Patent Publication Number: US-6342986-B2

Title: Servo writing with simultaneous biasing of magneto-resistive read elements

Description:
RELATED APPLICATIONS 
     This application is a divisional of co-pending U.S. patent application Ser. No. 09/326,092 filed, Jun. 4, 1999 and claims the benefit of U.S. Provisional Application Ser. No. 60/088,064, filed Jun. 5, 1998. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the field of disc drive storage devices, and more particularly, but not by way of limitation, to improving servo writing operations such as servo track write verification by biasing multiple magneto-resistive read elements of a disc drive. 
     BACKGROUND OF THE INVENTION 
     Hard disc drives are used in modern computer systems to enable users to store and retrieve vast amounts of data in a fast and efficient manner. A typical disc drive is generally composed of a head/disc assembly (HDA) which houses requisite mechanical portions of the drive and a printed wiring assembly (PWA) which supports requisite electronic portions of the drive. 
     The HDA includes a base deck to which various components are mounted and a top cover which cooperates with the base deck to form a sealed housing to reduce particulate contamination. Within the housing, a disc stack is formed from one or more magnetic recording discs which are axially aligned for rotation by a spindle motor at a constant, high speed, such as 10,000 revolutions per minute during normal disc drive operation. 
     A rotary actuator assembly is mounted adjacent the disc stack and includes a plurality of rigid arms which extend into the stack between adjacent discs, as well as above and below the top and bottom discs. The rigid arms support flexible suspension assemblies which in turn, support a corresponding number of read/write heads adjacent the surfaces of the discs. One head is provided for each disc surface to read data from and to write data to the corresponding surface. 
     Of particular interest are magneto-resistive (MR) heads, which utilize thinfilm inductive write elements to write data and MR read elements to read previously written data. A typical MR read element is formed from an alloy of materials so as to have a baseline electrical resistance which varies in the presence of a magnetic field of a selected orientation. By passing a bias current through the MR element, the selective magnetization of a corresponding track can be determined in relation to variations in voltage detected across the MR element. 
     A preamplifier/driver circuit is typically mounted to the side of the actuator assembly and includes a write driver circuit to apply write currents to a selected write element during a write operation. The preamplifier/driver circuit further includes a bias current source which is sequentially connected to the appropriate read element to effect a read operation. 
     The electronics provided on the disc drive PWA primarily serve to control the operation of the HDA and to communicate with a host computer in which the disc drive is mounted. Generally, the top level functional blocks on the PWA include a read/write channel which controls the reading and writing of data from and to the discs, a spindle motor control circuit which controls the rotation of the spindle motor, and a servo control circuit which controls the position of the heads. 
     Aspects of a typical servo control circuit are discussed in U.S. Pat. No. 5,262,907 issued to Duffy et al., assigned to the assignee of the present invention. The servo control circuit positions the heads relative to the tracks through the application of current to a coil of a voice coil motor (VCM) within the HDA, the coil being mounted to the actuator opposite the heads. The tracks are defined from servo data written to servo data fields on the surfaces of the discs during the manufacturing of the HDA. The servo data are stored as a series of radially extending servo wedges on each of the disc surfaces, with the servo wedges composed of adjacently aligned servo data fields, with each servo data field in each wedge defining a unique track. Hence, by periodically transducing the servo data associated with a particular track, the servo control circuit can adjust the current applied to the coil to adjust the position of the corresponding head. User data fields, which are used to store user data from the host computer, are subsequently defined between adjacent servo fields during a disc drive formatting operation. 
     Conventionally, the servo data are written using a servo track writing system, also commonly referred to as a servo track writer (STW). A typical servo track writer comprises a fixture on which each HDA in turn is mounted. Once mounted, the servo track writer proceeds to write the servo data using the heads of the HDA. Thus, a typical servo track writer includes control circuitry which generally emulates portions of the electronics disposed on the PWA, as well as a closed loop positioning system which both detects the radial position of the heads and mechanically advances the heads. Access to the actuator is achieved by providing an opening in the base deck of the HDA which is later sealed. 
