyperlinks and so on). However, with a Netpage pen, the user simply puts the nib on the interactive element in order to click on it. The optical sensor identifies the element via its unique page and page location while a force sensor registers a ‘pen down’ condition when the nib is pressed against the page. Registering ‘pen down’ and ‘pen up’ is also fundamental to capturing the users handwriting on Netpage input fields. 
     For optimal operation, the cartridge should be securely coupled to the force sensor. However, the cartridge should be easily de-coupled from the force sensor whenever it is replaced with a fresh cartridge. 
     Summary 
     Accordingly this aspect provides an ink cartridge for insertion into a stylus, the ink cartridge comprising: 
     an elongate body for containing a supply of ink, the elongate body having a nib end and an opposing end; and 
     an engagement formation at the opposing end for engaging the nib end of another ink cartridge of the same type in the stylus in order to extract it from the stylus. 
     By forming the customized removal tool on the cartridge itself, it will always be convenient to the user when a spent cartridge is to be replaced. The tool allows the user to grip the replacement cartridge for better purchase and more force when removing the existing cartridge from the stylus. With a greater extracting force, the releasable coupling between the cartridge and the force sensor can be tighter and more secure. Furthermore, extracting the cartridge through the nib end of the pen rather than the back (as is the case with many conventional pens) minimizes pen disassembly and the force sensor can remain in place. 
     Optionally, the nib end has a writing nib in fluid communication with the supply of ink. 
     Electronic Stylus with Recharging Contacts at Ink Cartridge Receptacle Opening 
     Background 
     The Netpage system involves the interaction between a user and a computer network (or stand alone computer) via a pen and paper based interface. The ‘pen’ is usually an electronic stylus with a writing nib and an optical sensor for reading a pattern of coded data on the paper (or other surface). 
     For convenience the electronics within the pen are powered by a rechargeable battery. Typically the pen is used frequently throughout the day with many intervening periods when the pen is not used. The battery can be sized to accommodate a full day&#39;s use before overnight recharging. However, it will be appreciated that battery size directly affects the overall size and weight of the pen. To keep the battery size down to a practical size, the user should be encouraged to connect the pen to the recharger when it is not in use. In light of this, connecting the pen to the recharger should be quick and simple for the user. 
     Summary 
     Accordingly this aspect provides an electronic stylus comprising: 
     an outer casing housing electronic components and a rechargeable battery; 
     a structure defining a receptacle for retaining an ink cartridge with a ball point nib at one end, the structure having an open end for axially receiving the ink cartridge as it is slid into the receptacle; wherein, 
     the open end of the structure is at least partially formed by electrical contacts configured for connection to complementary contacts within a battery recharger. 
     Putting the recharging contacts at the opening of the ink cartridge receptacle allows the pen to be simply placed into a cup style recharger when the pen is not being used. The internal shape of the recharger can be formed so that the outer casing and/or the nib accurately guides the recharging contacts into engagement with the complementary contacts. In this way, the recharger can effectively double as a pen holder on the user&#39;s desk. 
     Optionally, the structure has conductive sections connected to the electrical contacts for transmitting power to the battery. Optionally, the structure is tubular with an internal conductive layer insulated from an outer conductive layer by an insulating layer. 
     Optionally, the stylus further comprises a cap that fits over the nib, the cap having a conductive portion positioned such that fitting the cap over the nib switches the stylus to a low power inactive state. 
     Optionally, the stylus further comprises a printed circuit board (PCB) wherein the internal conductive layer and the outer conductive layer engage respective electrical contacts on the PCB. 
     Pre-Loaded Force Sensor 
     Background 
     The Netpage system involves the interaction between a user and a computer network (or stand alone computer) via a pen and paper based interface. The ‘pen’ is an electronic stylus with a marking or non-marking nib and an optical sensor for reading a pattern of coded data on the paper (or other surface). 
     One of the primary features of the Netpage pen is its ability to ‘click’ on interactive elements on a Netpage in the same way a mouse can click on screen-based interactive elements (e.g. hyperlinks and so on). However, with a Netpage pen, the user simply puts the nib on the interactive element in order to click on it. The optical sensor identifies the element via its unique page and location ID while a force sensor registers a ‘pen down’ condition when the nib is pressed against the page. Registering ‘pen down’ and ‘pen up’ is also fundamental to capturing the user&#39;s handwriting on Netpage input fields. Non-binary force signals are also captured for reproducing hand-drawn strokes with varying force-related width and opacity. Force variation can also be used as one of the dimensions examined during signature verification. 
     To accurately sense relatively light forces (such as the force of handwriting on a nib) the force sensor needs to be mounted against the nib or cartridge with very fine tolerances. With a full span movement of sensor being relatively small (typically less than 50 microns), positioning the force sensor and the replaceable cartridge with sufficiently accuracy can be prohibitively difficult and commercially impractical for a mass produced article. 
     Summary 
     Accordingly, this aspect provides a force sensor comprising: 
     a load bearing structure for contact with an input member subject to a force to be sensed; 
     a sensor circuit for converting a force applied to the load bearing structure into a signal indicative of the force; and, 
     a pre-load bias assembly for engaging the input member to bias it against the load bearing structure. 
     By keeping the input member biased against the load bearing structure of the sensor, accurately mounting the sensor next to the input member is no longer an issue. The biasing mechanism can be a simple spring structure while still providing a suitably consistent biasing force. Such a mechanism has relatively low production costs and avoids the need to adhere to fine tolerances. 
     Optionally, the pre-load bias assembly has a spring and engagement formations for releasably engaging the input member. Optionally, the sensor circuit is a piezoresistive bridge circuit. Optionally the sensor circuit is a capacitative or inductive force sensing circuit. Optionally, the sensor circuit senses forces up to about 500 grams (5 Newtons). Optionally the signal output from the sensor circuit supports a hand writing recognition facility. Optionally the load bearing structure has a 10 micron full span movement during operation. Optionally, the pre-load bias assembly applies a bias of about 10 grams to 20 grams (0.1 Newtons to 0.2 Newtons). 
     Optionally, the input member is an ink cartridge within a writing stylus. Optionally the load bearing structure has an elastomeric member to absorb shock loads to the input member. 
     In a closely related aspect, there is provided an electronic stylus comprising: 
     an elongate molding; 
     a nib at one end of the elongate molding; and, 
     a force sensor mounted to the elongate molding, the force sensor having a load bearing structure for contact with an input member connected to the nib, a sensor circuit for converting a force applied to the nib into a signal indicative of the force, and a pre-load bias assembly for engaging the input member to bias it against the load bearing structure. 
     Optionally, the nib is a ball point nib and the input member is an ink cartridge in fluid communication with the nib. 
     Pen with Side Loading Cartridge 
     Background 
     This Netpage system involves the interaction between a user and a computer network (or stand alone computer) via a pen and paper based interface. The ‘pen’ is an electronic stylus with a marking or non-marking nib and an optical sensor for reading a pattern of coded data on the paper (or other surface). 
     The Netpage pen is an electronic stylus with force sensing, optical sensing and Bluetooth communication assemblies. A significant number of electronic components need to be housed within the pen casing together with a battery large enough to provide a useful battery life. Despite this, the overall dimensions of the pen need to be small enough for a user to manipulate it as they would a normal pen. 
     If the Netpage pen has a ballpoint nib, the ink cartridge must be kept as small as possible to conserve space within the pen casing, yet not so small that it needs to be replaced too frequently. Furthermore, the force sensor is best located at the end of the cartridge axially opposite the nib. This effectively precludes retracting the cartridge through the top (non-writing end) of the pen without disassembling much of the pen. 
     Beyond the Netpage context, most ink pens have cartridges that need to be inserted or withdrawn through the ends of the tubular pen casing. This imposes structural restrictions of the shape of the cartridge and therefore its ink storage capacity. 
     Summary 
     Accordingly, this aspect provides a pen comprising: 
     an elongate chassis molding; and, 
     a cartridge with a nib and an elongate body; wherein, 
     the cartridge is configured for insertion and removal from the elongate chassis mold from a direction transverse to the longitudinal axis of the chassis molding. 
     Optionally, the cartridge is an ink cartridge and the elongate body houses an ink reservoir. 
     According to a closely related aspect, the present invention provides an ink cartridge for a pen, the ink cartridge comprising: 
     an elongate ink reservoir; and, 
     a writing nib in fluid communication with the ink reservoir; wherein, 
     the elongate ink reservoir has an enlarged transverse cross section along a portion of its length intermediate its ends. 
     By configuring the pen chassis and cartridge so that it can be inserted and removed from the side rather than through the ends, the capacity of the cartridge can be significantly increased. An enlarged section between the ends of the ink cartridge increases the capacity while allowing the relatively thin ends to be supported at the nib molding and opposing end of the pen chassis. In a Netpage pen, inserting the cartridge from the side avoids the need to remove the force sensor when replacing the cartridge. Again, the thinner sections at each end of the cartridge allow it to engage a ball point nib supported in the nib molding and directly engage the force sensor at the other end, while the enlarged middle portion increases the ink capacity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: 
         FIG. 1  shows the structure of a complete tag; 
         FIG. 2  shows a symbol unit cell; 
         FIG. 3  shows nine symbol unit cells; 
         FIG. 4  shows the bit ordering in a symbol; 
         FIG. 5  shows a tag with all bits set; 
         FIG. 6  shows a tag group made up of four tag types; 
         FIG. 7  shows the continuous tiling of tag groups; 
         FIG. 8  shows the interleaving of codewords A, B, C &amp; D within a tag; 
         FIG. 9  shows a codeword layout; 
         FIG. 10  shows a tag and its eight immediate neighbours labelled with its corresponding bit index; 
         FIG. 11  shows a nib and elevation of the pen held by a user; 
         FIG. 12  shows the pen held by a user at a typical incline to a writing surface; 
         FIG. 13  is a lateral cross section through the pen; 
         FIG. 14A  is a bottom and nib end partial perspective of the pen; 
         FIG. 14B  is a bottom and nib end partial perspective with the fields of illumination and field of view of the sensor window shown in dotted outline; 
         FIG. 15  is a partial perspective of the USB cable and USB socket in the top end of the pen; 
         FIG. 16  is an exploded perspective of the pen components; 
         FIG. 17  is a longitudinal cross section of the pen; 
         FIG. 18  is a partial longitudinal cross section of the cap placed over the nib end of the pen; 
         FIG. 19  is an exploded perspective of the optics assembly; 
         FIG. 20  is an exploded perspective of the force sensor assembly; 
         FIG. 21  is an exploded perspective of the ink cartridge tube and nib engaging removal tool; 
         FIG. 22  is a partially sectioned perspective of a new ink cartridge engaging the nib end of the currently installed ink cartridge; 
         FIG. 23  is a partial perspective of the packaged force sensor on the main PCB; 
         FIG. 24  is a longitudinal cross section of the force sensor and main PCB shown in  FIG. 15 ; 
         FIG. 25  is an exploded perspective of the cap assembly; 
         FIG. 26  is a circuit diagram of the pen USB and power CCT&#39;s; 
         FIG. 27A  is a partial longitudinal cross section of the nib and barrel molding; 
         FIG. 27B  is a partial longitudinal cross section of the IR LED&#39;s and the barrel molding; 
         FIG. 28  is a ray trace of the pen optics adjacent a sketch of the ink cartridge; 
         FIG. 29  is a side elevation of the lens; 
         FIG. 30  is a side elevation of the nib and the field of view of the optical sensor; 
         FIG. 31  is an exploded perspective of the pad; 
         FIG. 32  is a longitudinal cross section of the pad with the pen inserted; 
         FIG. 33  is a schematic representation of the force sensor assembly; 
         FIG. 34  is a schematic representation of a top-loading ink cartridge and force sensor; 
         FIG. 35  is a schematic representation of a top loading ink cartridge into a pen with a retaining cavity for the pre-load spring; 
         FIG. 36  is a block diagram of the pen electronics; 
         FIG. 37  show the charging and connection options for the pen and the pod; 
         FIGS. 38A to 38E  show the various components of the packaged force sensor; 
         FIG. 39  is a bottom perspective of the main PCB with the Bluetooth antenna shield removed; 
         FIG. 40  is a top perspective of the main PCB; 
         FIG. 41  is a bottom perspective of the chassis molding and elastomeric and cap; 
         FIG. 42A  is a perspective of the optics assembly lifted from the chassis molding; 
         FIG. 42B  is an enlarged partial perspective of the optics assembly seated in the chassis molding; 
         FIG. 43A  is a bottom perspective of the force sensor assembly partially installed in the chassis molding; 
         FIG. 43B  is a bottom perspective of the force sensing assembly installed in the chassis molding; 
         FIG. 44  is a bottom perspective of the battery and main PCB partially installed in the chassis molding; 
         FIG. 45  is a bottom perspective of the chassis molding with the base molding lifted clear; 
         FIGS. 46A and 46B  are enlarged partial perspectives showing the cold stake on the chassis molding being swaged and sealed to the base molding; 
         FIG. 47  is a bottom perspective of the product label being fixed to the base molding; 
         FIG. 48  is an enlarged partial perspective of the nib molding being inserted on the chassis molding; 
         FIG. 49  is a perspective of the tube molding being inserted over the chassis molding; 
         FIG. 50  is a perspective of the cap assembly being placed on the nib molding; 
         FIG. 51  is a diagram of the major power states of the pen; and, 
         FIG. 52  is a diagram of the operational states of the Bluetooth module. 
     
