Abstract:
A method and apparatus is disclosed for communicating with a host. In one embodiment, a smart card has an IC with voltage conditioning circuitry and a pull-up resistor. The smart card, when inserted in a smart card reader coupled to the host, is capable of signaling the host over a bus using the pull-up resistor selectively coupled to a voltage output of the voltage conditioning circuitry and a first output of the smart card. The voltage conditioning circuitry output is selectively coupled to the first output through the resistor, responsive to the device being powered by the bus but not transmitting. This tends to pull up the first output to the voltage level of the voltage source, which makes the smart card capable of being properly detected by the host upon the bus being driven by a host. Selectively disconnecting the pull-up resistor while the smart card is transmitting or receiving results in a more balanced differential output signal. Since the pull-up resistor and voltage conditioning circuitry supplying the proper voltage to the pull-up resistor are an integrated part of the IC, no separate contact is required to supply voltage to the resistor. This permits the smart card to be compatible with the contact configuration of certain existing smart cards, and eliminates a need for the pull-up resistor or voltage conditioning circuitry to be included in the smart card reader.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is related to the following U.S. application that is assigned to the same assignees as the present application, and is hereby incorporated herein by reference: 
     “Method and Device for Local Clock Generation Using Universal Serial Bus Downstream Received Signals DP and DM,” filing date Jul. 13, 2000, Ser. No. 09/614,736. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electronic devices for use with a universal serial bus (USB). More particularly, the invention relates to circuitry that improves common mode performance of transmitters or receivers, such as in USB-compatible devices, and facilitates connection of devices, such as USB devices, to a host. 
     2. Description of Related Art 
     A number of standard interfaces exist for communicating between a host and a device. Referring to FIG. 1, a conventional information handling system  100  is shown. The system  100  makes use of the universal serial bus (“USB”)  125  for connecting a host computer  170  (also known simply as a “host”) to a number of devices, known as USB devices, such as a display  135 , printer  140 , keyboard  145 , trackball  150 , optical scanner  155 , disk drive  160  and other such device  165 . Each one of the devices  135 ,  140 , etc. is coupled to the USB  125  via respective ports  130  of the hub  110 . 
     The USB is currently defined by the Universal Serial Bus Specification written and controlled by USB Implementers Forum, Inc., a non-profit corporation founded by the group of companies that developed the USB Specification. In particular, Universal Serial Bus Specification, revision 1.1, dated Sep. 23, 1998 (the “USB Specification”), Chapter 5 “USB Data Flow Model,” Chapter 7 “Electrical,” and Chapter 8 “Protocol Layer” are hereby incorporated herein by reference. 
     According to the USB Specification 1.1, USB devices may include both low speed and full speed devices. Low speed devices transfer data at a transmission rate of 1.5 MHz and full speed devices transfer data at a rate of 12 MHz. Data are transmitted on communication lines. That is, the USB device transmits a differential output signal or receives a differential input signal on these communication lines. In the low speed mode, the differential signal indicates a first logical state, referred to as the “J” state, if D+ is at a voltage level below that of D−, and a second state, the “K” state, if D+ is at a voltage level above that of D−. In the full speed mode, the differential signal indicates a first state, the “K” state, if D+ is at a voltage level below that of D−, and a second state, the “J” state, if D+ is at a voltage level above that of D−. The differential design gives better protection against ground shifts and noise since the received signal level is determined by comparing two voltage levels that are both subject to ground shifts or noise affecting both of the differential signals in a similar manner. 
     A host  170  detects the presence of a device, such as device  165 , on the USB  125  during an attachment phase, while drivers of the port  130  and device are in tri-state. Detection of the attachment is based on a certain connection on the port  130  of a pull-up resistor  210  associated with the device. Likewise, detection of whether the device is operating in low or full speed mode also depends on the pull-up resistor connection. 
     Referring now to FIG&#39;s  2  and  3 , FIG. 2 shows a transmitter  230  of a typical USB device, of the low speed variety, coupled to a receiver  240  on the corresponding port  130 . FIG. 3 shows a transmitter  230  of a typical USB device, of the full speed variety, coupled to a receiver  240  of the corresponding port  130 . The low speed device (FIG. 2) pull-up resistor  210  is connected between positive voltage contact  213  and the D− signal line  212 . The full speed device (FIG. 3) has the pull-up resistor  210  connected between the positive voltage contact  213  and the D+ signal line  215 . Note that according to the USB Specification, the voltage level of  211  supplying the pull-up resistor is different than that of the specified voltage supplied by the port  130  on the signal line  220  by V BUS . Thus, the resistor, for example, which is conventionally external to integrated circuitry of the USB device in the prior art, is supplied by its own voltage contact  213 , and not the V BUS  line  220 , unless additional circuitry is also included coupled to the line  220  to condition the voltage for supplying the pull-up resistor. 
