Abstract:
A method and apparatus for writing spiral servo information is provided. In one embodiment, a servo track writer having a digital signal processor is provided. The servo track writer also includes a clock head, which reads a clock track from a disk surface to generate a clock signal that is used to provide interrupt signals to the digital signal processor. A transducer, which writes spiral servo information onto the disk surface, is positioned under control of the digital signal processor based upon the interrupt signals. In one embodiment, a method of writing spiral servo information onto a disk surface using a servo track writer having a digital signal processor is provided. The method includes the steps of reading a clock track written onto the disk surface to generate interrupt signals; providing the interrupt signals to the digital signal processor; and, positioning a transducer over the disk surface under control of the digital signal processor based upon the interrupt signals provided thereto.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Priority is claimed from U.S. Provisional Patent Application Ser. No. 60/223,446 filed Aug. 4, 2000, which is incorporated by reference herein in its entirety. 
     INCORPORATION BY REFERENCE 
     U.S. patent application Ser. No. 09/853,093 filed May 9, 2001 is also incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to computer disk drives. More particularly, the present invention relates to writing servo information onto one or more disk surfaces of a disk drive, wherein the servo information is written in a spiral fashion by modifying existing servo track writing equipment. 
     BACKGROUND OF THE INVENTION 
     Computer disk drives store information on magnetic disks. Typically, the information is stored on each disk in concentric tracks that are divided into sectors. Information is written to and read from a disk by a transducer that is mounted on an actuator arm capable of moving the transducer radially over the disk. Accordingly, the movement of the actuator arm allows the transducer to access different tracks. The disk is rotated by a spindle motor at high speed which allows the transducer to access different sectors on the disk. 
     A conventional disk drive, generally designated  10 , is illustrated in  FIG. 1 . The disk drive comprises a disk  12  that is rotated by a spin motor  14 . The spin motor  14  is mounted to a base plate  16 . 
     The disk drive  10  also includes an actuator arm assembly  18 , which includes a transducer  20  (wherein the transducer has both a write head and a read head) mounted to a flexure arm  22 . The actuator arm assembly  18  is attached to an actuator arm  24  that can rotate about a bearing assembly  26 . A voice coil motor  28  cooperates with the actuator arm  24  and, hence, the actuator arm assembly  18 , to move the transducer  20  relative to the disk  12 . The spin motor  14 , voice coil motor  28  and transducer  20  are coupled to a number of electronic circuits  30  mounted to a printed circuit board  32 . The electronic circuits  30  typically include a read channel chip, a microprocessor-based controller and a random access memory (RAM) device. 
     The disk drive  10  typically includes a plurality of disks  12  and, therefore, a plurality of corresponding actuator arm assemblies  18 . However, it is also possible for the disk drive  10  to include a single disk  12  as shown in  FIG. 1 . 
       FIG. 2  is a functional block diagram which illustrates a conventional disk drive  10  that is coupled to a host computer  32  via an input/output port  34 . The disk drive  10  is used by the host computer  32  as a data storage device. The host  32  delivers data access requests to the disk drive  10  via port  34 . In addition, port  34  is used to transfer customer data between the disk drive  10  and the host  32  during read and write operations. 
     In addition to the components of the disk drive  10  shown and labeled in  FIG. 1 ,  FIG. 2  illustrates (in block diagram form) the disk drive&#39;s controller  36 , read/write channel  38  and interface  40 . Conventionally, data is stored on the disk  12  in substantially concentric data storage tracks on its surface. In a magnetic disk drive  10 , for example, data is stored in the form of magnetic polarity transitions within each track. Data is “read” from the disk  12  by positioning the transducer  20  (i.e., the read head) above a desired track of the disk  12  and sensing the magnetic polarity transitions stored within the track, as the track moves below the transducer  20 . Similarly, data is “written” to the disk  12  by positioning the transducer  20  (i.e., the write head) above a desired track and delivering a write current representative of the desired data to the transducer  20  at an appropriate time. 
     The actuator arm assembly  18  is a semi-rigid member that acts as a support structure for the transducer  20 , holding it above the surface of the disk  12 . The actuator arm assembly  18  is coupled at one end to the transducer  20  and at another end to the VCM  28 . The VCM  28  is operative for imparting controlled motion to the actuator arm  18  to appropriately position the transducer  20  with respect to the disk  12 . The VCM  28  operates in response to a control signal i control  generated by the controller  36 . The controller  36  generates the control signal i control  in response to, among other things, an access command received from the host computer  32  via the interface  40 . 