     Position detection and feedback are usually carried out using a laser inferometer or other precise optical displacement instrumentation. Mechanical advancement of the heads can be carried out by inserting a pusher pin assembly into the HDA to engage and move the actuator assembly. More recently, positioning systems have also been developed which apply current to the actuator coil, thereby utilizing the VCM to advance the position of the heads. The control and positioning systems are usually interfaced with a personal computer (PC) which provides a graphical user interface for the STW operator to control the operation of the system. 
     The writing of servo data is an important, but correspondingly time consuming, portion of the HDA manufacturing process. A typical STW operation can take up to several hours per HDA, so that disc drive manufacturers will often implement large numbers of STW stations to accommodate large scale disc drive production efforts. For purposes of efficiency, a servo track writer does not typically write all of the servo data on one disc surface before moving to the next surface; rather, each of the heads are selected in turn so that, after a selected head writes a portion of the data at a given radius, the next head is selected and used to write a corresponding portion of the servo data at a selected angular distance from the first portion, and so on. This results in the servo data on each disc surface being offset with respect to the data on adjacent surfaces, resembling steps of a spiral staircase. This is repeated multiple times around the circumference of the discs, as disc drives typically have from 30 to 90 servo wedges on each surface. 
     Once all of the servo data have been written at a given radius, the servo track writer proceeds to verify the accuracy of the writing operation. This generally entails sequentially applying a read bias current to each of the heads in turn to transduce and verify the servo data from the respective surfaces in the order in which the data were initially written to the surfaces. Because only one read bias current source is available in the preamplifier circuit, the elapsed time between successive servo data fields on successively read data surfaces must be long enough to allow the current to be switched from the first head to the second head and to allow the second head to reach an equilibrium state before the second servo data field is read. Hence, it may require a reduction in the rotational speed of the disc, or additional revolutions of the discs, in order to enable the servo track writer to verify all of the servo data. This can present a bottleneck in a disc drive manufacturing process, requiring additional investments in resources to meet the required process throughput. 
     Accordingly, there is a continued need for improvements in the manner in which data are tranduced from disc recording surfaces, such as during the read verification operations carried out during the writing of servo data. It is to such improvements that the present invention is directed. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an apparatus and method for providing improved read operations upon disc recording surfaces through the simultaneous biasing of multiple magneto-resistive servo writing. 
     In accordance with preferred embodiments, a disc drive includes a head/disc assembly (HDA) housing a plurality of disc recording surfaces and an actuator assembly which supports a corresponding plurality of heads adjacent the surfaces. A preamplifier circuit is mounted to the actuator assembly and includes a first read bias current source which generates a first read bias current of selected magnitude. The preamplifier circuit further includes a second read bias current source which generates a second read bias current of selected magnitude independently of the first read bias current. A head selection circuit of the preamplifier circuit selects first and second heads of the HDA so that the first and second read bias currents are simultaneously directed to the first and second heads, respectively. 
     In this way, a servo writing operation, such as a verification operation used to verify accuracy of servo data written to the disc recording surfaces during a servo track write operation, can be carried out efficiently by sequentially applying read bias currents to successive pairs of the heads. The read bias current applied to a selected one of each pair of the heads is used to transduce the data from the associated disc recording surface, while the read bias current applied to the remaining one of each pair of the heads is used to prepare the remaining one of each pair of the heads to subsequently transduce the data from the associated disc recording surface. 
     These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of a head/disc assembly (HDA) of a disc drive constructed in accordance with preferred embodiments of the present invention, with the HDA combinable with a PWA to complete the disc drive. 
     FIG. 2 illustrates the manner in which servo data are arranged on each of the disc surfaces in a plurality of radially extending wedges, each wedge comprising a plurality of adjacently disposed servo data blocks. 
     FIG. 3 shows a preferred configuration of one of the servo data blocks. 
     FIG. 4 provides a functional block diagram for a servo track writer used to write the servo data to the discs of the HDA in accordance with preferred embodiments. 
     FIG. 5 provides a functional block diagram of a servo control circuit used to effect head positional control by the disc drive, with the circuitry being disposed on the disc drive PWA and at least portions of which preferably emulated by the servo track writer. 