     The presence of the pull-up resistor on only the D− signal line  212 , for low speed peripherals, or on only the D+ signal line  215 , for full speed peripherals, introduces an imbalance in the symmetry of the differential signal from the USB transmitter  230 , that is, outputs on the signal lines. In other words, due to the resistor, the amplitudes of the signal swings on the signal lines are not the same and the signals do not change at the same rate. This asymmetry is problematic for several reasons, including increases in EMI/RFI radiation, received bit length variation and data stream skew. Aspects of these problems are addressed in U.S. Pat. Nos. 5,905,389 and 5,912,569 (the “Alleven patents”) by introducing a delay circuit in one of the two USB transmitters. While this mitigates the problems, it does not fully eliminate the imbalance in the differential signal arising from the single pull-up resistor. 
     The presence of the pull-up resistor on one of the communication lines also gives rise to other issues. One issue concerns power consumption by the peripheral. U.S. Pat. No. 6,076,119 (the “Maemura et al. patent”) introduces a switch between the pull-up resistor and a terminal voltage, wherein the switch selectively disconnects the pull-up resistor when a device is inoperative. This reduces power consumption, and also simplifies determination by a host computer that a physically connected USB device is inoperative, but it does not address the imbalance in differential signal arising from connection of the single pull-up resistor during operation of the device. 
     Another issue concerns suitability for use of “smart cards” in connection with a USB. Referring now to FIG. 4, a smart card  400  is shown which has an integrated circuit module (“ICM”)  420  affixed to a card  410 . Although conventional USB peripherals have the USB required pull-up resistor mounted externally, it is problematic to mount a resistor on the surface of a smart card, which is carried in a wallet or purse and repeatedly inserted and removed from a reader. Furthermore, since smart cards compatible with ISO7816 standard are in widespread use in Europe and Asia, legacy issues limit the number of contacts on the smart card which are available for USB applications of smart cards This also gives rise to difficulties in connecting an external resistor to a smart card. 
     In addition to the above described problems associated with surface mounting and terminal limitations on smart cards, the conventional USB pull-up resistor is also problematic for readers used with smart cards in single-user applications. For relatively centralized applications, such as transactions with payphones, automatic teller machines or point of sale terminals, the number of transactions per smart card reader is high. That is, in these applications each smart card reader is shared by many users, the frequency of transactions per reader is very high, and the cost of the readers is not a major factor. However, smart cards are also useful for widely distributed transactions conducted via the Internet, such as for financial transactions or for logging securely onto a network. For this application, transactions are commonly associated with individual use of computers in homes and offices, and accordingly, smart card readers in this application are used relatively much less frequently, so that the cost per reader is a significant feasibility factor since the solution cost is equal to the smart card reader cost plus the smart card cost. 
     To overcome the aforementioned problem of USB terminal limitations on ISO7816 compatible smart cards, the use of a conventional USB device&#39;s external pull-up resistor requires voltage conditioning circuitry external to the smart card, as described hereinabove. This pull-up resistor and voltage conditioning circuitry is conventionally located in the smart card reader. As has been stated, this is not an issue for a smart card reader shared by many users, but it is quite problematic for a smart card reader used by a single-user in Internet transactions, because it tends to drive up the cost of the smart card solution. 
     From the above discussion it should be understood that while advances have been made in USB devices, needs still exist for further improvements which address the problems of EMI/RFI radiation, variation in received bit lengths and skew in the received data stream, all of which arise from imbalance in differential USB signaling due to the USB required pull-up resistor. Furthermore, solutions to these problems and other problems related to the pull-up resistor are particularly difficult for smart cards performing as a USB device, so that the needs are particularly acute in this context. 
     SUMMARY OF THE INVENTION 
     The foregoing needs are addressed in the present invention. According to a method form of the invention, an apparatus communicates with a host by receiving a voltage at a first voltage level, on a. first one of a number of contacts coupled to an integrated circuit (“IC”). The contacts and the IC are part of a smart card. The received voltage is conditioned, by voltage conditioning circuitry on the IC., The voltage conditioning circuitry generates an output voltage at a second voltage level for signaling attachment to the host. This is signaled by the voltage conditioning circuitry output pulling up a second contact to the second voltage level, through a resistor of the IC. A signal is also driven on the second contact by a driver on the IC for further communicating to the host. That is, in an embodiment, the driver signal is for communication. 