     The read/write channel  38  is operative for appropriately processing the data being read from/written to the disk  12 . For example, during a read operation, the read/write channel  38  converts an analog read signal generated by the transducer  20  into a digital data signal that can be recognized by the controller  36 . The channel  38  is also generally capable of recovering timing information from the analog read signal. During a write operation, the read/write channel  38  converts customer data received from the host  32  into a write current signal that is delivered to the transducer  20  to “write” the customer data to an appropriate portion of the disk  12 . The read/write channel  38  is also operative for continually processing data read from servo information stored on the disk  12  and delivering the processed data to the controller  36  for use in, for example, transducer positioning. 
       FIG. 3  is a top view of a magnetic storage disk  12  illustrating a typical organization of data on the surface of the disk  12 . As shown, the disk  12  includes a plurality of concentric data storage tracks  42 , which are used for storing data on the disk  12 . The data storage tracks  42  are illustrated as center lines on the surface of the disk  12 ; however, it should be understood that the actual tracks will each occupy a finite width about a corresponding centerline. The data storage disk  12  also includes servo information in the form of a plurality of radially-aligned servo spokes  44  that each cross all of the tracks  42  on the disk  12 . The servo information in the servo spokes  44  is read by the transducer  20  during disk drive operation for use in positioning the transducer  20  above a desired track  42  of the disk  12 . The portions of the track between servo spokes  44  have traditionally been used to store customer data received from, for example, the host computer  32  and are thus referred to herein as customer data regions  46 . 
     It should be understood that, for ease of illustration, only a small number of tracks  42  and servo spokes  44  have been shown on the surface of the disk  12  of  FIG. 3 . That is, conventional disk drives include one or more disk surfaces having a considerably larger number of tracks and servo spokes. 
     During the disk drive manufacturing process, a special piece of equipment known as a servo track writer (STW) is used to write the radially-aligned servo information which forms servo spokes  44 . A STW is a very precise piece of equipment that is capable of writing servo information on the disk surface with a high degree of positional accuracy. In general, a STW is a very expensive piece of capital equipment. Thus, it is generally desirable that a STW be used as efficiently as possible during manufacturing operations. Even a small reduction in the amount of data needed to be written by the STW per disk surface can result in a significant cost and time savings. 
       FIG. 4  depicts, in block diagram form, certain portions of a conventional servo track writer  50  and a disk drive  10 . Only those components that are used to position the disk drive&#39;s actuator arm assembly  18  radially relative to the center of the disk surface are shown in  FIG. 4 . Among other things, the servo track writer  50  includes an STW digital signal processor (DSP)  52 , a STW voice-coil motor (VCM)  54 , a STW actuator arm assembly  56  and a push-pin system  58 . 
     In order to write servo information on to a disk surface  12 , the disk drive  10  is loaded onto the STW  50  and is held securely in place. One of a variety of push-pin systems  58  (e.g., a mechanical push-pin system or an optical push-pin system) is used to create an interface between the actuator arm assembly  18  of disk drive  10  and the actuator arm assembly  56  of the servo track writer  50 . By properly positioning the STW actuator arm assembly  56 , the actuator arm assembly  18  and, hence, the transducer  20  of the disk drive  10  may be positioned at an appropriate location relative to the center of the disk surface  12 . In order to effectuate this positioning, the STW  50  uses a servo loop formed by an external relative encoder (see block  70  in  FIG. 6 ), which cooperates with (or forms a part of) the STW VCM  54 , and a compensation circuit (see block  70  in  FIG. 6 ). 
     Once the transducer  20  is appropriately positioned relative to the disk surface  12 , servo information is then written by the transducer  20  onto the disk surface  12  at the particular radial location. Subsequently, the STW actuator arm assembly  56  is used to position the actuator arm assembly  18  of the disk drive  10  at a next radial location and servo information is written at this radial location. The process repeats until servo information is written at all predetermined radial locations across the disk surface  12 . 
     As shown in  FIG. 4 , the STW  50  also includes a crystal  60  and a divide-by-N circuit which are used to provide a series of interrupt signals  64  (see  FIG. 5 ) to the STW DSP  52  at predetermined sample times, T s . Upon receipt of an interrupt signal  64 , the STW DSP  52  performs an interrupt service routine (ISR)  66 , which lasts for a duration generally less than the sample time, T s , as indicated by the brackets shown in  FIG. 5 . 