    
    
     DETAILED DESCRIPTION 
     As discussed above, the invention is well suited for incorporation in the Assignee&#39;s Netpage system. In light of this, the invention has been described as a component of a broader Netpage architecture. However, it will be readily appreciated that electronic styli have much broader application in many different fields. Accordingly, the present invention is not restricted to a Netpage context. 
     Netpage Surface Coding 
     Introduction 
     This section defines a surface coding used by the Netpage system (described in co-pending application Ser. No. 11/193.479 as well as many of the other cross referenced documents listed above) to imbue otherwise passive surfaces with interactivity in conjunction with Netpage sensing devices (described below). 
     When interacting with a Netpage coded surface, a Netpage sensing device generates a digital ink stream which indicates both the identity of the surface region relative to which the sensing device is moving, and the absolute path of the sensing device within the region. 
     Surface Coding 
     The Netpage surface coding consists of a dense planar tiling of tags. Each tag encodes its own location in the plane. Each tag also encodes, in conjunction with adjacent tags, an identifier of the region containing the tag. In the Netpage system, the region typically corresponds to the entire extent of the tagged surface, such as one side of a sheet of paper. Each tag is represented by a pattern which contains two kinds of elements. The first kind of element is a target. Targets allow a tag to be located in an image of a coded surface, and allow the perspective distortion of the tag to be inferred. The second kind of element is a macrodot. Each macrodot encodes the value of a bit by its presence or absence. 
     The pattern is represented on the coded surface in such a way as to allow it to be acquired by an optical imaging system, and in particular by an optical system with a narrowband response in the near-infrared. The pattern is typically printed onto the surface using a narrowband near-infrared ink. 
     Tag Structure 
       FIG. 1  shows the structure of a complete tag  200 . Each of the four black circles  202  is a target. The tag  200 , and the overall pattern, has four-fold rotational symmetry at the physical level. 
     Each square region represents a symbol  204 , and each symbol represents four bits of information. Each symbol  204  shown in the tag structure has a unique label  216 . Each label  216  has an alphabetic prefix and a numeric suffix. 
       FIG. 2  shows the structure of a symbol  204 . It contains four macrodots  206 , each of which represents the value of one bit by its presence (one) or absence (zero). 
     The macrodot  206  spacing is specified by the parameter s throughout this specification. It has a nominal value of 143 μm, based on 9 dots printed at a pitch of 1600 dots per inch. However, it is allowed to vary within defined bounds according to the capabilities of the device used to produce the pattern. 
       FIG. 3  shows an array  208  of nine adjacent symbols  204 . The macrodot  206  spacing is uniform both within and between symbols  208 . 
       FIG. 4  shows the ordering of the bits within a symbol  204 . 
     Bit zero  210  is the least significant within a symbol  204 ; bit three  212  is the most significant. Note that this ordering is relative to the orientation of the symbol  204 . The orientation of a particular symbol  204  within the tag  200  is indicated by the orientation of the label  216  of the symbol in the tag diagrams (see for example  FIG. 1 ). In general, the orientation of all symbols  204  within a particular segment of the tag  200  is the same, consistent with the bottom of the symbol being closest to the centre of the tag. 
     Only the macrodots  206  are part of the representation of a symbol  204  in the pattern. The square outline  214  of a symbol  204  is used in this specification to more clearly elucidate the structure of a tag  204 .  FIG. 5 , by way of illustration, shows the actual pattern of a tag  200  with every bit  206  set. Note that, in practice, every bit  206  of a tag  200  can never be set. 
     A macrodot  206  is nominally circular with a nominal diameter of ( 5/9)s. However, it is allowed to vary in size by ±10% according to the capabilities of the device used to produce the pattern. 
     A target  202  is nominally circular with a nominal diameter of (17/9)s. However, it is allowed to vary in size by ±10% according to the capabilities of the device used to produce the pattern. 
     The tag pattern is allowed to vary in scale by up to ±10% according to the capabilities of the device used to produce the pattern. Any deviation from the nominal scale is recorded in the tag data to allow accurate generation of position samples. 
     Tag Groups 
     Tags  200  are arranged into tag groups  218 . Each tag group contains four tags arranged in a square. Each tag  200  has one of four possible tag types, each of which is labelled according to its location within the tag group  218 . The tag type labels  220  are  00 ,  10 ,  01  and  11 , as shown in  FIG. 6 . 
       FIG. 7  shows how tag groups are repeated in a continuous tiling of tags, or tag pattern  222 . The tiling guarantees the any set of four adjacent tags  200  contains one tag of each type  220 . 
     Codewords 
     The tag contains four complete codewords. The layout of the four codewords is shown in  FIG. 8 . Each codeword is of a punctured 2 4 -ary (8, 5) Reed-Solomon code. The codewords are labelled A, B, C and D. Fragments of each codeword are distributed throughout the tag  200 . 
     Two of the codewords are unique to the tag  200 . These are referred to as local codewords  224  and are labelled A and B. The tag  200  therefore encodes up to 40 bits of information unique to the tag. 
     The remaining two codewords are unique to a tag type, but common to all tags of the same type within a contiguous tiling of tags  222 . These are referred to as global codewords  226  and are labelled C and D, subscripted by tag type. A tag group  218  therefore encodes up to 160 bits of information common to all tag groups within a contiguous tiling of tags. 
     Reed-Solomon Encoding 
     Codewords are encoded using a punctured 2 4 -ary (8, 5) Reed-Solomon code. A 2 4 -ary (8, 5) Reed-Solomon code encodes 20 data bits (i.e. five 4-bit symbols) and 12 redundancy bits (i.e. three 4-bit symbols) in each codeword. Its error-detecting capacity is three symbols. Its error-correcting capacity is one symbol. 
       FIG. 9  shows a codeword  228  of eight symbols  204 , with five symbols encoding data coordinates  230  and three symbols encoding redundancy coordinates  232 . The codeword coordinates are indexed in coefficient order, and the data bit ordering follows the codeword bit ordering. 
     A punctured 2 4 -ary (8, 5) Reed-Solomon code is a 2 4 -ary (15, 5) Reed-Solomon code with seven redundancy coordinates removed. The removed coordinates are the most significant redundancy coordinates. 
     The code has the following primitive polynominal:
 
 p ( x )= x   4   +x+ 1  (EQ 1)
 
     The code has the following generator polynominal:
 
 g ( x )=( x+α )( x+α   2 ) . . . ( x+α   10 )  (EQ 2)
 
     For a detailed description of Reed-Solomon codes, refer to Wicker, S. B. and V. K. Bhargava, eds.,  Reed - Solomon Codes and Their Applications , IEEE Press, 1994, the contents of which are incorporated herein by reference. 
     The Tag Coordinate Space 
     The tag coordinate space has two orthogonal axes labelled x and y respectively. When the positive x axis points to the right, then the positive y axis points down. 
     The surface coding does not specify the location of the tag coordinate space origin on a particular tagged surface, nor the orientation of the tag coordinate space with respect to the surface. This information is application-specific. For example, if the tagged surface is a sheet of paper, then the application which prints the tags onto the paper may record the actual offset and orientation, and these can be used to normalise any digital ink subsequently captured in conjunction with the surface. 
     The position encoded in a tag is defined in units of tags. By convention, the position is taken to be the position of the centre of the target closest to the origin. 
     Tag Information Content 
     Table 1 defines the information fields embedded in the surface coding. Table 2 defines how these fields map to codewords. 
                                                 TABLE 1                   Field definitions            field   width   description                    per codeword               codeword type   2   The type of the codeword, i.e. one of               A (b′00′), B (b′01′), C (b′10′) and D               (b′11′).       per tag       tag type   2   The type 1  of the tag, i.e. one of               00 (b′00′), 01 (b′01′), 10 (b′10′) and 11               (b′11′).       x coordinate   13   The unsigned x coordinate of the tag 2 .       y coordinate   13   The unsigned y coordinate of the tag b .       active area flag   1   A flag indicating whether the tag is a               member of an active area. b′1′ indicates               membership.       active area map   1   A flag indicating whether an active area       flag       map is present. b′1′ indicates the presence               of a map (see next field). If the map is               absent then the value of each map entry is               derived from the active area flag (see               previous field).       active area map   8   A map 3  of which of the tag&#39;s immediate               eight neighbours are members of an active               area. b′1′ indicates membership.       data fragment   8   A fragment of an embedded data stream.               Only present if the active area map is               absent.       per tag group       encoding format   8   The format of the encoding.               0: the present encoding               Other values are TBA.       region flags   8   Flags controlling the interpretation and               routing of region-related information.               0: region ID is an EPC               1: region is linked               2: region is interactive               3: region is signed               4: region includes data               5: region relates to mobile application               Other bits are reserved and must be zero.       tag size   16   The difference between the actual tag size       adjustment       and the nominal tag size 4 , in 10 nm units,               in sign-magnitude format.       region ID   96   The ID of the region containing the tags.       CRC   16   A CRC 5  of tag group data.       total   320                 1 corresponds to the bottom two bits of the x and y coordinates of the tag         2 allows a maximum coordinate value of approximately 14 m         3 FIG. 29 indicates the bit ordering of the map         4 the nominal tag size is 1.7145 mm (based on 1600 dpi, 9 dots per macrodot, and 12 macrodots per tag)         5 CCITT CRC-16 [7]              FIG. 10  shows a tag  200  and its eight immediate neighbours, each labelled with its corresponding bit index in the active area map. An active area map indicates whether the corresponding tags are members of an active area. An active area is an area within which any captured input should be immediately forwarded to the corresponding Netpage server for interpretation. It also allows the Netpage sensing device to signal to the user that the input will have an immediate effect.
 
                                                                         TABLE 2                   Mapping of fields to codewords                    codeword           field           codeword   bits   field   width   bits                            A   1:0   codeword type   2   all                   (b′00′)               10:2    x coordinate   9   12:4                19:11   y coordinate   9   12:4            B   1:0   codeword type   2   all                   (b′01′)                2   tag type   1   0               5:2   x coordinate   4   3:0                6   tag type   1   1               9:6   y coordinate   4   3:0               10   active area flag   1   all               11   active area map   1   all                   flag               19:12   active area map   8   all               19:12   data fragment   8   all           C 00     1:0   codeword type   2   all                   (b′10′)               9:2   encoding format   8   all               17:10   region flags   8   all               19:18   tag size   2   1:0                   adjustment           C 01     1:0   codeword type   2   all                   (b′10′)               15:2    tag size   14   15:2                    adjustment               19:16   region ID   4   3:0           C 10     1:0   codeword type   2   all                   (b′10′)               19:2    region ID   18   21:4            C 11     1:0   codeword type   2   all                   (b′10′)               19:2    region ID   18   39:22           D 00     1:0   codeword type   2   all                   (b′11′)               19:2    region ID   18   57:40           D 01     1:0   codeword type   2   all                   (b′11′)               19:2    region ID   18   75:58           D 10     1:0   codeword type   2   all                   (b′11′)               19:2    region ID   18   93:76           D 11     1:0   codeword type   2   all                   (b′11′)               3:2   region ID   2   95:94               19:4    CRC   16   all                        
Note that the tag type can be moved into a global codeword to maximise local codeword utilization. This in turn can allow larger coordinates and/or 16-bit data fragments (potentially configurably in conjunction with coordinate precision). However, this reduces the independence of position decoding from region ID decoding and has not been included in the specification at this time.
 