     In another aspect of the method, the second contact is pulled up to the second voltage level through a switch on the IC, responsive to the apparatus being powered. 
     In still another aspect of the method, the second voltage level is decoupled from the second contact by the switch, responsive to a detach indication from control circuitry of the IC. 
     In yet another aspect, the IC also asserts a second driver signal for differential signal communication to the host, on a third one of the contacts. The voltage conditioning circuitry output voltage is decoupled from the second contact by the switch on the IC, responsive to the apparatus transmitting, i.e., transferring data to the host at the USB full or low speed data rate, to reduce an imbalance for the first and second driver signals. Further, the voltage conditioning circuitry output voltage is re-connected to the second contact by the switch, responsive to termination of the transmitting. 
     In another embodiment, the voltage conditioning circuitry output voltage is decoupled from the second contact by the switch on the IC, responsive to the apparatus receiving, i.e., transferring data from the host at the USB full or low speed data rate, to reduce an imbalance for signals driven by the host. In this embodiment, the voltage conditioning circuitry output voltage is re-connected to the second contact by the switch, responsive to termination of the receiving. 
     In another method aspect, receiving the voltage at the first voltage level includes receiving the smart card by a reader having solely passive components, and electrically coupling a connector of the reader to the contacts, for coupling the contacts to the host. 
     According to an apparatus form of the invention, a device has a driver and outputs for communicating with a host. In a first aspect, the device has voltage conditioning circuitry, and a pull-up resistor and is capable of signaling the host over a bus using the pull-up resistor coupled to a first one of the outputs and a voltage output of the voltage conditioning circuitry. 
     A switch is included in series with the voltage output, pull-up resistor and the aforementioned first output. The switch is capable of selectively connecting the voltage conditioning circuitry output, through the pull-up resistor, to the first output, responsive to the device being powered by the bus, but not transmitting. This tends to pull up the first output to the voltage level of the voltage conditioning circuitry output, which makes the device capable of being properly detected by the host upon the bus being driven by a host. 
     In the context of a USB embodiment, the device has two drivers for differential outputs and both the outputs are coupled to an output contact pair. One of this contact pair is for the aforementioned first output. Since the other one of the output contact pair has no corresponding pull-up resistor, it is advantageous to disconnect the pull-up resistor while the device drivers are transmitting, since this results in a more balanced differential output signal, and the benefits of less common-mode noise, reduced EMI/RFI, improved bit lengths and reduced skew in the received bit stream. 
     The pull-up resistor must at times be connected to the output, however. This is because, as described in the Background hereinabove, for USB applications the host determines if the device is attached to the USB and if the device is low speed or full speed by examining D− and D+ signal lines on the USB to which the output terminals of the device may be connected. The invention involves recognition that although the pull-up resistor must be connected for proper detection of the device by the host on the D+ or D− lines, the pull-up resistor can advantageously be disconnected when the device is driving those lines. 
     In an additional aspect, the apparatus of the invention includes an integrated circuit (“IC”), which is part of a smart card having a number of electrical contacts. (The contacts and the IC are preferably elements of the same integrated circuit module (“ICM”).) The IC is coupled to the ICM contacts, including a first output of the IC coupled to a first one of the ICM contacts for receiving a voltage supply from a USB port. The IC includes voltage conditioning circuitry coupled to a second one of the ICM contacts through a resistor of the IC. 
     In still another aspect, the apparatus includes a reader, having a connector for receiving the smart card and coupling connector contacts to the smart card contacts. Electrical components of the reader consist solely of passive components, that is, inactive components having resistance, inductance or capacitance characteristics, but no gain or directional function. 
     It is an advantage that the pull-up resistor and voltage conditioning circuitry supplying the proper voltage to the pull-up resistor are integrated on the IC, so no contact is required on the smart card to supply the voltage to the resistor. This permits the apparatus to be compatible with the contact configuration of existing smart cards. 
     It is still another advantage that the resistor being an integrated resistor of the IC eliminates the need for including the pull-up resistor or any voltage conditioning circuitry for the resistor as part of the reader, making the reader more suitable for low cost applications. 
    
    
     These and other advantages of the invention will be further apparent from the following drawings and detailed description. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 illustrates a conventional information handling system, with universal serial bus and USB devices. 
     FIG. 2 is a typical prior art configuration for a USB low speed device. 