       FIG. 6  depicts, in block diagram form, the steps of a conventional interrupt service routine. As shown in  FIG. 6 , the ISR broadly includes the steps of: profile generation (block  68 ), STW servo loop closure, whereby the generated profile is followed (block  70 ), and communication/housekeeping between the host computer  32  and the STW DSP  52  (block  72 ). 
     Although not shown in  FIG. 4 , the STW  50  also includes an external clock head assembly and a phase-locked loop (PLL). The external clock head is used for reading a clock track that has been written on the disk surface  12  using conventional techniques (e.g., the Monte Carlo technique). The phase-locked loop (PLL) is provided to maintain very accurate physical transitions relative to the disk surface  12 . Importantly, in the conventional STW  50 , the transducers  20  of the disk drive  10  are “placed” and “held” at radial positions relative to the center of the disk  12  completely independently from the clock PLL. It is only after the transducers have been “placed” at a radial position that the transducers  20  write the appropriate servo pattern clocked out by the PLL clock via a pattern generator, which keeps track of the circumferential position. After the servo pattern has been written, the transducers  20  are moved to the next radial position (again, independent from the clock PLL) and the process is repeated. Eventually, servo information is written across the entire disk surface to form the servo spokes  44  shown in  FIG. 3 . 
     Because servo information is currently written by placing transducers at radial locations across the disk surface and then writing servo information which is used to define a track, the time for writing servo information increases as the total number of tracks able to be placed on a disk surface increases. Since the number of tracks per inch (TPI) continues to increase, manufacturing times are likely to continue to increase, unless more servo track writers are supplied. However, as alluded to above, the purchase of additional servo track writers involves a significant capital expense. 
     In order to solve this problem and to expedite the manner by which servo information is written onto a disk surface (among other things), it has been determined that it would be beneficial to write servo information in spiral patterns (see, U.S. patent application Ser. No. 09/853,093 filed May 9, 2001, which is incorporated herein by reference in its entirety).  FIG. 7  is a simplified diagrammatic representation of first and second spiral patterns  100 ,  102  written onto a disk surface  12 . Each of the spiral patterns  100 ,  102  is written while the transducer  20  is dynamically moved across the disk surface  12  at a constant velocity. The spiral patterns  100 ,  102  may include a constant frequency pattern with synch marks (represented by black squares in  FIG. 7 ) imbedded therein. During operation of the disk drive  10 , the synch marks are used to position a transducer  20  over the disk surface  12  and, hence, forms (at least a part of) the servo information. 
     Writing servo information in such a manner presents a number of new problems. For example, since the transducer  20  is not “placed” and “held” at a particular radius relative to the center of the disk surface  12  before servo information is written, it would be desirable to develop a method for ensuring that corresponding synch marks along different spirals are located along the same radius. Furthermore, it would be desirable to develop a method for ensuring that the circumferential distance between adjacent synch marks along the same radius is equivalent. Reference is made to  FIG. 8 , which is diagrammatic representation of a fragmentary top view of a disk surface  12  having two spiral patterns written thereon, to illustrate these points. 
     As shown in  FIG. 8 , portions of Spiral N and Spiral N+1 are written on disk surface  12 . A first synch mark  104  associated with Spiral N is written along Spiral N near the outer diameter of the disk surface  12 . Similarly, a first synch mark  106  associated with Spiral N+1 is written along Spiral N+1 near the outer diameter of the disk surface  12 . For the servo information to properly perform its function, sync mark X of Spiral N and synch mark X of Spiral N+1 should lie on the same radius R relative to the center  108  of the disk  12 . Furthermore, the circumferential distance between adjacent synch marks along the same radius should be the same. For example, the circumferential distance between adjacent synch marks that lie along radius R should be equal to the circumferential distance D between synch mark X of Spiral N and synch mark X of Spiral N+1. 
     A further problem is that, as mentioned above, servo track writers are extremely expensive instruments. Accordingly, replacing existing servo track writers with new servo track writers that are used to write servo information in spiral patterns would be extremely expensive. Thus, it would be beneficial to develop a method for writing servo information using spiral patterns by minimally modifying existing servo track writers, rather than requiring altogether new servo track writers. 