Embedded Data
 
     If the “region includes data” flag in the region flags is set then the surface coding contains embedded data. The data is encoded in multiple contiguous tags&#39; data fragments, and is replicated in the surface coding as many times as it will fit. 
     The embedded data is encoded in such a way that a random and partial scan of the surface coding containing the embedded data can be sufficient to retrieve the entire data. The scanning system reassembles the data from retrieved fragments, and reports to the user when sufficient fragments have been retrieved without error. 
     As shown in Table 3, a 200-bit data block encodes 160 bits of data. The block data is encoded in the data fragments of A contiguous group of 25 tags arranged in a 5 5 square. A tag belongs to a block whose integer coordinate is the tag&#39;s coordinate divided by 5. Within each block the data is arranged into tags with increasing x coordinate within increasing y coordinate. 
     A data fragment may be missing from a block where an active area map is present. However, the missing data fragment is likely to be recoverable from another copy of the block. 
     Data of arbitrary size is encoded into a superblock consisting of a contiguous set of blocks arranged in a rectangle. The size of the superblock is encoded in each block. A block belongs to a superblock whose integer coordinate is the block&#39;s coordinate divided by the superblock size. Within each superblock the data is arranged into blocks with increasing x coordinate within increasing y coordinate. 
     The superblock is replicated in the surface coding as many times as it will fit, including partially along the edges of the surface coding. 
     The data encoded in the superblock may include more precise type information, more precise size information, and more extensive error detection and/or correction data. 
                                                 TABLE 3                   Embedded data block            field   width   description                    data type   8   The type of the data in the superblock.               Values include:               0: type is controlled by region flags               1: MIME               Other values are TBA.       superblock width   8   The width of the superblock, in blocks.       superblock   8   The height of the superblock, in blocks.       height       data   160   The block data.       CRC   16   A CRC 6  of the block data.       total   200                 6 CCITT CRC-16 [7]            
Cryptographic Signature of Region ID
 
     If the “region is signed” flag in the region flags is set then the surface coding contains a 160-bit cryptographic signature of the region ID. The signature is encoded in a one-block superblock. 
     In an online environment any signature fragment can be used, in conjunction with the region ID, to validate the signature. In an offline environment the entire signature can be recovered by reading multiple tags, and can then be validated using the corresponding public signature key. 
     MIME Data 
     If the embedded data type is “MIME” then the superblock contains Multipurpose Internet Mail Extensions (MIME) data according to RFC 2045 (see Freed, N., and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME)—Part One: Format of Internet Message Bodies”, RFC 2045, November 1996), RFC 2046 (see Freed, N., and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME)—Part Two: Media Types”, RFC 2046, November 1996) and related RFCs. The MIME data consists of a header followed by a body. The header is encoded as a variable-length text string preceded by an 8-bit string length. The body is encoded as a variable-length type-specific octet stream preceded by a 16-bit size in big-endian format. 
     The basic top-level media types described in RFC 2046 include text, image, audio, video and application. 
     RFC 2425 (see Howes, T., M. Smith and F. Dawson, “A MIME Content-Type for Directory Information”, RFC 2045, September 1998) and RFC 2426 (see Dawson, F., and T. Howes, “vCard MIME Directory Profile”, RFC 2046, September 1998) describe a text subtype for directory information suitable, for example, for encoding contact information which might appear on a business card. 
     Encoding and Printing Considerations 
     The Print Engine Controller (PEC) supports the encoding of two fixed (per-page) 2 4 -ary (15, 5) Reed-Solomon codewords and six variable (per-tag) 2 4 -ary (15, 5) Reed-Solomon codewords. Furthermore, PEC supports the rendering of tags via a rectangular unit cell whose layout is constant (per page) but whose variable codeword data may vary from one unit cell to the next. PEC does not allow unit cells to overlap in the direction of page movement. 
     A unit cell compatible with PEC contains a single tag group consisting of four tags. The tag group contains a single A codeword unique to the tag group but replicated four times within the tag group, and four unique B codewords. These can be encoded using five of PEC&#39;s six supported variable codewords. The tag group also contains eight fixed C and D codewords. One of these can be encoded using the remaining one of PEC&#39;s variable codewords, two more can be encoded using PEC&#39;s two fixed codewords, and the remaining five can be encoded and pre-rendered into the Tag Format Structure (TFS) supplied to PEC. PEC imposes a limit of 32 unique bit addresses per TFS row. The contents of the unit cell respect this limit. PEC also imposes a limit of 384 on the width of the TFS. The contents of the unit cell respect this limit. 
     Note that for a reasonable page size, the number of variable coordinate bits in the A codeword is modest, making encoding via a lookup table tractable. Encoding of the B codeword via a lookup table may also be possible. Note that since a Reed-Solomon code is systematic, only the redundancy data needs to appear in the lookup table. 
     Imaging and Decoding Considerations 
     The minimum imaging field of view required to guarantee acquisition of an entire tag has a diameter of 39.6 s (i.e. (2×(12+2))√{square root over (2)}s), allowing for arbitrary alignment between the surface coding and the field of view. Given a macrodot spacing of 143 μm, this gives a required field of view of 5.7 mm. 
     Table 4 gives pitch ranges achievable for the present surface coding for different sampling rates, assuming an image sensor size of 128 pixels. 
                                                 TABLE 4                   Pitch ranges achievable for present surface       coding for different sampling rates; dot pitch =       1600 dpi, macrodot pitch = 9 dots, viewing distance =       30 mm, nib-to-FOV separation = 1 mm, image sensor       size = 128 pixels                sampling rate   pitch range                            2     ~ 40 to. 49           2.5     ~ 27 to. 36           3     ~ 10 to. 18                        
Given the present surface coding, the corresponding decoding sequence is as follows:
         locate targets of complete tag   infer perspective transform from targets   sample and decode any one of tag&#39;s four codewords   determine codeword type and hence tag orientation   sample and decode required local (A and B) codewords   codeword redundancy is only 12 bits, so only detect errors   on decode error flag bad position sample   determine tag x-y location, with reference to tag orientation   infer 3D tag transform from oriented targets   determine nib x-y location from tag x-y location and 3D transform   determine active area status of nib location with reference to active area map   generate local feedback based on nib active area status   determine tag type from A codeword   sample and decode required global (C and D) codewords (modulo window alignment, with reference to tag type)   although codeword redundancy is only 12 bits, correct errors; subsequent CRC verification will detect erroneous error correction   verify tag group data CRC   on decode error flag bad region ID sample   determine encoding type, and reject unknown encoding   determine region flags   determine region ID   encode region ID, nib x-y location, nib active area status in digital ink   route digital ink based on region flags
 
Note that region ID decoding need not occur at the same rate as position decoding. Note that decoding of a codeword can be avoided if the codeword is found to be identical to an already-known good codeword.
       

     Netpage Pen 
     Functional Overview 
     The Netpage pen is a motion-sensing writing instrument which works in conjunction with a tagged Netpage surface (see Netpage Surface Coding and Netpage Surface Coding Security sections above). The pen incorporates a conventional ballpoint pen cartridge for marking the surface, a motion sensor for simultaneously capturing the absolute path of the pen on the surface, an identity sensor for simultaneously identifying the surface, a force sensor for simultaneously measuring the force exerted on the nib, and a real-time clock for simultaneously measuring the passage of time. 
     While in contact with a tagged surface, as indicated by the force sensor, the pen continuously images the surface region adjacent to the nib, and decodes the nearest tag in its field of view to determine both the identity of the surface, its own instantaneous position on the surface and the pose of the pen. The pen thus generates a stream of timestamped position samples relative to a particular surface, and transmits this stream to a Netpage server (see Netpage Architecture section in co-pending application Ser. No. 11/193,479). The sample stream describes a series of strokes, and is conventionally referred to as digital ink (DInk). Each stroke is delimited by a pen down and a pen up event, as detected by the force sensor. 
     The pen samples its position at a sufficiently high rate (nominally 100 Hz) to allow a Netpage server to accurately reproduce hand-drawn strokes, recognise handwritten text, and verify hand-written signatures. 
     The Netpage pen also supports hover mode in interactive applications. In hover mode the pen is not in contact with the paper and may be some small distance above the surface of the paper (or tablet etc.). This allows the position of the pen, including its height and pose to be reported. In the case of an interactive application the hover mode behaviour can be used to move the cursor without marking the paper, or the distance of the nib from the coded surface could be used for tool behaviour control, for example an air brush function. The pen includes a Bluetooth radio transceiver for transmitting digital ink via a relay device to a Netpage server. When operating offline from a Netpage server the pen buffers captured digital ink in non-volatile memory. When operating online to a Netpage server the pen transmits digital ink in real time. 
     The pen is supplied with a docking cradle or “pod”. The pod contains a Bluetooth to USB relay. The pod is connected via a USB cable to a computer which provides communications support for local applications and access to Netpage services. 
     The pen is powered by a rechargeable battery. The battery is not accessible to or replaceable by the user. Power to charge the pen can be taken from the USB connection or from an external power adapter through the pod. The pen also has a power and USB-compatible data socket to allow it to be externally connected and powered while in use. The pen cap serves the dual purpose of protecting the nib and the imaging optics when the cap is fitted and signalling the pen to leave a power-preserving state when uncapped. 
     Pen Form Factor 
     The overall weight (45 g), size and shape (159 mm×17 mm) of the Netpage pen fall within the conventional bounds of hand-held writing instruments. 
     Ergonomics and Layout 
       FIG. 11  shows a rounded triangular profile gives the pen  400  an ergonomically comfortable shape to grip and use the pen in the correct functional orientation. It is also a practical shape for accommodating the internal components. A normal pen-like grip naturally conforms to a triangular shape between thumb  402 , index finger  404  and middle finger  406 . 
     As shown in  FIG. 12 , a typical user writes with the pen  400  at a nominal pitch of about 30 degrees from the normal toward the hand  408  when held (positive angle) but seldom operates a pen at more than about 10 degrees of negative pitch (away from the hand). The range of pitch angles over which the pen  400  is able to image the pattern on the paper has been optimised for this asymmetric usage. The shape of the pen  400  helps to orient the pen correctly in the user&#39;s hand  408  and to discourage the user from using the pen “upside-down”. The pen functions “upside-down” but the allowable tilt angle range is reduced. The cap  410  is designed to fit over the top end of the pen  400 , allowing it to be securely stowed while the pen is in use. Multi colour LEDs illuminate a status window  412  in the top edge (as in the apex of the rounded triangular cross section) of the pen  400  near its top end. The status window  412  remains un-obscured when the cap is stowed. A vibration motor is also included in the pen as a haptic feedback system (described in detail below). 
     As shown in  FIG. 13 , the grip portion of the pen has a hollow chassis molding  416  enclosed by a base molding  528  to house the other components. The ink cartridge  414  for the ball point nib (not shown) fits naturally into the apex  420  of the triangular cross section, placing it consistently with the user&#39;s grip. This in turn provides space for the main PCB  422  in the centre of the pen and for the battery  424  in the base of the pen. By referring to  FIG. 14   a , it can be seen that this also naturally places the tag-sensing optics  426  unobtrusively below the nib  418  (with respect to nominal pitch). The nib molding  428  of the pen  400  is swept back below the ink cartridge  414  to prevent contact between the nib molding  428  and the paper surface when the pen is operated at maximum pitch. 
     As best shown in  FIG. 14   b , the imaging field of view  430  emerges through a centrally positioned IR filter/window  432  below the nib  418 , and two near-infrared illumination LEDs  434 ,  436  emerge from the two bottom corners of the nib molding  428 . The use of two illumination LEDs  434 ,  436  ensures a more uniform illumination field  438 ,  440 . As the pen is hand-held, it may be held at an angle that causes reflections from one of the LED&#39;s that are detrimental to the image sensor. By providing more than one LED, the LED causing the offending reflections can be extinguished. 
     Pen Feedback Indications 
       FIG. 17  is a longitudinal cross section through the centre-line if the pen  400  (with the cap  410  stowed on the end of the pen). The pen incorporates red and green LEDs  444  to indicate several states, using colours and intensity modulation. A light pipe  448  on the LEDs  444  transmit the signal to the status indicator window  412  in the tube molding  416 . These signal status information to the user including power-on, battery level, untransmitted digital ink, network connection on-line, fault or error with an action. 
     A vibration motor  446  is used to haptically convey information to the user for important verification functions during transactions. This system is used for important interactive indications that might be missed due to inattention to the LED indicators  444  or high levels of ambient light. The haptic system indicates to the user when:
         The pen wakes from standby mode   There is an error with an action   To acknowledge a transaction
 
Pod Feedback Indications
       

     Turning briefly to the recharging pod  450  shown in  FIGS. 31 and 32 , red and green LEDs  452  to indicate various states using colours and intensity modulation. The light from the LEDs is transmitted to the exterior of the pod via the polymer light pipe molding  454 . These signal status information to the user including charging state, and untransmitted digital ink by illuminating/pulsating one LEDs  452  at a time. 
     Features and Accessories 
     As shown in  FIG. 15 , the pen has a power and data socket  458  is located in the top end  456  of the pen, hidden and moisture-sealed behind an elastomeric end-cap  460 . The end-cap can be pried open to give access to the socket  458  and reset switch (at the bottom of recess  464 ) and remains open while the cable  462  is in use. The USB power and data cable  462  allows the pen to be used for periods that exceed the battery life. 
     The usual method of charging the pen  400  is via the charging pod  450  shown in  FIGS. 31 and 32 . As will be described in greater detail below, the pod  450  includes a Bluetooth transceiver connected by USB to a computer and several LEDs to indicate for charging status. The pod is compact to minimise its desktop footprint, and has a weighted base for stability. Data transfer occurs between the pen and the pod via a Bluetooth radio link. 
     Market Differentiation 
     Digital mobile products and quality pens are usually considered as personal items. This pen product is used by both genders from 5 years upwards for personal, educational and business use, so many markets have to be catered for. The pen design allows for substantial user customisation of the external appearance of the pen  400  and the pod  450  by having user changeable parts, namely the cap  410 , an outer tube molding  466  (best shown in  FIGS. 16 and 49 ) and the pod jacket  468  (best shown in  FIGS. 31 and 32 ). These parts are aquagraphic printed (a water based transfer system) to produce a variety of high quality graphic images and textures over all surfaces of these parts. 
     These parts are accessories to the pen, allowing the user to change the appearance whenever they wish. A number of licensed images provide enhancers for the sale of accessories as an additional business model, similar to the practice with mobile phone covers. 
     Pen Mechanical Design 
     Parts and Assemblies 
     Referring to  FIG. 16 , the pen  400  has been designed as a high volume product and has four major sub-assemblies:
         an optical assembly  470 ;   a force sensing assembly  474 ;   a cap assembly  472 ; and,   the main assembly  476 , which holds the main PCB  422  and battery  424 .
 