     FIG. 3 is a typical prior art configuration for a USB full speed device. 
     FIG. 4 illustrates a conventional smart card. 
     FIG. 5 illustrates details of an integrated circuit module according to the present embodiment. 
     FIG. 6 shows a smart card reader according to an embodiment of the present invention. 
     FIG. 7 shows details of the reader of FIG.  6 . 
     FIG. 8 illustrates, in flow chart format, logic for connecting and disconnecting a resistor on the integrated circuit module of FIG.  5 . 
     FIG. 9 illustrates aspects of a low speed USB device according to an embodiment. 
     FIG. 10 illustrates aspects of a full speed USB device according to an embodiment. 
     FIG. 11 shows an alternative embodiment of a series circuit of FIGS. 8 and 9. 
     FIG. 12 shows details of the integrated resistor of FIG.  9  and FIG.  10 . 
     FIG. 13 illustrates additional aspects of a USB device according to an embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings illustrating embodiments in which the invention may be practiced. It should be understood that other embodiments may be utilized and changes may be made without departing from the scope of the present invention. 
     Referring to FIG. 5, an ICM  420 , according to an embodiment of the present invention, is shown in more detail than the conventional ICM  420  shown in FIG.  4 . In particular, electrical contacts  522  through  529  are shown. These electrical contacts  522  through  529  associated with the ICM  420  of the smart card are coupled via wires  519  to an IC  530  of the ICM  420 , and are used for the host to communicate with the IC by inserting the card into a reader  600  (not shown). 
     Referring to FIG. 6, a smart card  400  is shown with an embodiment of the USB compatible reader  600 . The reader  600  has a slot  651  for receiving the card  410  which includes the ICM  420 . The reader  600  includes a cable  652  and connector  653  for connecting the reader  600  to the port  130  (FIG. 1) on hub  110  (FIG.  1 ). The reader  600  includes a passthrough connector  620 . The combination of the reader  600  and the smart card  400  of the present embodiment can be used as a smart card USB device, such as device  165  in FIG.  1 . 
     Referring to FIG. 7, further details are shown of the device  165 . The reader  600  has a passthrough connector  620  with contacts  722  through  729 , which make contact with the contacts  522  through  529  of the smart card  400 , and couples them to bus  125  (FIG. 1) via the cable  652  and connector  653 , plugged into the port  130  (FIG.  1 ). Only four contacts  722 ,  725 ,  726  and  729  are relevant. It should be appreciated from the foregoing, it is significant that according to the embodiment shown, the reader  600  contains no active electronic components. Smart card  400  includes IC  530  having the active components required to transmit information to the host and receive information from the host. Power to operate the IC on the smart card is supplied from the port  130  via the connector  653 . the cable  652  and the reader  600  when the smart card is inserted into the reader slot  651 . Inserting the smart card  400  into the reader slot  651  causes contacts  722 ,  725 ,  726 , and  729  on the smart card to couple with contacts  522 ,  525 ,  526 , and  529  in the reader  600 . The coupling of the smart card contacts with the reader contacts when the smart card is inserted into the reader slot supplies necessary power to the IC on the smart card and also enables necessary signals to flow between the IC and the host via the cable and the connector, using the USB. 
     FIG. 9 is a diagram showing an operation mode transfer system in accordance with an embodiment of the present invention. The embodiment in FIG. 9 illustrates a low speed USB device  165  coupled to the bus  125 , which connects the device to a receiver  240  portion of a port  130  of a hub  110  connected to a host computer  170  (not shown). 
     The IC  530  includes a first driver  965  and second driver  970 . The drivers are coupled to respective contacts  525  and  529  to drive signal lines D+ and D−, respectively, on the USB  125 , at the USB low speed 1.5 MHz data transfer rate. Since the device in FIG. 9 is a low speed device, it has pull-up resistor  210  coupled to the D− differential signal line  212 . According to the embodiment, the resistor  210  is an integrated device in the IC, rather than external. It is electrically coupled to the D− line  212  through the contact  529  of the smart card (not shown) and a switch  940 . The resistor  210  is also coupled to an output  930  of voltage conditioning circuitry  935 . The output  930 , the resistor  210 , and switch  940  are in a series circuit  960 . The voltage conditioning circuitry is coupled to VCC contact  522 , and the V BUS  line  220 , for receiving the V BUS  supply voltage (nominally 5 volts) from the port  130 , and generates a VTERM voltage supply (nominally 3.3 volts) particularly for the pull-up resistor  210 . 