     FIG. 6 is an elevational view of a disc stack of the HDA, generally illustrating the manner in which servo data are sequentially ordered on successive disc surfaces. 
     FIG. 7 shows a basic construction of one of the MR heads of the HDA. 
     FIG. 8 provides a representation of a prior art preamplifier/driver circuit used to supply read bias currents to a plurality of MR heads. 
     FIG. 9 provides a representation of a preamplifier/driver circuit having multiple read bias current sources to simultaneously apply read bias currents to a plurality of MR heads in accordance with preferred embodiments of the present invention. 
     FIG. 10 provides a representation of an alternative construction for the preamplifier/driver circuit of FIG. 9 in accordance with preferred embodiments of the present invention. 
     FIG. 11 is a flow chart illustrating a SERVO TRACK WRITE OPERATION routine, generally indicative of steps preferably carried out by the servo track writer of FIG. 4 in accordance with preferred embodiments of the present invention. 
     FIG. 12 is a flow chart illustrating a VERIFY SERVO DATA subroutine preferably carried out as part of the routine of FIG. 11 in accordance with preferred embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring first to FIG. 1, shown therein is a top plan view of a disc drive  100 , constructed and formatted in accordance with preferred embodiments of the present invention. The disc drive  100  is formed of two primary assemblies: a head/disc assembly (HDA)  101  which composes substantially all of the mechanical portions of the disc drive, and a printed wiring assembly (PWA) which supports electronics used to control the operation of the HDA. The PWA is mounted to the underside of the HDA  101  and is thus not visible in FIG.  1 . 
     The HDA  101  includes a base deck  102  to which various disc drive components are mounted. A top cover, which has been omitted from FIG. 1 to facilitate the present discussion, cooperates with the base deck  101  to form a sealed housing for the HDA  101 . A spindle motor  104  is provided to rotate a stack of discs  106  at a constant high speed during normal disc drive operation, with a disc clamp  108  securing the discs to the spindle motor  104 . 
     To access the discs  106 , a controllably positionable actuator assembly  110  (also referred to as an “E-block”) is provided which rotates about a cartridge bearing assembly  112  in response to currents applied to a coil (a portion of which is shown at  113 ) of a voice coil motor (VCM)  114 . The actuator assembly  110  includes a plurality of actuator arms from which corresponding flexure assemblies extend, the topmost of which are identified at  116  and  118  respectively. Heads  120  are provided at distal ends of the flexure assemblies  116 ,  118  and are supported over the discs  106  by air bearings established by air currents set up by the rotation of the discs  106 . The heads  120  are positionably located over data tracks (not shown) of the discs  106  in order to read data from and write data to the tracks, respectively. As discussed more fully below, the heads  120  are characterized as magneto-resistive (MR) heads, with each head including a thin film inductive write element and a MR read element. 
     A latch assembly  121  secures the heads  120  over texturized landing zones (not shown) at the innermost diameters of the discs  106  when the HDA  101  is not in an operational mode (i.e., when the discs  106  are brought to rest). A flex circuit assembly  122  facilitates electrical communication between the actuator assembly  110  and the PWA. The flex circuit assembly  122  includes a flex circuit board  124  which supports an encapsulated preamplifier/driver circuit  130  which applies read and write currents to the heads  120 . Preferred construction and operation of the preamplifier/driver circuit  130 , also referred to herein as the “preamplifier circuit” and the “preamp,” will be discussed in greater detail below. 
     FIG. 2 provides a generalized representation of a preferred manner in which servo data are arranged onto the surfaces of the discs  106 . Particularly, a number of servo wedges (one denoted at  150 ) radially extend from innermost to outermost radii of recording surface portions of the discs  106 . Each of the servo wedges comprises a plurality of adjacently positioned and aligned servo data fields, as shown at  152  in FIG.  3 . 
     The servo data field  152  includes an automatic gain control (AGC) field  154  which stores an oscillating pattern (such as a  2 T pattern) to prepare servo control circuitry (not shown) of the disc drive for receipt of remaining portions of the servo field  152 . A synchronization (sync) field  156  provides timing information to the servo control circuitry. Index and Gray code fields  158 ,  160  respectively, indicate the angular and radial position of the servo field  152 . A position field  162  provides inter-track positioning information and serves to define track boundaries on the disc surface. 