Wherever possible, moldings have been designed as line-of-draw to reduce cost and promote longevity in the tooling.
       

     These assemblies and the other major parts can be identified in  FIG. 17 . As the form factor of the pen is to be as small as possible these parts are packed as closely as practical. The electrical components in the upper part of the pen, namely the force sensor assembly  474  and the vibration motor  446  all have sprung contacts ( 512  of  FIGS. 24 and 480  of  FIG. 38A  respectively) directly mating with contact pads  482  and  484  respectively (see  FIG. 40 ) on the PCB  422 . This eliminates the need for connectors and also decouples these parts from putting any stress onto the main PCB. 
     Although certain individual molded parts are thin walled (0.8 to 1.2 mm) the combination of these moldings creates a strong structure. The pen is designed not to be user serviceable and therefore has a cold stake under the exterior label to prevent user entry. Non-conducting plastics moldings are used wherever possible to allow an omnidirectional beam pattern to be formed by the Bluetooth radio antenna  486  (see  FIG. 40 ). 
     Optics Assembly 
     The major components of the optical assembly are as shown in  FIGS. 18 and 19 . The axial alignment of the lens  488  to the image sensor  490  is toleranced to be better than 50 μm to minimise blur at the image. The barrel molding  492  is therefore has high precision with tight tolerancing. It has a molded-in aperture  494  near the image sensor  490 , which provides the location for the lens  488 . As the effect of thermal expansion is very small on a molding this size, it is not necessary to use a more expensive material. 
     The flex PCB  496  mounts two infrared LEDs  434  and  436 , a wire bonded Chip-on-Flex image sensor  490  and some chip capacitors  502 . The flex PCB  496  is 75 micron thick polyimide, which allows the two infrared LEDs  434  and  436  to be manipulated. Stiffeners are required in certain areas on the flex as backing for the attached components. The flex PCB  496  is laser cut to provide accuracy for mounting onto the barrel molding  492  and fine pitch connector alignment. 
     Force Sensing Assembly and Ink Cartridge 
       FIGS. 20 ,  23 ,  24  and  40  show the components and installation of the force sensing assembly. The force sensing assembly  474  is designed to accurately measure force put on the ink cartridge  414  during use. It is specified to sense between 0 and 500 grams force with enough fidelity to support handwriting recognition in the Netpage services. This captive assembly has two coaxial conductive metal tubes  498 , a retainer spring  504  and a packaged force sensor  500 . 
     Conductive Metal Tube 
     The conductive metal tubes  498  has an insert molded insulation layer  506  between two metal tubes (inner tube  508  and outer tube  510 ), which each have a sprung gold plated contact finger ( 512  and  514  respectively). Power for charging the battery is provided by two contacts  516  (see  FIG. 31 ) in the charging pod  450  and is conducted by these two tubes directly to recharging contacts  518  and  520  (see  FIG. 40 ) on the main PCB  422 , via a spring contact ( 512  and  514  respectively) on each tube. 
     When the pen cap assembly  472  is placed on the front of the pen  400 , a conductive elastomeric molding in the pen cap mates with the ends of both concentric tubes in the conductive metal tube part, completing the circuit and signalling the cap presence to the pen electronics (see  FIG. 18 ). 
     Force Sensor Operating Principles 
       FIG. 33  schematically illustrates the operation of the force sensing assembly  474 . The spring  700  applies a pre-load to the force sensor IC  526  (via a ball bearing  524 ) before the cartridge  414  is subject to any force at the nib  418 . The cartridge  414  itself is not pushed against the force sensor as it passes through the spring. Instead, the spring pushes a boot  702  against the force sensor, and the boot is coupled to the end of the cartridge. The boot  702  is a compromise between allowing easy manual insertion and removal of cartridge  414 , and ensuring the cartridge is held securely without travel. The use of a boot  702  also allows the inclusion of a stop surface  698 . The stop limits the travel of the boot  702  thereby protecting the spring  700  from overload. 
     Packaged Force Sensor 
       FIGS. 38A to 38E  are perspectives of the various components of the packaged force sensor  500 .  FIG. 38A  shows a steel ball  524  protruding from the front of a sensor IC (chip)  526 . The ball  524  is the point contact used to transmit force directly to the chip. Wire bonds  604  connect the chip  526  to the spring contacts  478 . The chip sits in the recess  564  formed in the rear molding  566  shown in  FIG. 38B . A pressure relief vent  584  in the base of the recess  564  allows air trapped by the chip  526  to escape. The front molding  606  shown in  FIG. 38C , has slots  608  in its underside for the sprung contacts  478  and a central aperture  610  to hold the ball  524 . Location details  612  mate with corresponding details in the coaxial conductive tubes  498  as shown in  FIG. 24 . 
     As there is only 10 microns full span movement in this system, the mounting of this assembly in the pen and use of axial preload is tightly toleranced. The force sensing assembly is mounted in the top of the pen so that it can only stress the pen chassis molding  416  (see  FIG. 16 ), and force will not be transmitted to the main PCB  422 . The force sensor is a push fit onto the end of the inner conductive metal tube  508  also trapping the retainer spring  504 , which makes a simple dedicated assembly  500 . 
     Retainer Spring 
     Turning to  FIGS. 20 and 24 , the retainer spring  504  is the equivalent to the boot  702  described in  FIG. 33 . It is a high precision stamping out of thin sheet metal with an insulating layer  708  at the point where it contacts the ball  524 . This inhibits electrical interference with the force sensor IC  526  caused by external electrostatic discharge via the ink cartridge  414 . The metal retainer spring  504  is formed into four gripping arms  530  and two spring arms  532 . 
     A spent cartridge removal tool  534  is secured to the open end of the cartridge  414  with an interference fit. The gripping arms  530  grip a complementary external grip profile  704  on the removal tool  534 . The spring arms  532  extend beyond the end of gripping arms  530  to press against the stepped section  706  in the coaxial tube assembly  498 . This in turn pushes insulated base  708  against the ball  524  to put an accurate axial preload force of between 10 and 20 grams onto the force sensor. 
     Ink Cartridge 
     The pen ink cartridge  414  is best shown in  FIGS. 21A and 21B . Research shows that industry practice is for the ballpoint nib  418  to be made by one source and the metal tube  536  to be made by another, along with assembly and filling. There are no front loading standard ink cartridges that meet the design capacity and form factor requirements so a custom cartridge has been developed. This ink cartridge  414  has a 3 mm diameter tube  536  with a standard ballpoint nib inserted. The spent cartridge removal tool  534  is a custom end molding that caps the open end of the metal tube  536 . 
     The removal tool  536  contains an air vent  538  for ink flow, a location detail  540  and a co-molded elastomeric ring  542  around a recess  544  detail used for extracting the spent ink cartridge. The tool is levered down to engage the nib of the old cartridge and then drawn out through the nib end of the pen as shown in  FIG. 21B . The elastomer ring  542  reduces the possibility that a hard shock could damage the force sensor if the pen is dropped onto a hard surface. 
     The location detail  540  allows the ink cartridge  414  to accurately seat into the retainer spring  504  in the force sensing assembly  474  and to be preloaded against the force sensor  500 . The removal tool (apart from the co-molded elastomeric ring) is made out of a hard plastic such as acetal and can be molded in color to match the ink contents. The ink capacity is 5 ml giving an expected write-out length comparable with standard ballpoint ink cartridges. This capacity means that refill cycles will be relatively infrequent during the lifetime of the pen. 
     Force Sensing Method 
     Pressing the nib  418  against a surface will transfer the force to the ball  524  via the gripping arms  530 . The force from the nib adds to the preload force from the spring arms  532 . The force sensor is a push fit into the end of the coaxial tube assembly  498  and both directly connect to the PCB with spring contacts ( 478  and  512  respectively). 
       FIG. 24  shows the limited space available for an axial force sensor, hence a packaged design is required as off-the-shelf items have no chance of fitting in this space envelope in the required configuration. 
     This force sensing arrangement detects the axial force applied to the cartridge  414 , which is the simplest and most accurate solution. There is negligible friction in the system as the cartridge contacts only on two points, one at either end of the conductive metal barrel  498 . The metal retainer spring  504  will produce an accurate preload force up to 20 grams onto the force sensor  500 . This is seen to be a reliable system over time, as the main parts are metal and therefore will not suffer from creep, wear or stiction during the lifetime of the pen. 
     This design also isolates the applied force by directing it onto the packaged force sensor, which pushes against the solid seat in the chassis molding  416  of the pen. This allows the force sensing assembly  474  to float above the main PCB  422  (so as not to put strain on it) whilst transmitting data via the spring contacts  478  at the base of the packaged force sensor  500 . The resulting assembly fits neatly into the pen chassis molding  416  and is easy to hand assemble. 
     Top/Side Loading Cartridge 
     As discussed above, the pen will require periodic replacement of the ink cartridge during its lifetime. While the front loading ink cartridge system is convenient for users, it can have some disadvantages. Front loading limits the capacity of the ink reservoir in the cartridge, since the diameter of the cartridge along its full length is limited to the minimum cartridge diameter, as dictated by the constraints of the pen nose. 
     The cartridge  414  must be pushed against the force sensor IC  526  (via the steel ball  524 ) by a pre-load spring  700  (see  FIG. 33 ). However, the cartridge  414  itself does not provide the face against which the spring pushes, since the cartridge must pass through the spring. This necessitates the boot  702  or retaining spring  504  discussed above. The boot is necessarily a compromise between allowing easy manual insertion and removal of cartridge, and ensuring the cartridge is held securely without travel. 
     A ‘top-loading’ cartridge, as illustrated in  FIG. 34 , can overcome these disadvantages. It will be appreciated that ‘top loading’ is a reference to insertion of the cartridge from a direction transverse to the longitudinal axis of the pen. Because of the other components within the pen, it is most convenient to insert the cartridge from the ‘top’ or apex  420  of the pen&#39;s substantially triangular cross section (see  FIG. 13 ). 
     The pre-load spring  700  can be placed toward the nib  418  of the cartridge  414 , thus providing a convenient mechanism for seating the cartridge against the force sensor ball  524  after insertion. A cartridge travel stop  712  is formed on the chassis molding  416  to prevent overloading the force sensor  526 . Since the cartridge itself provides the face against which the pre-load spring pushes, the boot is eliminated and the cartridge couples directly with the force sensor. 
     As the cartridge is no longer constrained to a single diameter along its full length, its central section can be wider and accommodate a much larger ink reservoir  710 . 
     The currently proposed pen design has an internal chassis  416  and an external tube molding  466 . The external molding  466  is user replaceable, allowing the user to customise the pen  400 . Removing the external molding  466  also provides the user with access to the pen&#39;s product label  652  (see  FIG. 47 ). Skilled workers in this field will appreciate that the chassis molding  416  and the base molding  528  could be modified to provide the user with access to a replaceable battery. 
     Referring again to  FIG. 34 , removing the external molding  466  (not shown) can also provide the user with access to the top-loading pen cartridge  414 . Once the external molding is removed, most of the length of the pen cartridge  414  is exposed. The user removes the cartridge by sliding it forwards against the pre-load spring  700  to extract its tail  718  from the force sensor aperture  720 , then tilting it upwards to free the tail  718  from the cartridge cavity  722 , and finally withdrawing the cartridge  710  from the pre-load spring  700  and cavity  722 . The user inserts a new cartridge by following the same procedure in reverse. 
     Since a top-loading cartridge can have a much greater capacity than a front-loading cartridge, it is not unreasonable to require the user to remove the external molding  466  to replace the cartridge  414 , since the user will have to replace a top-loading cartridge much less often than a front-loading cartridge. 
     Referring to  FIG. 35 , the pre-load spring  700  can be provided with its own cavity  716  and retaining ring  714  to make it easier to insert the cartridge  414 . 
     Cap Assembly 
     The pen cap assembly  472  consists of four moldings as shown in  FIG. 25 . These moldings combine to produce a pen cap which can be stowed on the top end of the pen  456  during operation. When capped, it provides a switch to the electronics to signal the capped state (described in ‘Cap Detection Circuit’ section below). A conductive elastomeric molding  522  inside the cap  410  functions as the cap switch when it connects the inner  512  and outer  514  metal tubes to short circuit them (see  FIG. 26 ). The conductive elastomeric molding  522  is pushed into a base recess in the cap molding  410 . It is held captive by the clip molding  544  which is offered into the cap and snaps in place. A metallised trim molding  546  snaps onto the cap molding  410  to complete the assembly  472 . 
     The cap molding  410  is line-of-draw and has an aquagraphic print applied to it. The trim  546  can be metallised in reflective silver or gold type finishes as well as coloured plastics if required. 
     Pen Feedback Systems—Vibratory 
     The pen  400  has two sensory feedback systems. The first system is haptic, in the form of a vibration motor  446 . In most instances this is the primary user feedback system as it is in direct contact with the users hand  408  and the ‘shaking’ can be instantly felt and not ignored or missed. 
     Pen Feedback Systems—Visual 
     The second system is a visual indication in the form of an indicator window  412  in the tube molding  466  on the top apex  420  of the pen  400 . This window aligns with a light pipe  448  in the chassis molding  416 , which transmits light from red and green indicator LEDs  452  on the main PCB  422 . The indicator window  412  is positioned so that it is not covered by the user&#39;s hand  408  and it is also unobstructed when the cap  410  is stowed on the top end  456  of the pen. 
     Optical Design 
     The pen incorporates a fixed-focus narrowband infrared imaging system. It utilises a camera with a short exposure time, small aperture, and bright synchronised illumination to capture sharp images unaffected by defocus blur or motion blur. 
                                   TABLE 5               Optical       Specifications                                    Magnification     ~ 0.225           Focal length of lens   6.0 mm           Viewing distance   30.5 mm           Total track length   41.0 mm           Aperture diameter   0.8 mm           Depth of field   . ~ /6.5 mm 7             Exposure time   200 us           Wavelength   810 nm 8             Image sensor size   140 × 140 pixels           Pixel size   10 um           Pitch range 9       ~ 15 to. 45 deg           Roll range     ~ 30 to. 30 deg           Yaw range   0 to 360 deg           Minimum sampling   2.25 pixels per           rate   macrodot           Maximum pen   0.5 m/s           velocity                         7 Allowing 70um blur radius             8 Illumination and filter             9 Pitch, roll and yaw are relative to the axis of the pen.            
Pen Optics and Design Overview
 