     The switch  940  is controlled by a signal at output  955  from control circuitry  950 . The switch closes responsive to the signal on the output  955  of control circuitry  950  being asserted, which couples the voltage conditioning circuitry output  930  and resistor  210  to the contact  529 , and thereby to the D− line  212 . The switch  940  opens responsive to the signal on the output  955  of control circuitry  950  being deasserted, which decouples the voltage conditioning circuitry output  930  and resistor  210  from the contact  529 , and thereby from the D− line  212 . 
     FIG. 13 illustrates a receiver  250  for a USB device  165  according, to an embodiment of the present invention. The receiver  250  includes a single input amplifier A 6  on the IC  530 , coupled to contact  529  for receiving a D− input signal from the host transmitter  260 , a single input amplifier A 8  on the IC  530 , coupled to contact  525  for receiving a D+ input signal from the host transmitter  260 , and a differential amplifier A 7  on the IC  530 , coupled to both the contacts for receiving both the D+ and D− input signals. According to one embodiment of the present invention, the device  165  includes both the receiver  250  of FIG. 13, and the low speed transmitter  230  of FIG. 9 described above. According to another embodiment, the device  165  includes both the receiver  250  of FIG. 13, and the full speed transmitter  230  of FIG. 10 described hereinbelow. 
     Referring now to FIG. 8, logic is described for control circuitry  950  selectively connecting and disconnecting the pull up resistor  210  of device  165 . At step  805 , the device is inserted in reader  600 , which is coupled to port  130 , and the port  130  powers the device. Then, at step  810 , control circuitry  950  determines whether the voltage level VBUS received by device  165  is adequate for the device to be attached to the bus  125 , that is, whether the D− contact  529  should be pulled up to the voltage level VTERM of output  930  through resistor  210 . If not, then the circuitry  950  continues to monitor, at step  810 , until the voltage level V BUS  is adequate. Once the control circuitry detects adequate voltage on V BUS , an output signal on output  955  is asserted to close switch  940  and pull up the contact  529 , at step  815 . At this point, with signal line D− pulled up the device is capable of being detected by the host as a low speed USB device  165 , and the device may communicate its identity to the host, and the host may enumerate the device. 
     Next, at step  820 , the control circuitry determines whether there is any indication that the device should be detached, such as to be re-enumerated. If there is an indication that the device should be detached, at step  825  the circuitry  950  deasserts the signal at output  955 , so that the contact  529  is no longer pulled up. If no, the circuitry continues to assert the signal at output  955  to hold up the voltage level of contact  529 ; provided, however, that once the control circuitry determines that the device is transmitting or receiving, i.e., transferring data between the host and the device at the USB low or full speed data rate, at step  830 , the circuitry temporarily deasserts the signal on output  955 , at step  835 , and continues to monitor, at step  830 , for transmitting or receiving to end. Once transmitting or receiving by the device has ended, control circuitry  950  re-asserts the signal on output  955 , at step  840 , to once again pull up the contact  529 . 
     A result of this arrangement is that the switch is open during data transfers, causing the differential signal line D− to match the differential signal line, insofar as neither of the differential signal lines has a pull-up resistor connected, which results in improved differential signal quality. Also, the switch is closed during an interval when the device is powered and not transmitting, provided that there is no detachment demand, permitting the host to determine the operating mode of the device. 
     There may be numerous other conditions, not shown in FIG. 8, for deasserting the control circuitry output  955  signal in order to open the switch. For example, the switch  940  may also be used to decouple the pull up resistor  210  at times other than during data transmissions. Such additional detachments may be done, for example, to conserve power or to reduce communications overhead processing by the host, and are compatible with also decoupling the pull up resistor during data transmission. The switch may be opened during times when the V BUS  voltage supply to the device is out of USB specifications, or if electrical contacts  522 ,  525 ,  526  or  529  of device  165  are not properly coupled to the USB. Instances when the voltage is too unstable or too low for reliable operation may be sensed by the control circuitry  950 , based on a test for voltage on the VCC contact  529 , and presence of pull-down resistors  216  and  217 . Reasons for attachment and detachment are further described in the USB Specification. 
     Many of the above described features can be achieved independently of the order of connection of the resistor and the switch between the V TERM  voltage and the D− signal line  212 . Accordingly, in another embodiment of the present invention shown in FIG. 11, the series circuit  960  is modified such that the order of connection of the resistor  210  and the switch  940  is reversed. That is, the resistor  210  is connected to the switch  940  on one side and to one of the contacts  525  or  529  on the other side, and the switch  940  is connected to the resistor  210  and to the output  930  on the other side. The output  930  is connected to the voltage conditioning circuitry  935  (FIG.  9 ). 