     The servo data are written to the discs  106  during manufacturing of the HDA  101  using a servo track writer (STW)  170 , such as generally represented in FIG.  4 . The HDA  101  is mounted over a mounting fixture  172  which serves as a mechanical reference for the servo track writer  170 . The mounting fixture  172  typically comprises a granite block having a calibrated, level top surface on which a mounting plate is placed. The mounting plate includes clamps which locate and secure the HDA  101  to the mounting fixture  172 . As such mounting fixtures  172  are well known, additional discussion will not be provided herein except to state that the HDA  101  is mounted in such a manner so as to provide access to the actuator assembly  110  (shown in FIG. 1) by the STW  170  during a servo write operation. 
     Continuing with FIG. 4, a pusher block assembly  174  is mounted to the mounting fixture  172  underneath the HDA  101  so as to engage the actuator assembly  110  in order to selectively position the actuator assembly  1   10  relative to the discs  106 . Typically, an opening (not shown) is provided in the bottom of the base deck  102  of the HDA  101  so that the pusher block assembly  174  can access the internal environment of the HDA  101  by extending up through the opening in the base deck  102 . Once the servo data have been written, the opening is typically covered by an adhesive label or other means to seal the internal environment of the HDA  101 . 
     The STW  170  further comprises a positioning system  176  mounted to the mounting fixture  172  relative to the HDA  101 . The positioning system  176  rotates the pusher block assembly  174  about a central axis in order to advance the position of the actuator assembly  110 . The position of the pusher block assembly  174  is controlled by the positioning system  176  through a detection system (not separately designated) which detects the position of the pusher block assembly  174  and provides correction signals to a motor (also not separately designated) in order to rotate the pusher block assembly  174  accordingly. 
     A control circuit  178  emulates portions of the disc drive PWA in order to control the operation of the HDA  101 , including the transfer of servo data to the HDA  101  from a personal computer (PC)  180 . The PC  180  provides a graphical user interface for a user to control the STW operation. 
     During operation of the STW  170 , the user mounts the HDA  101  to the mounting fixture  172  and instructs the STW  170  (by way of the PC  180 ) to commence writing the servo data to the discs  106 . The control circuit  178  instructs the HDA  101  to commence rotation of the discs  106  by way of the spindle motor  104 (FIG. 1) and instructs the pusher block assembly  174  to place the actuator assembly  174  at a starting position, such as near the outer radii of the discs  106 . The STW  101  thereafter instructs the HDA  101  to write the servo data to each of the surfaces of the discs  106 , while mechanically advancing the pusher block assembly  174  so as to define each new successive track on the discs  106 . Each of the heads  120  in turn write the servo data to the corresponding discs  106  at each incrementally selected disc radius. 
     The time required to complete the writing of the servo data to a typical HDA depends upon the amount of servo data to be written to the discs, the rotational speed of the discs, the number of individual disc surfaces, and the time required to validate, or verify, that the servo data were written correctly. For reference, it can take several hours to complete the STW operation for a single high capacity HDA. Hence, there are substantial economic benefits to not only writing the data correctly the first time (to avoid the time and expense of rewriting the data), but also to carrying out all aspects of the STW operation as efficiently as possible. 
     Before turning to a discussion of preferred methodologies by which the STW  170  of FIG. 4 advantageously carries out readback verification functions of the written servo data, reference is made to FIG. 5, which provides a functional block diagram of a servo circuit  182  used to effect head positional control for each of the heads  120  with respect to the corresponding disc surfaces. It will be recognized that most of the servo circuit  182  is disposed on the aforementioned disc drive PWA and used during normal disc drive operation. However, since the STW operation is carried out on the HDA  101  before the PWA is mounted thereto, the control circuit  178  preferably includes this circuitry so as to emulate functional aspects of this circuit. Although the STW  170  of FIG. 4 utilizes a pusher pin assembly  172  to mechanically advance the actuator assembly  110 , the STW  170  can be readily provided with an alternative configuration wherein the control circuit  178  utilizes circuitry such as set forth by FIG. 5 to apply current to the actuator coil  113  to sequentially advance the actuator assembly  110 . 