     Cross sections showing the pen optics are provided in  FIGS. 27A and 27B . An image of the Netpage tags printed on a surface  548  adjacent to the nib  418  is focused by a lens  488  onto the active region of an image sensor  490 . A small aperture  494  ensures the available depth of field accommodates the required pitch and roll ranges of the pen  400 . 
     First and second LEDs  434  and  436  brightly illuminate the surface  549  within the field of view  430 . The spectral emission peak of the LEDs is matched to the spectral absorption peak of the infrared ink used to print Netpage tags to maximise contrast in captured images of tags. The brightness of the LEDs is matched to the small aperture size and short exposure time required to minimise defocus and motion blur. 
     A longpass IR filter  432  suppresses the response of the image sensor  490  to any coloured graphics or text spatially coincident with imaged tags and any ambient illumination below the cut-off wavelength of the filter  432 . The transmission of the filter  432  is matched to the spectral absorption peak of the infrared ink to maximise contrast in captured images of tags. The filter also acts as a robust physical window, preventing contaminants from entering the optical assembly  470 . 
     The Imaging System 
     A ray trace of the optic path is shown in  FIG. 28 . The image sensor  490  is a CMOS image sensor with an active region of 140 pixels squared. Each pixel is 10 μm squared, with a fill factor of 93%. Turning to  FIG. 29 , the lens  488  is shown in detail. The dimensions are:
         D=3 mm   R 1 =3.593 mm   R 2 =15.0 mm   X=0.8246 mm   Y=1.0 mm   Z=0.25 mm
 
This gives a focal length of 6.15 mm and transfers the image from the object plane (tagged surface  548 ) to the image plane (image sensor  490 ) with the correct sampling frequency to successfully decode all images over the specified pitch, roll and yaw ranges. The lens  488  is biconvex, with the most curved surface facing the image sensor. The minimum imaging field of view  430  required to guarantee acquisition of an entire tag has a diameter of 39.6 s (s=spacing between macrodots in the tag pattern) allowing for arbitrary alignment between the surface coding and the field of view. Given a macrodot spacing, s, of 143 μm, this gives a required field of view of 5.7 mm.
       

     The required paraxial magnification of the optical system is defined by the minimum spatial sampling frequency of 2.25 pixels per macrodot for the fully specified tilt range of the pen  400 , for the image sensor  490  of 10 μm pixels. Thus, the imaging system employs a paraxial magnification o{tilde over (f)} 0.225, the ratio of the diameter of the inverted image (1.28 mm) at the image sensor to the diameter of the field of view (5.7 mm) at the object plane, on an image sensor  490  of minimum 128×128 pixels. The image sensor  490  however is 140×140 pixels, in order to accommodate manufacturing tolerances. This allows up to +/−120 μm (12 pixels in each direction in the plane of the image sensor) of misalignment between the optical axis and the image sensor axis without losing any of the information in the field of view. 
     The lens  488  is made from Poly-methyl-methacrylate (PMMA), typically used for injection moulded optical components. PMMA is scratch resistant, and has a refractive index of 1.49, with 90% transmission at 810 nm. The lens is biconvex to assist moulding precision and features a mounting surface to precisely mate the lens with the optical barrel molding  492 . 
     A 0.8 mm diameter aperture  494  is used to provide the depth of field requirements of the design. 
     The specified tilt range of the pen i{tilde over (s)} 15.0 to 45.0 degree pitch, with a roll range o{tilde over (f)} 30.0 to 30.0 degrees. Tilting the pen through its specified range moves the tilted object plane up to 6.3 mm away from the focal plane. The specified aperture thus provides a corresponding depth of field o{tilde over (f)}/6.5 mm, with an acceptable blur radius at the image sensor of 16 μm. 
     Due to the geometry of the pen design, the pen operates correctly over a pitch range o{tilde over (f)} 33.0 to 45.0 degrees. 
     Referring to  FIG. 30 , the optical axis  550  is pitched 0.8 degrees away from the nib axis  552 . The optical axis and the nib axis converge toward the paper surface  548 . With the nib axis  552  perpendicular to the paper, the distance A between the edge of the field of view  430  closest to the nib axis and the nib axis itself is 1.2 mm. 
     The longpass IR filter  432  is made of CR-39, a lightweight thermoset plastic heavily resistant to abrasion and chemicals such as acetone. Because of these properties, the filter also serves as a window. The filter is 1.5 mm thick, with a refractive index of 1.50. Each filter may be easily cut from a large sheet using a CO 2  laser cutter. 
     The Illumination System 
     The tagged surface  548  is illuminated by a pair of 3 mm diameter LEDs  434  and  436 . The LEDs emit 810 nm radiation with a divergence half intensity, half angle o{tilde over (f)}/15 degrees in a 35 nm spectral band (FWHM), each with a power of approximately 45 mW per steradian. 
     Pod Design and Assembly 
                               TABLE 6               Pod Mechanical Specifications                                Size   h63 × w43 × d46 mm       Mass   50 g       Operating   −10~+55 C.       Temperature       Operating Relative   10-90%       Humidity       Storage Temperature     ~ 20 to +60 C. worst case       Storage Relative   5-95%       Humidity       Shock and Vibration   Drop from 1 m onto a hard surface without           damage. Mechanical shock 600 G, 2.5 ms, 6           axis.       Serviceability   Replaceable jacket (part of customisation kit).           No internal user serviceable parts - the case is           not user openable.       Power   USB: 500 mA.           External power adapter: 600 mA at 5.5 VDC.                    
Pod Design
 
     The pen  400  is supplied with a USB tethered pod, which provides power to the pen and a Bluetooth transceiver for data transfer between the pen and the pod. Referring to  FIG. 31 , the pod  450  is a modular design and is comprised of several line of draw moldings. The pod tower molding  554  holds the pen at a  15  degree from vertical angle, which is both ergonomic from a pen stowing and extraction perspective, but also is inherently stable. 
     Pod Assembly 
     The assembly sequence for the pod  450  is as follows: 
     An elastomeric stop molding  556  is push fitted into the pod tower molding  554  to provide a positive stop for the pen when inserted into the pod. 
     The pod tower molding  554  has two metal contacts  516  pushed onto location ribs under the stop. These contacts  516  protrude into a void  558  where the nib molding  428  is seated as shown in  FIG. 32 . When a pen is present, they contact the coaxial metal barrels  498  around the ink cartridge  414 . These act as conductors to provide charge to the battery  424 . The pod PCB  560  is offered up into the pod tower molding  554  and snapped into place. Sprung charging contacts  562  on the metal contact piece  516  align with power pads on the pod PCB  560  during assembly. The underside of the pod PCB  450  includes several arrays of red, green and blue LEDs  564  which indicate several charging states from empty to full. Blue is the default ‘charging’ and ‘pod empty’ status color and they are transmitted via a translucent elastomeric light pipe  566  as an illuminated arc around the pod base molding  568 . 
     Despite a reasonable centre of gravity with a pen inserted, a cast weight  570  sits in the base molding  568  to increase stability and lessen the chance of the pod  450  falling over when knocked. The base molding  568  screws into the tower molding  554  to hold the weight  570 , light pipe  566  and PCB  560  after the tethered USB/power cable  572  is connected to the pod PCB  560 . 
     Personalisation 
     In line with the market differentiation ability of the pen, the pod includes a pod jacket molding  468 . This user removable molding is printed with the same aquagrahic transfer pattern as the tube and cap moldings of the pen it is supplied with as a kit. 
     Therefore the pattern of the pen, cap and pod are three items that strongly identify an individual users pen and pod to avoid confusion where there are multiple products in the same environment. They also allow this product to become a personal statement for the user. 
     The pod jacket molding  468  can be supplied as an aftermarket accessory in any number of patterns and images with the cap assembly  472  and the tube molding  466  as discussed earlier. 
     Electronics Design 
                               TABLE 7               Electrical Specifications                                Processor   ARM7 (Atmel AT91FR40162) running at           80 MHz           with 256 kB SRAM and 2 MB flash memory       Digital ink storage   5 hours of writing       capacity       Bluetooth   1.2       Compliance       USB Compliance   1.1       Battery standby time   12 hours (cap off), &gt;4 weeks (cap on)       Battery writing time   4 hours of cursive writing (81% pen down,           assuming easy offload of digital ink)       Battery charging   2 hours       time       Battery Life   Typically 300 charging cycles or 2 years           (whichever occurs first) to 80% of initial           capacity.       Battery   ~340 mAh at 3.7 V, Lithium-ion Polymer (LiPo)       Capacity/Type                    
Pen Electronics Block Diagram
 
       FIG. 36  is a block diagram of the pen electronics. The electronics design for the pen is based around five main sections. These are:
         the main ARM7 microprocessor  574 ,   the image sensor and image processor  576 ,   the Bluetooth communications module  578 ,   the power management unit IC (PMU)  580  and   the force sensor microprocessor  582 .
 
ARM7 Microprocessor
       

     The pen uses an Atmel AT91FR40162 microprocessor (see Atmel,  AT 91  ARM Thumb Microcontrollers—AT 91 FR 40162  Preliminary , http://www.keil.com/dd/docs/datashts/atmel/at91fr40162.pdf) running at 80 MHz. The AT91FR40162 incorporates an ARM7 microprocessor, 256 kBytes of on-chip single wait state SRAM and 2 MBytes of external flash memory in a stack chip package. 
     This microprocessor  574  forms the core of the pen  400 . Its duties include:
         setting up the Jupiter image sensor  584 ,   decoding images of Netpage coded impressions, with assistance from the image processing features of the image sensor  584 , for inclusion in the digital ink stream along with force sensor data received from the force sensor microprocessor  582 ,   setting up the power management IC (PMU)  580 ,   compressing and sending digital ink via the Bluetooth communications module  578 , and   programming the force sensor microprocessor  582 .       