     SUMMARY OF THE INVENTION 
     The present invention is designed to reduce the aforementioned problems and meet the aforementioned, and other, needs. 
     Instead of the servo loop of the STW acting independently and asynchronously with regard to the disk surface and the clock track, in the present invention, the clock PLL is divided down to produce the required sample time as an external interrupt to the servo track writer&#39;s digital signal processor. Accordingly, a sample-time/servo-sector interrupt, which is tied to the physical disk surface, is created. Since each interrupt occurs at a known disk location (i.e., the next servo sector), a position profile can be specified at each interrupt that will guarantee that spirals (and, hence, synch marks) will be placed at exact positions on the disk and relative to each other. 
     Preferably, servo track writing is performed in a closed-loop fashion for all profile movements. In such case, the position of the actuator (and, hence, the transducer) relative to the disk surface is known at all times (e.g., both during movements or stationary periods). This command side processing is as exact as the underlying clock PLL jitter and/or STW DSP interrupt service routine processing repeatability. 
     Other embodiments, objects, features and advantages of the invention will be apparent from the following specification taken in conjunction with the following drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic representation illustrating a conventional disk drive with its top cover removed; 
         FIG. 2  is a functional block diagram which illustrates a conventional disk drive that is coupled to a host computer via an input/output port; 
         FIG. 3  is a diagrammatic representation of a top view of a magnetic storage disk illustrating a typical organization of data on the surface of a disk; 
         FIG. 4  is a block diagram illustrating portions of a conventional servo track writer; 
         FIG. 5  is a diagrammatic representation illustrating a series of interrupt signals which occur at predetermined sample times, T s ; 
         FIG. 6  is a block diagram illustrating a conventional interrupt service routine; 
         FIG. 7  is a simplified diagrammatic representation of first and second spiral patterns written onto a disk surface; 
         FIG. 8  is simplified diagrammatic representation of a fragmentary top view of a disk surface having two spiral patterns written thereon; 
         FIG. 9  is a simplified diagrammatic representation of a modified servo track writer for writing spiral servo information in accordance with the present invention; 
         FIG. 10  illustrates an equation showing the relationship between spindle speed X (in units of revolutions per minute), the interrupt rate Y (in units of seconds per interrupt) and the number of interrupts per revolution Z (in units of interrupts per revolution), along with two illustrative example calculations; 
         FIG. 11  is a diagrammatic representation illustrating acceleration and velocity curves along a disk surface for one embodiment of a “write portion” of a spiral profile; 
         FIG. 12  is a diagrammatic representation illustrating acceleration and position curves relative to interrupts for one embodiment of a spiral profile; 
         FIG. 13  is a simplified diagrammatic representation of a top view of a disk surface which illustrates a sequential manner of writing spirals of servo information on a disk surface; and, 
         FIG. 14  is a simplified block diagram illustrating a switch, which permits a STW DSP to receive interrupts based upon a clock signal while writing spirals of servo information and to receive conventional fixed interrupts based upon a signal from a crystal during other operations. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While this invention is susceptible of embodiments in many different forms, there are shown in the drawings and will herein be described in detail, preferred embodiments of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspects of the invention to the embodiments illustrated. 
       FIG. 9  is a diagrammatic representation of a modified servo track writer  200  for writing spiral servo information in accordance with the present invention. Like the conventional STW shown in  FIG. 4 , the modified servo track writer  200  includes a STW digital signal processor (DSP)  252 , a STW voice-coil motor (VCM)  254 , a STW actuator arm assembly  256  and a push-pin system  258 . However, in contrast to the conventional STW  50  shown in  FIG. 4 , the modified STW  200  of the present invention uses signals read from a clock track  260  written on the disk surface  12  to provide a series of interrupt signals to the STW DSP  252 . 
     More specifically, a clock head  262  is used to read information stored in the clock track  260  and generates an analog clock signal that is delivered to clock head amplifier  264 . An amplified analog clock signal is then delivered to a pattern generator/PLL  266  to generate a digital clock signal. The pattern generator/PLL  266  preferably also includes a divide-by-M circuit  268 , which is used to divide down the digital clock signal, to provide a series of interrupt signals to the STW DSP  254  at sample times, T s , that are “tied” to the disk surface  12 . 