     As set forth by FIG. 5, servo data are transduced from a selected disc surface and, after preamplification by the preamp/driver circuit  130  (previously shown in FIG.  1  and herein also referred to as the “preamp”), the servo data are passed to a demodulator circuit (demod)  184  which conditions the servo data for presentation to a digital signal processor (DSP)  186 . During servo circuit position control operations, the DSP  186  operates in accordance with programming stored in DSP memory  188  to output current command signals to a coil driver  190 , which in turn applies current to the coil  113  to position the head  120  as desired. 
     Additionally, the DSP  186  communicates with the preamp  130  to select the desired head  120  as well as to select the desired bias current for the head. FIG. 6 shows a schematic representation of a selected head  120 , generally illustrating the presence of both a write element  192  and a read element  194 . As will be understood, the write element  192  is used to write data to the discs  106  and preferably comprises a thin film inductive element with a conductor wrapped multiple times around a horseshoe shaped core with an air gap brought into proximity to the disc surface. As pulsed write currents are passed through the conductor, magnetic fringing occurs across the gap, selectively magnetizing the disc surface. 
     The read element  194  preferably comprises an MR element formed from an alloy of cobalt, nickel and iron and possesses a nominal direct current (dc) electrical resistance (such as 40 ohms). When the MR element is subjected to a magnetic field of selected orientation, the element undergoes a change in the baseline electrical resistance. Hence, data are read from a disc surface by passing a bias current through the MR element and monitoring changes in voltage thereacross induced by the magnetization of the disc surface. This produces the readback signal which can then be decoded by read/write circuitry to reproduce the originally stored data. The bias current is applied by the preamp  130  and has a magnitude selected by the DSP  186 . 
     It will be recognized that all of the heads  120  have the same nominal construction as shown in FIG.  6 . Although MR head technology has been presented herein in accordance with preferred embodiments, it will be further recognized that the present invention, as claimed below, is not necessarily limited to such construction, but can readily be used with other, similar technologies wherein bias currents are applied to read elements to transduce magnetic data, such as giant magneto-resistive (GMR) and spin-valve technologies. 
     FIG. 7 shows the generally stair-step manner in which servo data are written to successive disc recording surfaces. Particularly, FIG. 7 is an elevational representation of a plurality of discs  106  of the HDA  101  and corresponding heads  120  which are supported adjacent the surfaces of the discs  106  by flexure assemblies  118 , as discussed above in FIG.  1 . The servo wedges  150  (FIG. 2) are angularly displaced as shown so that, as discussed above, the heads  120  are cycled to write the servo data at different angular locations while being maintained at the same radial location with respect to the discs  106 . 
     FIG. 8 shows a generalized functional block diagram of relevant portions of a prior art preamplifier/driver circuit (“preamp”)  200  which is operably coupled to a total of four read/write heads  202 ,  204 ,  206  and  208 . The heads  202 ,  204 ,  206  and  208  are nominally identical to the heads  120  discussed above, each thereby possessing an MR construction. It will be understood that the use of four heads is merely for purposes of illustration, in that prior art preamps such as  200  are commercially available to service a wide variety in the number of heads (such as up to 20 heads). Moreover, it will be understood that the preamp  200  further includes circuitry used to apply write currents to the write elements  192  (FIG. 6) of the heads, but such has been omitted for clarity. 
     The functional blocks of the prior art preamp  200  set forth in FIG. 8 include a buffer  210 , a bias current source  212 , a head selection circuit  214 , switching circuitry represented as a multiplexer (mux)  216 , and a read amplifier (amp)  218 . During operation, a multi-bit input word is supplied to the buffer  210  which indicates, among other things, a desired head to be selected and a desired magnitude of bias current to be applied thereto. This input word is input along path  220  and can be supplied, for example, by a DSP such as illustrated at  186  in FIG.  5 . 