     The ARM7 microprocessor  574  runs from an 80 MHz oscillator. It communicates with the Jupiter image sensor  576  using a Universal Synchronous Receiver Transmitter (USRT)  586  with a  40  MHz clock. The ARM7  574  communicates with the Bluetooth module  578  using a Universal Asynchronous Receiver Transmitter (UART)  588  running at 115.2 kbaud. Communications to the PMU  580  and the Force Sensor microprocessor (FSP)  582  are performed using a Low Speed Serial bus (LSS)  590 . The LSS is implemented in software and uses two of the microprocessor&#39;s general purpose IOs. 
     The ARM7 microprocessor  574  is programmed via its JTAG port. This is done when the microprocessor is on the main PCB  422  by probing bare pads  592  (see  FIG. 39 ) on the PCB. 
     Jupiter Image Sensor 
     The Jupiter Image Sensor  584  (see U.S. Ser. No. 10/778,056 listed in the cross referenced documents above) contains a monochrome sensor array, an analogue to digital converter (ADC), a frame store buffer, a simple image processor and a phase lock loop (PLL). In the pen, Jupiter uses the USRT&#39;s clock line and its internal PLL to generate all its clocking requirements. Images captured by the sensor array are stored in the frame store buffer. These images are decoded by the ARM7 microprocessor  574  with help from the Callisto image processor contained in Jupiter. 
     Jupiter controls the strobing of two infrared LEDs  434  and  436  at the same time as its image array is exposed. One or other of these two infrared LEDs may be turned off while the image array is exposed to prevent specular reflection off the paper that can occur at certain angles. 
     Bluetooth Communications Module 
     The pen uses a CSR BlueCore4-External device (see CSR,  BlueCore 4- External Data Sheet rev c,  6 Sep. 2004) as the Bluetooth controller  578 . It requires an external 8 Mbit flash memory device  594  to hold its program code. The BlueCore4 meets the Bluetooth v1.2 specification and is compliant to v0.9 of the Enhanced Data Rate (EDR) specification which allows communication at up to 3 Mbps. 
     A 2.45 GHz chip antenna  486  is used on the pen for the Bluetooth communications. 
     The BlueCore4 is capable of forming a UART to USB bridge. This is used to allow USB communications via data/power socket  458  at the top of the pen  456 . 
     Alternatives to Bluetooth include wireless LAN and PAN standards such as IEEE 802.11 (Wi-Fi) (see IEEE, 802.11  Wireless Local Area Networks , http://grouper.ieee.org/groups/802/11/index.html), IEEE 802.15 (see IEEE, 802.15  Working Group for WPAN , http://grouper.ieee.org/groups/802/15/index.html), ZigBee (see ZigBee Alliance, http://www.zigbee.org), and WirelessUSB Cypress (see  WirelessUSB LR  2.4- GHz DSSS Radio SoC , http://www.cypress.com/cfuploads/img/products/cywusb6935.pdf), as well as mobile standards such as GSM (see GSM Association, http://www.gsmworld.com/index.shtml), GPRS/EDGE,  GPRS Platform , http://www.gsmworld.com/technology/gprs/index.shtml), CDMA (see CDMA Development Group, http://www.cdg.org/, and Qualcomm, http://www.qualcomm.com), and UMTS (see 3rd Generation Partnership Project (3GPP), http://www.3gpp.org). 
     Power Management Chip 
     The pen uses an Austria Microsystems AS3603 PMU  580  (see Austria Microsystems,  AS 3603  Multi - Standard Power Management Unit Data Sheet v 2.0). The PMU is used for battery management, voltage generation, power up reset generation and driving indicator LEDs and the vibrator motor. 
     The PMU  580  communicates with the ARM7 microprocessor  574  via the LSS bus  590 . The PMU uses one of two sources for charging the battery  424 . These are the power from the power and USB jack  458  at the top of the pen  456  (see  FIG. 15 ) and the power from the pod  450  via the two conductive tubes  498  (see  FIG. 24 ). The PMU charges the pen&#39;s lithium polymer battery  424  using trickle current, constant current and constant voltage modes with little intervention required by the ARM7 microprocessor  574 . The PMU also includes a fuel gauge which is used by the ARM7 microprocessor to determine how much battery capacity is left. 
     The PMU  580  generates the following separate voltages:
         3.0V from an LDO for the ARM7 IO voltage and the Jupiter IO and pixel voltages.   3.0V from an LDO for the force sensor and force sensor filter and amplifier (3.0V for the force sensor microprocessor is generated from an off chip LDO since the PMU contains no LDOs that can be left powered on).   3.0V from an LDO for the BlueCore4 Bluetooth device.   1.8V from a buck converter for the ARM7 core voltage.   1.85V from an LDO for the Jupiter core voltage.   5.2V from a charge pump for the infrared LED drive voltage.
 
At power up or reset of the PMU, the ARM7 IO voltage and 1.8V core voltage are available. The other voltage sources need to be powered on via commands from the ARM7  574  via the LSS bus  590 .
       

     Indicator LEDs  444  and the vibrator motor  446  are driven from current sink outputs of the PMU  580 . 
     The PMU  580  can be put into ultra low power mode via a command over the LSS bus  590 . This powers down all of its external voltage sources. The pen enters this ultra low power mode when its cap assembly  472  is on. 
     When the cap  472  is removed or there is an RTC wake-up alarm, the PMU  580  receives a power on signal  596  from the force sensor microprocessor  582  and initiates a reset cycle. This holds the ARM7 microprocessor  574  in a reset state until all voltages are stable. A reset cycle can also be initiated by the ARM7  574  via a LSS bus message or by a reset switch  598  which is located at the top of the pen next to the USB and power jack  458  (see  FIG. 15 ). 
     Force Sensor Subsystem 
     The force sensor subsystem comprises a custom Hokuriku force sensor  500  (based on Hokuriku,  HFD -500  Force Sensor , http://www.hdk.co.jp/pdf/eng/e1381AA.pdf), an amplifier and low pass filter  600  implemented using op-amps and a force sensor microprocessor  582 . 
     The pen uses a Silicon Laboratories C8051F330 as the force sensor microprocessor  582  (see Silicon Laboratories,  C 8051 F 330/1  MCU Data Sheet, rev  1.1). The C8051F330 is an  8051  microprocessor with on chip flash memory, 10 bit ADC and 10 bit DAC. It contains an internal 24.5 MHz oscillator and also uses an external 32.768 kHz tuning fork. 
     The Hokuriku force sensor  500  is a silicon piezoresistive bridge sensor. An op-amp stage  600  amplifies and low pass (anti-alias) filters the force sensor output. This signal is then sampled by the force sensor microprocessor  582  at 5 kHz. 
     Alternatives to piezoresistive force sensing include capacitive and inductive force sensing (see Wacom, “Variable capacity condenser and pointer”, US Patent Application 20010038384, filed 8 Nov. 2001, and Wacom,  Technology , http://www.wacom-components.com/english/tech.asp). 
     The force sensor microprocessor  582  performs further (digital) filtering of the force signal and produces the force sensor values for the digital ink stream. A frame sync signal from the Jupiter image sensor  576  is used to trigger the generation of each force sample for the digital ink stream. The temperature is measured via the force sensor microprocessor&#39;s  582  on chip temperature sensor and this is used to compensate for the temperature dependence of the force sensor and amplifier. The offset of the force signal is dynamically controlled by input of the microprocessor&#39;s DAC output into the amplifier stage  600 . 
     The force sensor microprocessor  582  communicates with the ARM7 microprocessor  574  via the LSS bus  590 . There are two separate interrupt lines from the force sensor microprocessor  582  to the ARM7 microprocessor  574 . One is used to indicate that a force sensor sample is ready for reading and the other to indicate that a pen down/up event has occurred. 
     The force sensor microprocessor flash memory is programmed in-circuit by the ARM7 microprocessor  574 . 
     The force sensor microprocessor  582  also provides the real time clock functionality for the pen  400 . The RTC function is performed in one of the microprocessor&#39;s counter timers and runs from the external 32.768 kHz tuning fork. As a result, the force sensor microprocessor needs to remain on when the cap  472  is on and the ARM7  574  is powered down. Hence the force sensor microprocessor  582  uses a low power LDO separate from the PMU  580  as its power source. The real time clock functionality includes an interrupt which can be programmed to power up the ARM7  574 . 
     The cap switch  602  is monitored by the force sensor microprocessor  582 . When the cap assembly  472  is taken off (or there is a real time clock interrupt), the force sensor microprocessor  582  starts up the ARM7  572  by initiating a power on and reset cycle in the PMU  580 . 
     Pen Design 
     Electronics PCBs and Cables 
     There are two PCBs in the pen, the main PCB  422  ( FIG. 39 ) and the flex PCB  496  ( FIG. 19 ). The other separate components in the design are the battery  424 , the force sensor  500 , the vibrator motor  446  and the conductive tubes  498  ( FIG. 16 ) which function as the power connector to the pod  450  ( FIG. 31 ). 
     Main PCB 
       FIGS. 39 and 40  show top and bottom perspectives respectively of the main PCB  422 . The main PCB  422  is a 4-layer FR4  1 . 0  mm thick PCB with minimum trace width and separation of 100 microns. Via specification is 0.2 mm hole size in a 0.4 mm pad. The main PCB  422  is a rectangular board with dimensions 105 mm×11 mm. 
     The major components which are soldered to the main PCB are the Atmel ARM7 microprocessor  574 , the AMS PMU  580 , the Silicon Labs force sensor microprocessor  582 , the op-amps for force sensor conditioning amplifier  600  and the CSR Bluetooth chip  578  and its flash memory  594 , antenna  486  and shielding can  612 . 
     The force sensor  500 , the vibrator motor  446  and the coaxial conductive tubes  498  use sprung contacts to connect to pads on the main PCB  422 . All of these items are pushed down onto the main PCB  422  by the chassis molding  416  of the pen. 
     There are three connectors soldered onto the main PCB  422 ; the flex PCB connector  612 , the power and USB jack  458  at the top of the pen  456 , and the battery cable harness connector  616 . The cable harness to the battery is the only wired cable inside the pen. Also soldered onto the main PCB  422  is the reset switch  598 . This is in the recess  464  shown in  FIG. 5 . 
     Flex PCB 
     The Jupiter image sensor  576  is mounted on the flex PCB  496  as shown in  FIG. 19 . As the critical positioning tolerance in the pen is between the optics  426  and the image sensor  490 , the flex PCB  496  allows the optical barrel molding  492  to be easily aligned to the image sensor  490 . By having a flexible connection between the image sensor and the main PCB  422 , the positioning tolerance of the main PCB is not critical for the correct alignment of the optics  426 . 
     The image sensor  490 , the two infrared LEDs  434  and  436 , and five discrete bypass capacitors  502  are mounted onto the flex PCB  496 . The flex is a 2-layer polyimide PCB, nominally 75 microns thick. The PCB is specified as flex on install only, as it is not required to move after assembly of the pen. Stiffener  612  is placed behind the discrete components  502  and behind the image sensor  490  in order to keep these sections of the PCB flat. Stiffener is also placed at the connection pads  620  to make it the correct thickness for the connector  614  the main PCB  422  (see  FIG. 28 ). The PCB design has been optimised for panel layout during manufacture by keeping it roughly rectangular in overall shape. 
     The flex PCB  496  extends from the main PCB, widening around the image sensor  490  and then has two arms  622  and  624  that travel alongside the optical barrel  492  to the two infrared LEDs  434  and  436 . These are soldered directly onto the arms  622  and  624  of flex PCB. The total length of the flex PCB is 41.5 mm and at its widest point it is 9.5 mm. The image sensor  490  is mounted onto the flex PCB  496  using a chip on flex PCB (COF) approach. In this technology, the bare Jupiter die  628  is glued onto the flex PCB  496  and the pads on the die are wire-bonded onto target pads on the flex PCB. These target pads are located beside the die. The wire-bonds are then encapsulated to prevent corrosion. Two non-plated holes  626  in the flex PCB next to the die  628  are used to align the PCB to the optical barrel  492 . The optical barrel is then glued in place to provide a seal around the image sensor  470 . The horizontal positional tolerance between the centre of the optical path and the centre of the imaging area on the Jupiter die  628  is +/−50 microns. The vertical tolerance due to the thickness of the die, the thickness of the glue layer and the alignment of the optical barrel  492  to the front of the flex PCB  496  is +/−5 microns. In order to fit in the confined space at the front of the pen, the Jupiter die  628  is designed so that the pads required for connection in the Netpage pen are placed down opposite sides of the die. 
     Pod and External Cables 
     There are three main functions that are required by the pod and external cabling. They are:
         provide a charging voltage so that the pen can recharge its battery,   provide a relay mechanism for transferring stored digital ink to the Netpage server via its Bluetooth/USB adapter and   provide a relay mechanism for downloading new program code to the pen via its Bluetooth/USB adapter.
 
POD
       

     Again referring to  FIGS. 31 and 32 , when the pen  400  is inserted into the pod  450 , power is provided by way of two sprung contacts  516  in the pod which connect to the two coaxial conductive tubes  498  that hold the ink cartridge tube  536  in the pen. The power for the pod  450  and the pen  400  charging is provided by USB bus power. 
     The pod has a tethered cable  572  which ends in two connectors. One is a USB “A” plug. The other is a 4-way jack socket. This 4-way jack socket is the same one present at the top of the pen (see socket  458  in  FIG. 15 ). When the 4-way jack is inserted into the pod&#39;s cable, it provides power for the pod and to the pen for charging. Otherwise, the power for the pod and the pen charging is provided by the USB bus power. 
     Three indicator LEDs  452  are present in the pod. They indicate the status of pen charging and communications. 
     POD PCB 
     The pod PCB  560  contains a CSR BlueCore4-External device. This is the same type of Bluetooth device as used in the pen  400 . The BlueCore4 device functions as a USB to Bluetooth bridge. 
     Cabling 
     Three cables are provided with the pen. The first cable  572  is tethered to the pod. At the other end of the cable is a USB A connector and a 4-way jack socket. There are six wires going into the pod, the four USB wires and two from the 4-way jack socket. 
     The second cable is a USB cable  462  ( FIG. 15 ) with a USB A connector on one end and a 4-way jack on the other end. The 4-way jack can be connected to either the pod or the top of the pen. 
     The third cable is a plug pack power cable (not shown) which plugs into a power outlet at one end and has a 4-way jack on the other end. This 4-way jack can be connected to either the pod  450  or the top of the pen  456 . 
     Connection Options 
       FIG. 37  shows the main charging and connection options for the pen and pod:
         Option  1  shows a USB connection from a host  630  to the pod  450 . The pen  400  is in the pod  450 . The pod  450  and the pen  400  communicate via Bluetooth. The pod is powered by the USB bus power. The pen is charged from the USB bus power. As a result the maximum USB power of 500 mA must be available in order to charge the pen.   Option  2  shows a USB connection from the host  630  to the pod  450  and a plug pack  632  attached to the pod cable  572 . The pen  400  is in the pod  450 . The pod and the pen communicate via Bluetooth. The pod is powered by the plug pack. The pen is charged from the plug pack power.   Option  3  shows a USB connection from the host  630  to the pod  450  and a plug pack  632  attached to the pen  400 . The pen  400  is in the pod  450 . The pod and the pen communicate via Bluetooth. The pod is powered by the USB bus power. The pen is charged from the plug pack power.   Option  4  shows a plug pack  632  attached to the pod cable  572 . The pen  400  is in the pod  450 . There is no communication possible between the pod and the pen. The pod is powered by the plug pack. The pen is charged from the plug pack power.   Option  5  shows a USB connection from the host  630  to the pen  400 . The pen  400  is not in the pod  450 . The host  630  and the pen  400  communicate via USB, allowing a wired, non-RF communication link. The pen is charged from the USB bus power. As a result the maximum USB power of 500 mA must be available in order to charge the pen.   Option  6  shows the plug pack  632  attached to the pen  400 . The pen  400  is not in the pod  450 . The pen is charged from the plug pack power.   Other connection options are not shown. However, it should be kept in mind that the pod is powered via its 4-way jack connector (and not from the USB bus power) if there is a connector in this jack. Also, the pen is powered from its 4-way jack (and not from its pod connection) when there is a connector in this jack.
 