     As shown in  FIG. 10 , the spindle speed X (in units of revolutions per minute), the interrupt rate Y (in units of seconds per interrupt) and the number of interrupts per revolution Z (in units of interrupts per revolution) are related to one another, as set forth in Equation 1. Thus, by setting any two of the parameters X, Y or Z, one can solve for the unknown parameter. 
     In one embodiment, the number of interrupts per revolution Z is equal to the number of servo samples per revolution (i.e., the number of spiral crossings, or synch marks, at a particular radius). It should be understood, however, that the number of servo samples per revolution divided by the number of interrupts per revolution Z can be any natural number. In general, the servo sample rate (i.e., the time between adjacent and equidistant spiral crossings, or synch marks, at a particular radius) typically should be in the 15–20 kHz range to allow for a 600–700 Hz bandwidth. Thus, for a disk surface having 160 servo samples per revolution and which is spinning at a rate of 5700 revolutions per minute, the servo sample rate will be 15.2 kHz. 
       FIG. 10  gives two examples of calculating one of parameters X, Y or Z given that two of the parameters are known. In both examples, the number of servo samples per revolution is equal to the number of interrupts per revolution. 
     In Example 1, the number of interrupts per revolution Z has been selected to be 160 and interrupt rate Y has been selected to be 68 microseconds per interrupt. In such case, by using Equation 1, the spindle speed X can be determined to be 5514.705 revolutions per minute. 
     In Example 2, spindle speed X has been selected to be 5700 revolutions per minute and the number of interrupts per revolution Z has been selected to be 160. In such case, by using Equation 1, the interrupt rate Y can be calculated to be 65.789 microseconds per interrupt. 
     As will be understood by those skilled in the art, if the filter coefficients associated with the compensator of the STW servo loop are fixed based upon a particular servo sample rate, then the sample rate may be maintained by slightly adjusting the STW write speed. However, if the write speed has been chosen and is fixed, the new filter coefficients associated with the compensator of the STW servo loop may be calculated “on the fly.” 
     As in the case of the conventional STW  50 , upon receipt of an interrupt signal, the STW DSP  252  performs an interrupt service routine (ISR). However, in contrast to the conventional STW  50 , special profiles are generated in order to write spiral servo patterns. Generation of special profiles (or spiral profiles) will now be discussed. 
     As will be understood by those skilled in the art, in order to take advantage of the position-based interrupts, a position-type profile is implemented. Since the interrupts are “tied” to the physical disk surface by the clock PLL (i.e., digital clock signal), the profile is placed precisely relative to the disk surface  12 . 
     Preferably, spiral patterns are written onto a disk surface by moving a transducer across the disk surface at a constant velocity (e.g., 10–20 inches per second). Furthermore, guardbands (e.g., locations where information is not stored) are provided at both the inner and outer diameters of the disk surface. Thus, a spiral profile includes a “write portion,” which is based upon the total radial distance that the transducer is required to move, as well as the constant velocity and guardband requirements. 
       FIG. 11  is a diagrammatic representation illustrating acceleration and velocity curves along a disk surface for one embodiment of a “write portion” of a spiral profile. The “write portion” of the spiral profile shown in  FIG. 11  is known as a constant accelerate “bang, coast, bang” profile. In such case, accelerate/decelerate times (i.e., the “bangs”) occur as the transducer  20  moves across the guardband portions (referenced by brackets in the figure) of the disk surface  12 . Preferably, the accelerate/decelerate times are as small as possible. As shown in the figure, during the coast segment of the “write portion” of the spiral profile, the transducer  20  moves at a constant velocity. 
     The spiral profile also includes a “post-write pad portion,” which allows for a settle time after the “write portion.” The spiral profile yet further includes a “re-trace portion,” to specify the manner by which the transducer is to return near its starting point, so that the next spiral servo pattern may be written. Preferably, the transducer returns to its starting point as quickly as possible in a manner consistent with available maximum energy and system component characteristics. Finally, the special profile includes a “post-re-trace pad portion,” which allows for a settle time after the “re-trace portion” and which allows for any special processing requirements. 
       FIG. 12  is a diagrammatic representation illustrating acceleration and position curves relative to interrupts for one embodiment of a spiral profile. In  FIG. 12 , a one-to-one relation exists between the number of interrupts and the predetermined number of servo samples. For illustrative purposes, eight spirals are to be written (i.e., there are eight servo samples per revolution and, hence, eight interrupts per revolution); however, in practice, many more spirals would be written (e.g., 160 spirals). 