     In response to the input word, the bias current source  212  selects and outputs the appropriate read bias current along path  222  to the mux  216 . At generally the same time, the head selection circuit  214  uses the input word to select (via path  224 ) the desired head from the population of heads  202 ,  204 ,  206  and  208  (for example, the top head  202 ). In this manner, the bias current source  212 , head selection circuit  214  and mux  216  cooperate to pass the desired magnitude of read bias current through the MR element of the selected head  202 . As the head  202  transduces the selective magnetization of the corresponding disc surface, variations in the voltage across the MR element of the head  202  are sensed and amplified by the read amp  218  to output an amplified read signal on path  226 . The amplified read signal is thereafter decoded to reconstruct the previously stored data. 
     While operable, the prior art preamp  200  of FIG. 8 presents limitations in the ability to transduce data from multiple heads. For example, switching from the first head  202  to the second head  204  requires the input of a new input word to the buffer  210 , the decoding of this word by the bias current source  212  and the head selection circuit  214 , the adjustment of the magnitude of the bias current to the new level and application of the new bias current magnitude to the second head  204 , and the thermal stabilization of the second head  204 . The foregoing actions are necessary before data can be transduced using the second head  204 . Thus, these and other considerations limit the speed at which data can be transduced from successively selected disc surfaces. In the context of a STW operation, the rotational speed of the discs  106  may need to be slowed, or the servo wedges  150  spaced sufficiently apart on successive surfaces, to allow these actions to be carried out without requiring multiple revolutions of the discs  106 . 
     Accordingly, FIG. 9 provides a functional block diagram of relevant portions of the preamp  130 , constructed in accordance with preferred embodiments of the present invention. For ease of illustration, the preamp  130  is contemplated as being connected to the four heads  202 ,  204 ,  206  and  208  discussed in FIG.  8 . It will be understood, however, that the present invention as claimed below is not limited to the particular configuration shown in FIG.  9 . 
     As with the prior art preamp  200  of FIG. 8, the preamp  130  of FIG. 9 generally includes functional blocks that carry out bias current generation, head selection, switching and read signal amplification. Significantly, however, the preamp  130  includes multiple read bias current sources (identified at  232  and  234 , respectively), each of which independently and simultaneously outputs a bias current of a selected magnitude. 
     Generally, during operation a multi-bit word is provided to a buffer  236 , indicative of not only the presently desired head to be selected (such as the first head  202 ) and the associated magnitude of bias current to be applied thereto, but also of the next head to be successively selected (such as the second head  204 ) and the associated magnitude of bias current for the next head. In response to the input word, the bias current sources  232 ,  234  each select and output the respective magnitudes of bias current on paths  238 ,  240  respectively, to first, second and third multiplexers (muxs)  242 ,  244  and  246 , as shown. It will be noted that the first and second muxs  242  and  244  are each connected to each of the heads  202 ,  204 ,  206  and  208 . Although these interconnections are shown to be outside the dotted line box representing the preamp  130 , it will be understood that these interconnections are contemplated as being formed within the preamp  130  so that only two conductors are preferably extended from the preamp  130  to each MR read element of the heads  202 ,  204 ,  206  and  208 . 
     A head selection circuit  248  is operably coupled to the buffer  236  and the first and second muxs  242 ,  244  to channel the first and second bias currents (from the first and second bias current sources  232 ,  234 ) to the appropriate heads; in this example the first and second heads  202  and  204 , respectively. Thus, the preamp  130  advantageously operates to simultaneously apply read bias currents to multiple heads. However, as the data transduced from the discs  106  is output serially (i.e., one disc at a time), the third mux  246  switches between the first and second heads  202 ,  204  by a selection input on path  250  to connect the appropriate readback signal to the read amp  218 . This results in the generation of an amplified readback signal on path  226 , as before. The selection input on path  250  can be provided, for example, by a sequencer (not shown) of the read channel circuitry which outputs read gate signals indicative of times when data are expected to be received from the preamp  130 . 
     In this way, with reference again to FIG. 7, while the servo data are verified on the top surface of the top recording disc  106 , the appropriate read bias current is simultaneously applied to the next head so that, when the servo data on the bottom surface of the top recording disc  106  reaches the next head, the next head is ready to transduce this next servo data. This operation continues to each successive head in turn, allowing efficient read verification of the servo data. It will be noted that the preamp  130  is particularly suited for operation during servo write verification, since the selection of heads occurs in accordance with a predetermined order. 