Battery and Power Consumption
       

     Referring to  FIG. 44 , the pen  400  contains a Lithium polymer battery  424  with a nominal capacity of 340 mAh. It&#39;s dimensions are 90.5 mm long×12 mm wide×4.5 mm thick. Based on the pen design, Table 8 shows the current requirements for various pen and Bluetooth states. 
                                                 TABLE 8                   Battery drain currents for all Pen states.                    Total mA @       State   Notes   VBatt 1                      Pen Capped   Pen is off   0.110       Pen Active   Pen Down   92.7       Pen Hover-1   Pen up, trying to decoded tags   31.7       Pen Hover-2   Pen up, decoding tags   62.9       Pen Idle   Pen up, not trying to decode tags   28.8       Bluetooth Not   Bluetooth IC off   0.0       Connected       Bluetooth Connection   Bluetooth connected in low power,   0.6       Timeout   no digital ink to download       Bluetooth Connected   Bluetooth connected in low power   4.1       (Sniff)   Sniff state       Bluetooth Connected   Bluetooth connected in high power   50.1       (Active)   Active state       Bluetooth Connecting   Bluetooth trying to connect   15.1           Network Access Point                 1 Sum of all current drains at battery. The Bluetooth currents can be concurrent with and additive to the Pen-state currents.            
Pen Usage Scenarios
 
     Some general usage scenarios are summarised here, showing the energy requirements needed to fulfil these scenarios. 
     Worst Case Scenario 
     Summary: The pen is used intensively for 4 hours (cursive writing) and will sit capped for one month (31 days), trying to offload stored digital ink. 
     The energy requirement for this scenario is 968 mAh. The nominal 340 mAh hour battery would achieve 35% of energy requirement for this scenario. 
     Single Working Week Case Scenario 
     Summary: The pen is used for cursive writing for a total of one hour a day for five days. and is capped for the remaining time. Total time for scenario is seven days.
         The energy requirement for this scenario is 456 mAh. The nominal 340 mAh hour battery would achieve 75% of energy requirement for this scenario.
 
Single Working Week not Capped During Working Hours Case Scenario
       

     Summary: The pen is used for cursive writing for a total of one hour a day for five days. and is capped for the remaining time. Total time for scenario is seven days. 
     The energy requirement for this scenario is 1561 mAh. The nominal 340 mAh hour battery would achieve 22% of energy requirement for this scenario. 
     Software Design 
     Netpage Pen Software Overview 
     The Netpage pen software comprises that software running on microprocessors in the Netpage pen  400  and Netpage pod  450 . 
     The pen contains a number of microprocessors, as detailed in the Electronics Design section described above. The Netpage pen software includes software running on the Atmel ARM7 CPU  574  (hereafter CPU), the Force Sensor microprocessor  582 , and also software running in the VM on the CSR BlueCore Bluetooth module  578  (hereafter pen BlueCore). Each of these processors has an associated flash memory which stores the processor specific software, together with settings and other persistent data. The pen BlueCore  578  also runs firmware supplied by the module manufacturer, and this firmware is not considered a part of the Netpage pen software. 
     The pod  450  contains a CSR BlueCore Bluetooth module (hereafter pod BlueCore). The Netpage pen software also includes software running in the VM on the pod BlueCore. As the Netpage pen  400  traverses a Netpage tagged surface  548 , a stream of correlated position and force samples are produced (see Netpage Overview above). This stream is referred to as DInk. Note that DInk may include samples with zero force (so called “Hover DInk”) produced when the Netpage pen is in proximity to, but not marking, a Netpage tagged surface. 
     The CPU component of the Netpage pen software is responsible for DInk capture, tag image processing and decoding (in conjunction with the Jupiter image sensor  576 ), storage and offload management, host communications, user feedback and software upgrade. It includes an operating system (RTOS) and relevant hardware drivers. In addition, it provides a manufacturing and maintenance mode for calibration, configuration or detailed (non-field) fault diagnosis. The Force Sensor microprocessor  582  component of the Netpage pen software is responsible for filtering and preparing force samples for the main CPU. The pen BlueCore VM software is responsible for bridging the CPU UART  588  interface to USB when the pen is operating in tethered mode. The pen BlueCore VM software is not used when the pen is operating in Bluetooth mode. 
     The pod BlueCore VM software is responsible for sensing when the pod  450  is charging a pen  400 , controlling the pod LEDs  452  appropriately, and communicating with the host PC via USB. 
     A more detailed description of the software modules is set out below. 
     The Netpage pen software is field upgradable, with the exception of the initial boot loader. The field upgradable portion does include the software running on the Force Sensor microprocessor  582 . Software upgrades are delivered to the pen via its normal communication mechanisms (Bluetooth or USB). After being received and validated, a new software image will be installed on the next shutdown/startup cycle when the pen contains no DInk pending offload. 
     Netpage System Overview 
     The Netpage pen software is designed to operate in conjunction with a larger software system, comprising Netpage relays and Netpage servers. The following is a brief overview of these systems in relation to the Netpage pen—a detailed discussion of the software for these systems and the specification of its interface to Netpage pen software is set out in the cross referenced documents. 
     Netpage relays are responsible for receiving DInk from pens, and transmitting that DInk to Netpage servers or local applications. The relay is a trusted service running on a device trusted by the pen (paired in Bluetooth terminology). The relay provides wide area networking services, bridging the gap between the pen and DInk consumers (such as Netpage servers or other applications). The primary relay device will be a desktop/laptop computer equipped with a Netpage pod. Bluetooth equipped mobile phones and PDAs can also be used as relays. Relays provide the pen with access to WAN services by bridging the Bluetooth connection to GPRS, WiFi or traditional wired LANs. 
     Netpage servers persist DInk permanently, and provide both application services for DInk based applications (such as handwriting recognition and form completion), and database functionality for persisted DInk (such as search, retrieval and reprinting). 
     Local applications may receive the DInk stream from the Netpage relay and use it for application specific purposes (such as for pointer replacement in image creation/manipulation applications). 
     Internal Design 
     The Netpage pen software is divided into a number of major modules:
         Image Processing   DInk storage and offload management   Host Communications   User Feedback   Power Management   Software Upgrade   Real Time Operating System   Hardware Drivers   Manufacturing and Maintenance mode   Force Sensor Microprocessor software   Pen BlueCore VM software   Pod BlueCore VM software
 
The remainder of this section gives a brief overview of these major software modules.
 
Image Processing
       

     The position information in the DInk stream produced by traversing a Netpage tagged surface is produced by performing an analysis of tagged images captured by the Jupiter Image Sensor  576 . 
     The Image Processing module is responsible for analysing images captured by Jupiter, identifying and decoding tags, estimating the pose of the pen, and combining this information to obtain position samples. 
     DInk Storage and Offload Management 
     Any DInk which corresponds to physical marking of a Netpage tagged surface (e.g. excluding Hover DInk) must be reliably and transactionally recorded by the Netpage system to allow for accurate reproduction of the Netpage tagged surface. Ensuring such DInk is recorded is the responsibility of the DInk storage and offload management software. It persists DInk in flash memory on the Netpage pen, and arranges for offload of DInk to a Netpage server via a Netpage relay. This offload process is transactional—the pen software maintains its record of DInk until it can guarantee that DInk has been received and persisted by a Netpage server. 
     DInk may be streamed in real time to applications requiring real time response to DInk (for example applications which use the pen as a replacement for a mouse or table pointer, such as graphics editing applications). This may be normal DInk or Hover DInk (for applications supporting hover), and the ability of the Netpage pen software to stream DInk to such applications is orthogonal to the storage and offload requirements for persistent DInk. 
     Host Communications 
     The Netpage pen software communicates with the Netpage relay either through wireless Bluetooth communication, or through a wired USB connection. Bluetooth connectivity is provided by the pen BlueCore. USB connectivity is provided by using the Bluetooth module in “pass through” mode. 
     The Communications module of the software is responsible for reliably transmitting DInk from the DInk storage and offload management module to the relay. It also provides management functionality such as maintaining a persistent list of known, trusted relays, and allows pairing with devices according to user specification. The communications module includes third party software (namely the ABCSP stack, see CSR,  ABCSP Overview, AN 11) provided by CSR for communication with the pen BlueCore. Bluetooth communication is only performed with Bluetooth paired devices, and uses the Bluetooth encryption facilities to secure these communications. 
     User Feedback 
     The Netpage pen provides two LEDs (red and green) and a vibration motor for user feedback. The user feedback software module is responsible for converting signals from other software modules into user feedback using the provided mechanisms. 
     Power Management 
     The Netpage pen has a limited power budget, and its design allows for dynamic power saving in a number of ways. For example, the CPU can disable peripherals when they are not in use to save power, and the pen BlueCore can be placed into a deep sleep mode or powered down when it is not required. The CPU itself can be powered down when the pen is not performing higher functions. Indeed, the only always-on components are the Force Sensor microprocessor  582  and Power Management Chip  580  which can power on the CPU in response to external stimuli. 
     The Power Management module  580  is responsible for analysing the current pen state and optimizing the power usage by switching off un-needed peripherals and other components as required. That is, this module intelligently manages the facilities offered by the Power Management module to provide optimal power usage given the required pen functionality. 
     Software Upgrade 
     The Netpage pen software is field upgradable, obtaining new software images via its Bluetooth or USB connections. The Software Upgrade module is responsible for managing the download of complete images via the Communications module, validating these images against included checksums, and arranging for the pen to boot from a revised image when it has been validated. 
     The Software Upgrade process happens largely concurrently with normal pen behaviour. The download of new images can happen concurrently with normal pen operation and DInk offload. However, the actual switch to boot from a new software image is only performed when no outstanding DInk remains to be offloaded. This simplifies management of the internal DInk formats, allowing them to be upgraded as necessary in new software loads. Existing pairing arrangements with relays are expected to survive software upgrade, although under some circumstances it may be necessary to repeat pairing operations. It should also be noted that small parts of the Netpage pen software, such as basic boot logic, are not field upgradable. These parts of the software are minimal and tightly controlled. 
     Note that the Software Upgrade module also manages software images for the Force Sensor microprocessor. Images for the latter form a part of the Netpage pen software load, and the Software Upgrade module reprograms the Force Sensor microprocessor in the field when a new image contains revisions to the Force Sensor microprocessor software. 
     Real Time Operating System 
     The Netpage pen software includes a Real Time Operating System (RTOS) for efficient management of CPU resources. This allows optimal handling of concurrent DInk capture, persistence, and offload despite the latencies involved in image capture, flash manipulation, and communication resources. 
     The RTOS for the Netpage pen software is the uC/OS II RTOS from Micrium Systems (see Labrosse, J. L.,  MicroC OS II: The Real Time Kernel,  2 nd Edition , CMP Books, ISBN 1578201039). This part of the Netpage pen software is comprised largely of third party code supplied by Micrium, tailored and customized for the needs of the pen. 
     Hardware Drivers 
     The Netpage pen software includes hardware drivers for all peripherals (both internal to the CPU and external to it) required for operation of the Netpage pen  400 . This includes USRT  586 , UART  588  and LSS  590  drivers for external bus communication, as well as higher level drivers for managing the Jupiter Image Sensor  576 , the pen BlueCore  578 , the Force Sensor microprocessor  582 , the Power Management IC  580 , and other internal systems. 
     Manufacturing and Maintenance Mode 
     The Netpage pen  400  may be put into a special manufacturing and maintenance mode for factory calibration or detailed non-field failure analysis. A deployed pen will never enter manufacturing and maintenance mode. It is a configuration, diagnostic and rectification mode that is only expected to be used by Silverbrook engineers under controlled conditions. The mechanism for placing the Netpage pen software into maintenance mode is not described here. 
     Force Sensor Microprocessor Software 
     The Force Sensor microprocessor  582  is an independent CPU tasked with filtering and resampling the force data obtained from the Force Sensor  500  proper to produce a stream of force samples to be included into the DInk stream as recorded by the pen. It is also responsible for initiating a wakeup of the CPU  574  in response to a pen down, uncap, or timer event, in the case that the CPU has been switched off for power saving purposes. 
     Pen Bluecore VM Software 
     The pen BlueCore is capable of running a small amount of software in a virtual machine (VM). Such VM software is highly resource limited, but can access the Bluetooth functionality, the I/O ports, and a small number of GPIO pins on the pen BlueCore. A small part of the Netpage pen software will run on the pen BlueCore in order to manage bridging the CPU UART to the USB connection provided by the pen BlueCore. 
     Pod Bluecore VM Software 
     The Netpage pod  450  contains a CSR BlueCore Bluetooth module, but no general purpose microprocessor. The pod BlueCore runs Netpage pen software in its VM. This software is responsible for sensing when the pod  450  is charging a pen  400 , controlling the pod LEDs  452  to indicate charging and communications status, and managing the USB communication link between the pod BlueCore and the host PC. Note that BlueCore provides a split stack model for the Bluetooth network stack, and the majority of the Bluetooth network stack will in fact be running on the host PC (where it has considerably greater access to resources). 
     Pen Assembly Sequence 
     The various sub-assemblies and components are manually inserted into the pen chassis molding  416  (see  FIG. 41 ). There are no special tools required to insert any of the assemblies as there is extensive use of snap fits and bumps on moldings for location. The only assembly tool needed is a cold staking procedure required after a testing to seal the pen assembly. 
     The assembly sequence for the pen is as follows: 
     Pen Chassis Assembly 
     The elastomeric end cap  460  is fed through an aperture  634  at the end of the chassis molding  416  and a tab  636  pulled through to secure it in place. 
     Optics Assembly 
     The optics assembly sequence is as follows:
         The lens is offered up to the aperture stop in the barrel and adhered in place.   The infrared filter is pushed into place in the front of the barrel molding.   The flex with image sensor is offered up to the top of the barrel molding and accurately located onto two pins.   Epoxy is applied around the base of the barrel molding to bond the flex into place and seal the image sensor from light and particulate contaminants.
 