     As shown in  FIG. 12 , from interrupt  1  to interrupt  4  of the first revolution, the transducer accelerates (e.g., over the guardband portion of the disk surface). Next, from interrupts  4 — 8  of the first revolution, the transducer moves over the disk surface at a constant velocity, so the spiral pattern is written. Subsequently, from interrupt  8  of the first revolution to interrupt  3  of the second revolution, the transducer decelerates. A pad time is provided between interrupt  3  of the second revolution to interrupt  7  of the second revolution. From interrupt  7  of the second revolution to interrupt  1  of the third revolution the transducer accelerates (in a direction opposite to the direction while writing) as part of the re-trace. From interrupt  1  to interrupt  4  of the third revolution, the transducer moves at a constant velocity. From interrupt  4  to interrupt  6  of the third revolution, the transducer decelerates (again, in a direction opposite to the direction while writing). A pad time is then provided from interrupt  6  of the third revolution for a period of 12 interrupts, so that the next spiral may be written beginning at interrupt  2  of revolution  5 . This process repeats until all 8 spirals have been written. 
     It should be noted that, instead of generating a single spiral profile that includes a “write portion,” “post-write pad portion,” “re-trace portion” and “post-re-trace pad portion,” one or more of the aforementioned portions may be considered to be separate profiles that are performed sequentially. However, the single profile approach is preferred. If no post spiral write processing is required, the single profile may be cycled repeatedly until all spirals are written (e.g., as in  FIG. 12 ). 
       FIG. 13  is a simplified diagrammatic representation of a top view of a disk surface which illustrates a sequential manner of writing spirals of servo information on a disk surface. For sake of clarity, in  FIG. 13 , twelve spirals are to be written, although many more spirals are written in practice. 
     In  FIG. 13 , by following a “write, post-write pad, re-trace, post-re-trace pad” profile (for example), a transducer begins writing spiral  1  at the predetermined position of servo sample  1  and, after a post-write pad time and re-trace, the transducer will be located at the predetermined position of servo sample  11 . Presuming a one-to-one correlation exists between the number of servo sectors and the number of interrupts, spiral  2  would be written after waiting for the occurrence of three interrupts (e.g., during the post-re-trace pad). (It should be noted that, in practice, a longer duration than three interrupts may be required.) The process would repeat until all twelve of the spirals were written. 
     Although the spirals have been described as being written from an outer diameter to the inner diameter, it should be understood that the spirals may be written from the inner diameter to the outer diameter. Furthermore, it should be understood that a sequential manner of writing spirals is not necessary. Instead, the spirals may be written in any order and, in an extreme opposite case to the sequential manner of writing spirals, the spirals may be written in a random order. 
     In the case of writing spirals in a sequential manner, in one embodiment, the entire profile (e.g., “write, post-write pad, re-trace, post re-trace pad”) should be equal to the predetermined total number of spiral sectors per revolution plus one. Thus, when the cycle repeats, the next spiral will begin at exactly the next predetermined servo sector location relative to the immediately previously written spiral. Accordingly, once this algorithm is started, all spirals will be written sequentially from start to finish. (It should be understood that many other algorithms are possible.) 
     If, for example, the entire profile doesn&#39;t equal an integer number of servo sectors per revolution plus 1, it is a relatively simple matter to wait for the appropriate physical disk location by keeping track of the number of interrupts that have occurred since the spiral writing process began. 
     It should be understood that, after the spirals of servo information have been written, it is no longer necessary to be locked to the clock.  FIG. 14  is a simplified block diagram illustrating a switch  272 , which permits the STW DSP  252  to receive interrupts based upon a clock signal  270  while writing spirals of servo information and to receive conventional fixed interrupts based upon a signal from the crystal  60  during other operations. 
     Finally, with reference again to  FIG. 9 , it should be understood that the divide-by-M circuit  268  could be physically separate from the pattern generator/PLL  266 . 
     While an effort has been made to describe some alternatives to the preferred embodiment, other alternatives will readily come to mind to those skilled in the art. Therefore, it should be understood that the invention may be embodied in other specific forms without departing from the spirit or central characteristics thereof. The present examples and embodiments, therefore, are to be considered in all respects as illustrative and not restrictive, and the invention is not intended to be limited to the details given herein.