     Hence, with reference again to FIG. 9, it will be noted that in one preferred embodiment, the buffer  220  receives successive input words which allow the head selection circuit  248  and the bias current sources  232 ,  234  to sequentially apply the desired bias currents to the desired heads in the desired order. In another preferred embodiment, the preamp  130  is configured to operate in a special STW mode (set by a particular bit in the input word on path  220 ), which instructs the preamp  130  to automatically cycle among a series of predetermined head/bias current combinations suitable for a particular STW operation. 
     FIG. 10 provides an alternative preferred construction for the preamp  130 , similar to that set forth by FIG. 9 except that each of the heads  202 ,  204 ,  206  and  208  is provided with a separate bias current source (identified at  252 ,  254 ,  256  and  258 , respectively). Again, only four heads have been shown in FIG. 10 for purposes of illustration, but the claimed invention is not so limited. 
     A head selection circuit  260  operates to selectively direct bias currents of selected magnitude from the bias current sources  252 ,  254 ,  256  and  258  to the heads  202 ,  204 ,  206  and  208 , respectively. The voltages across the heads  202 ,  204 ,  206  and  208  are presented to a suitable switching network, such as a multiplexer  262 , to sequentially apply these voltages to the read amp  218 . Although it is contemplated that all of the bias current sources  252 ,  254 ,  256  and  258  could be simultaneously applied to the heads  202 ,  204 ,  206  and  208 , such operation could undesirably increase the power requirements (and hence, the heat dissipation) of the preamp  130 . 
     Hence, the head selection circuit  260  preferably operates to direct bias currents to only two heads at a time: the head that is presently being used to transduce data and the next head in line to transduce data. Once each head in turn has finished reading a particular servo data field  152  (FIG.  3 ), that head is deselected in favor of the next head in line. For example, the head selection circuit  260  operates to apply read bias currents to the first and second heads  202  and  204 ; after the data are transduced by the first head  202 , the preamp  130  proceeds to transduce the data by the second head  204  while applying read bias current to the third head  206 , and so on. As before, it is contemplated that the head selection circuit  260  can operate in response to input words on path  220 , or can be configured to operate in a special STW mode to enable automatic cycling of the heads in a predetermined order. Selection inputs on path  250  can be used to indicate times when data are expected to be received from the preamp  130 . 
     FIGS. 11 and 12 summarize preferred operation of the preamp configurations of FIGS. 9 and 10 during STW operations. As discussed above, the preamp configurations of FIGS. 9 and 10 are particularly suited to servo track write verification operations, since the order in which the heads are to be successively selected are predetermined in relation to the arrangement of the servo data on the disc surfaces. It will be appreciated, however, that to the extent that it is known which head is to be selected next during normal disc drive operations, the preamp configurations of FIGS. 9 and 10 can be also used in a similar fashion to improve data transfer performance. 
     Beginning with FIG. 11, shown therein is generalized flow chart for a SERVO TRACK WRITE OPERATION routine  300 , generally indicative of steps carried out by the STW  170  of FIG. 4 in accordance with preferred embodiments. 
     At step  302 , the HDA  101  is first mounted to the mounting fixture  172  (FIG. 4) and necessary preparations are made to write the desired servo data to the HDA  101 . Such preparations can include the insertion of a clock head (not shown) into the base deck  102  in order to write a clock track to an outer radius of a selected disc  106  in order to provide timing information during the STW operation. 
     Next, appropriate magnitudes of read bias and write currents are selected at step  304 . As will be recognized, although MR heads are nominally identical, each will generally provide optimal performance at slightly different magnitudes of read bias and write currents. Thus conventional operations can be first carried out to select appropriate current magnitudes for each of the heads; alternatively, based on historical data, it may be determined that the servo data can be adequately written to and transduced from each of the disc surfaces using the same magnitudes of currents for each head. 