     The order of connection as shown in FIG. 9 is advantageous in that this arrangement results in improved IC protection against ESD. The Maemura et al. patent discloses a switch and resistor for a USB device coupled in the reverse order of resistor and switch shown in FIG. 9 herein. Furthermore, the resistor as disclosed in the “Maemura et al. patent” is not integrated in the device IC, as in the present invention, which presents difficulties in the context of smart cards, as has been described hereinabove. Also, according to Maemura et al. the switch is open while the device is in the inoperative state, but closed when the device is communicating with the host; whereas, according to the present invention the switch is open while the device is communicating. 
     In another embodiment of the present invention, the switch  940  is replaced by an array of switches connected to multiple control signals, from control circuitry like circuitry, to implement more complex logic functions causing the switch to be open. In one embodiment, a second switch, like switch  940  in FIG. 9, is placed between the V TERM  voltage and the resistor  210 , such that the resistor is connected to a switch on either side. 
     Referring now to FIG. 10, an embodiment is shown of an operation mode transfer system, in accordance with the present invention, for a USB device  165  of the full speed variety coupled via USB  125  to a port  130  of hub  110  (FIG.  1 ). In the device of FIG. 10 pull-up resistor  210  is coupled to contact  525 , and thereby to the differential signal line  215 , through switch  940 . The resistor is also connected to the voltage conditioning circuitry output  930  voltage V TERM  , so that for a full speed device as shown in FIG. 10, the host  170  detects that V TERM  is present on the signal line D+, which determines the presence of a device of the full speed variety. Aside from the data transfer rate, and the resistor  210  being coupled to the D+ contact  525 , instead of the D− contact  529 , the full speed device of FIG. 10, operates like that of the low speed device of FIG.  9 . 
     As was stated hereinabove, legacy issues limit the number of terminals which are available for smart cards, and this also gives rise to difficulties in connecting an external resistor to a smart card for a USB application. To elaborate, smart cards in widespread use are conventionally manufactured according to ISO7816 Specifications, which specifies the location and function of the electrical terminals on the cards as well as protocol. The six contacts,  522 ,  523 ,  524 ,  526 ,  527  and  528  shown in FIG. 5 for the smart card of FIG. 4 are currently used for functions defined according to the ISO7816 Specifications. The other two contacts  525  and  529  are designated by the ISO standard as being “reserved.”(In one of the embodiments, it is assumed that the smart card is performing as a USB device when inserted in a reader  600 . Mode selection between USB and ISO7816, if any, is not described herein.) However, as shown in FIGS. 2 and 3, it is conventional for a USB device  165  to have the required pull-up resistor mounted externally, which requires either i) circuitry in the device package, such as internal to an IC in the device, to condition the voltage VCC to supply the proper voltage level, i.e., a different voltage level than, for the pull-up resistor  210 , in which case an extra terminal  213  is also required for the voltage VCC supplied to the resistor, or else ii) external circuitry coupled to the terminal for conditioning the voltage to supply for the pull-up resistor. 
     One issue that arises from integrating the resistor on the IC, according to the present invention, concerns how to manufacture the resistor to sufficiently precise tolerances. That is, the USB Specification calls for the resistor to be within a predetermined tolerance range, which is narrower than conventionally achieved by ordinary fabrication methods. FIG. 12 illustrates the pull-up resistor  210 , which includes a resistive ladder  1210  (or simply “resistor”) integrated on the IC  530  (FIG.  5 ), having a structure suited for meeting the USB Specification tolerance limit. Resistive ladder  1210  is connected to a terminal point  1220  and to several switches  1230  distributed along the length of the resistive ladder. One of the switches  1230  is connected between the resistive ladder and the output connection point  1240  and the remaining switches  1230  are opened. The selection of which switch  1230  to connect is determined by testing at the time of manufacture of the device and stored in non volatile memories. This structure and method allows a resistor with an acceptable resistance tolerance to be fabricated with a process that produces resistive material varying widely in resistance per square unit. In an alternative embodiment, the switches are replaced with other suitable devices, such as fusable links. 
     The description of the present embodiment has been presented for purposes of illustration, but is not intended to be exhaustive or to limit the invention to the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention. Various other embodiments having various modifications may be suited to a particular use contemplated, but may be within the scope of the present invention.