Optics Assembly Insertion
       

     As shown in  FIG. 42A , the optics assembly  470  with the unfolded flex PCB  496  protruding is inserted into the chassis molding  416  and snapped into place. The IR LEDs  434  and  436  are then manipulated into cradles  638  either side of the barrel molding  492  as shown in  FIG. 42B . 
     Force Sensing Assembly Insertion 
     As shown in  FIGS. 43A and 43B , the force sensing assembly  474  is fed through between the chassis molding  416  and the optical barrel molding  492 . The assembly  474  is pivoted down and the force sensor is secured in the correct orientation into the chassis molding between ribs  640  and a support detail  642 . 
     The vibration motor  446  with elastomeric boot  644  is assembled into an aperture in the chassis  416 . The boot  644  has negative draft on the support detail  642 , which secures the motor  446  into the chassis  416  and orients it correctly. 
     A light pipe molding  448  is placed into the chassis molding  416  and is a force fit. 
     PCB and Battery Insertion 
     The end of the optics flex PCB  496  is offered into the flex connector  614  on the main PCB  422  and secured. 
     The main PCB  422  and LiPo battery  424  are then connected together as the socket is on the upper side of the PCB  422  and is not accessible when the board is in the chassis molding  416 . The battery  424  has foam pads to protect the components on the lower side of the PCB and to inhibit movement of the battery when it is fully assembled. 
     Referring to  FIG. 45 , the main PCB  422  and battery  424  can now be swung into place in the chassis molding  416 , with care being taken not to unduly stress the flex PCB  496 .  FIGS. 46A and 46B  shows a cold stake tool  646  sealing a cold stake pin  648  to an aperture  650  the base molding  528 . The cold stake  648  is used to help locate the PCB  422  into the chassis molding  416  and with gentle pressure the walls of the chassis  416  expand enough to allow snap fits to engage with the PCB and hold it securely. The PCB can still be extracted by flexing the chassis walls in the same manner if necessary. The battery can be tacked in place with adhesive tape if required. 
     The base molding  528  is hinged onto the chassis molding  416  and is fully located when the cold stake  648  appears in the aperture  650 . 
     Testing and Staking 
     At this point the assembly is complete enough to perform an optical and electronic diagnostic test. If any problems occur, the assembly can easily be stripped down again. Once approved, a cold stake tool  646  is applied to the pin  648  from the chassis molding  416  swaging it over to hold the base molding  528  captive ( FIG. 46B ). This prevents any user access to internal parts. 
     Product Label 
       FIG. 47  shows a product label  652  being applied to the base molding  416 , which covers the cold stake  648 . This label carries all necessary product information for this class of digital mobile product. It is exposed when the customisable tube molding  466  (see  FIG. 49 ) is removed by the user. 
     Nib Molding Insertion 
     As shown in  FIG. 48 , the nib molding  428  is offered up to the pen assembly and is permanently snapped into place against the chassis  416  and the base moldings  528  to form a sealed pen unit. 
     Tube Molding Assembly 
     As shown in  FIG. 49 , the tube molding  466  is slid over the pen assembly. The tube  466  is a transparent molding drafted from the centre to allow for thin walls. An aquagraphic print is applied to the surface with a mask used to retain a window  412 , which looks through to the light pipe  448  in the pen during use. A location detail  656  on the chassis molding  416  provides positive feedback when the molding is pushed home. The user can remove the tube molding by holding the nib end and pulling without gaining access to the pen assembly. 
     Cap Insertion 
     The cap assembly is fitted onto the pen to complete the product as shown in  FIG. 50 . 
     Netpage Pen Major Power States 
       FIG. 51  shows the various power states that the pen can adopt, as well as the pen functions during those power states. 
     Capped 
     In the Capped state  656 , the Pen does not perform any capture cycles. 
     Corresponding Pen Bluetooth states are Connected, Connecting, Connection Timeout or Not Connected. 
     Hover1 
     In the Hover1 state  658 , the Pen is performing very low frequency capture cycles (of the order of 1 capture cycle per second). Each capture cycle is tested for a valid decode, which indicates that the user is attempting to use the Pen in hover mode. 
     Valid Pen Bluetooth states are Connected or Connecting. 
     Hover2 
     In the Hover2 state  660 , the Pen is performing capture cycles of a lower frequency than in the Active state  662  (of the order  50  capture cycles per second). Each capture cycle is tested for a valid decode, which indicates that the user is continuing to use the Pen in hover mode. After a certain number of failed decodes, the Pen is no longer considered to be in hover mode. 
     Valid Pen Bluetooth states are Connected or Connecting. 
     Idle 
     In the Idle state  664 , the Pen is not performing any capture cycles, however, the Pen is active in as much as it is able to start the first of a number of capture cycles within  5  ms of a pen down event. 
     Valid Pen Bluetooth states are Connected or Connecting. 
     Active 
     In the Active state  662 , the Pen is performing capture cycles at full rate (100 capture cycles per second). 
     Valid Pen Bluetooth states are Connected or Connecting. 
     Netpage Pen Bluetooth States 
       FIG. 52  shows Netpage Pen power states that are related to the Bluetooth wireless communications subsystem in order to respond to digital ink offload requirements. Additionally, the Pen can accept connections from devices in order to establish a Bluetooth Pairing. 
     Each of the possible Pen Bluetooth related states are described in the following sections. 
     Connected 
     In the Connected state  666  the primary task for the Pen is to offload any digital ink that may be present within Pen storage, or to stream digital ink as it is being captured. Whilst in the Connected state it should also be possible for other devices to discover and connect to the pen for the purposes of Bluetooth Pairing. 
     In order to reduce power consumption whilst connected, it is desirable to take advantage of the relatively low bandwidth requirements of digital ink transmission and periodically enter a Bluetooth low power mode. A useful low power mode will typically be Sniff mode, wherein the periodic Bluetooth activity required of the Pen is reduced based on the Sniff interval, with the Sniff interval being determined by the current bandwidth requirements of digital ink transmission. 
     Connecting 
     Whilst in the Connecting state  668 , the Pen attempts to establish a connection to one of a number of known NAPs (Network Access Points) either to offload digital ink stored within Pen memory, or in anticipation of a sequence of capture cycles. 
     Upon entry into the Connecting state  668 , the Pen attempts an Inquiry/Page of each device in round-robin fashion with a relatively high frequency. If the connection is unsuccessful, the frequency of Inquiry/Page is reduced successively in a number of steps in order to reduce overall power consumption. 
     An Inquiry can last for 10.24 s and is repeated at a random interval. Initially the Inquiry may be repeated on average at 5 s intervals for the first 3 attempts, followed by 30 s for the next 5 attempts and then 5 minute intervals for the next 10 attempts and 10 minute intervals for subsequent attempts. 
     Connection Timeout 
     In the Connection Timeout state  670 , the Pen maintains the current Bluetooth connection by entering a Bluetooth low power Sniff state with relatively long sniff interval (e.g. 2.56 seconds) for a period of at least  2  minutes before disconnecting. Re-establishment of the connection is not attempted, should the connection be dropped before  2  minutes have elapsed. 
     Not Connected 
     In the Not Connected state  672 , the Pen does not hold any digital ink in its internal memory, and is capped. There is no Bluetooth activity, and no Bluetooth connection exists. 
     Discoverable and not Discoverable 
     The Pen is only discoverable  674  during the major states of Hover1  658  and Idle  664 . The Pen periodically enters the inquiry scan and page scan states whilst in Hover1  658  or Idle  664 , in order to respond to connection requests from other devices. 
     Cap Detection Circuit 
     Referring once again to  FIG. 26 , a cap detection circuit diagram is shown. As discussed above, the presence or absence of the cap assembly  472  on the nib molding  428  can directly determine the Pen power state and the Bluetooth state. The cap assembly  472  serves the dual purposes of protecting the nib  418  and the imaging optics  426  when the pen  400  is not in use, and signalling, via its removal or replacement, the pen to leave or enter a power-preserving state. 
     As described in the ‘Pod Assembly’ section above, the pen  400  has coaxial conductive tubes  498  that provide a set of external contacts—power contacts  678  and data contacts  680 . These mate with contacts  516  in the pod  450  to provide the pen with charging power and a USB connection. When placed over the nib molding  428 , the conductive elastomeric molding  522  short-circuits the pen&#39;s power contacts  678  to signal the presence of the cap. 
     The pen has three capping states:
         capon   cap off, not in pod   cap off, in pod
 
In the cap on state, the CAP_ON signal  682  is high. The pen will be powered off, subject to other pending activities such as digital ink offload, as described above in the NetPage Pen Bluetooth States section.
       

     In the cap off, not in pod state, the CAP_ON signal  682  is low. The pen will be powered on. 
     In the cap off, in pod state, the CAP_ON signal  682  is low. The pen will be powered on. 
     The CAP_ON signal  682  triggers transitions to and from the Capped state  656 , as described in the NetPage Pen Power States section above, via the power management unit  580  and the Amtel ARM7 microprocessor  574  (see Pen Design section above). 
     The battery charger can use the VCHG signal  684  to charge the battery. The VCHG signal  684  can be connected to the USB VBUS voltage (nominally 5V) to allow the battery to be charged at up to 500 mA (based on the USB specification). The VCHG signal can also be connected to a higher voltage generated by boosting the USB VBUS voltage (maximum charging current would be lower than 500 mA). Alternatively, the VCHG signal can be connected to a different voltage, e.g. from a DC plug pack  632  (see Connection Options section) connected to the pod  450 . In this case, the pen is a self-powered USB device from the point of view of the USB host  630 . 
     When the cap assembly  472  is removed, the CAP_ON signal  682  is pulled low via transistor Q 1   686 . The switching time of Q 1 , and hence the latency of cap removal detection, is a function of the stray capacitance of Q 1  and the value of resistor R 1   688 . A value of 1 Mohm results in a latency of about 0.5 ms. The cap removal detection latency must be balanced against the discharge rate of the battery in the capped state. A value of 1 Mohm yields a trivial discharge rate of 3 μA. Diode D 1   690  stops the battery being charged from the VCHG voltage  684  through R 1   688 . 
     The external USB host  630  (see  FIG. 37 ) is connected to the USB device  692  in the pen  400  via the USB  694  and US{tilde over (B)}  696  signals. Although the circuit in  FIG. 26  is shown with reference to a four-wire USB interface, the cap detection function of the circuit only relates to the two-wire power interface, and the pen can have a two-pin external power interface rather than a four-pin external USB interface depending on product configuration. 
     The above description is purely illustrative and the skilled worker in this field will readily recognize many variations and modifications that do not depart from the spirit and scope of the broad inventive concept.