     At step  306 , the STW  170  proceeds to position the actuator assembly  110  to the desired location so that the heads are ready to commence writing, which occurs at step  308 . More particularly, the STW  170  (FIG. 4) operates to write the desired servo data to a selected radial location on the discs  106 , cycling through each of the disc surfaces as illustrated in FIG.  7 . Although STW operations can vary, it is common to write the servo data in multiple increments for each track (such as in one-half track increments) so that multiple passes are made to complete all the servo fields  152  (FIG. 3) in a given cylinder (i.e., all tracks on all the discs  106  at a given radius). Hence, in a preferred embodiment one complete cylinder of servo fields  152  are written during the step  308  before readback verification takes place. 
     The servo data are next verified at step  310 , which is set forth more fully by the flow of FIG. 12, discussed below. Once the verification step is completed, decision step  312  determines whether all of the servo data have been written to the discs  106 ; if not, the routine continues to step  314  wherein the head position is incremented to prepare the heads to write the next cylinder of servo data, and the routine returns to step  308 . Once all of the servo data have been written to the discs, the routine passes from the decision step  312  to end at step  316 . 
     FIG. 12 provides a flow chart for a VERIFY SERVO DATA subroutine, preferably constituting steps carried out by the step  310  of FIG.  11 . 
     At step  318 , the routine operates to select a pair of heads identified as the “present head” and the “next head,” with the present head comprising the head from which servo data are to be transduced and the next head comprising the next head in line for use in transducing servo data. In accordance with the foregoing examples of FIGS. 9 and 10, the present head would initially be the first head  202  and the next head would initially be the second head  204 . During step  318 , the appropriate bias currents are simultaneously applied to the present head and the next head, in that the next head receives read bias current while the present head receives read bias current. 
     Next, the present head is used to transduce the servo data on the corresponding disc surface, as indicated by step  320 . As before, a readback signal is generated as the servo data pass under the present head, with this readback signal being amplified by the read amp  218  and output on path  226 . During the operation of step  320 , any servo data fields  152  having errors are logged for subsequent rereading and, if such persist, appropriate corrective actions. Once the present head has completed reading the servo data, the present head is deselected at step  322 . This entails the removal of the read bias current from the present head by the preamp  130 . 
     Decision step  324  next determines whether all of the servo data at the selected position of the heads have been verified; if not, the present head and the next head are incremented at step  326 . For example, if the heads are identified as K 0  through K n  and the present head is initially set to head K 0  and the next head is initially set to K 1 , then the operation of step  326  serves to adjust the present head to K 1  and the next head to K 2 . The subroutine continues in like fashion until all of the servo data at the selected head location (such as the selected cylinder) have been verified. For example, if there are  90  servo wedges  150  (FIG. 2) on each disc surface, then the subroutine  310  will cycle back through  90  times to read verify each of the associated servo data fields  152 . It is contemplated that this operation can be readily carried out during one revolution of the discs  106 . Once all of the servo data have been verified, the routine returns at step  328 . 
     In view of the foregoing, it will be recognized that the present invention is directed to an apparatus and method for servo writing with simultaneous biasing of multiple magneto-resistive read elements in a head/disc assembly (HDA) of a disc drive. 
     In accordance with preferred embodiments, an HDA  101  includes a preamplifier circuit  130  having a first read bias current source  232 ,  252  which generates a first read bias current of selected magnitude and a second read bias current source  234 ,  254  which generates a second read bias current of selected magnitude independently of the first read bias current. A head selection circuit  248 ,  260  selects first and second heads  202 ,  204  of the HDA disc drive so that the first and second read bias currents are simultaneously directed to the first and second heads, respectively. 
     For purposes of the appended claims, the phrase “disc drive” will be understood consistent with the foregoing discussion to describe a data storage device of the type used to store computerized data, such as  100 . The phrase “head/disc assembly,” as denoted above by reference numeral  101 , will be understood to describe a mechanical assembly of a disc drive housing one or more rotatable discs  106  and an actuator assembly  110  which supports a plurality of heads  120 ,  202 ,  204 ,  206 ,  208  adjacent the discs  106 . The term “magneto-resistive” will be understood to cover head construction technologies which utilize read bias currents to transduce magnetically stored data. 
     It will be clear that the present invention is well adapted to attain the ends and advantages mentioned as well as those inherent therein. While a presently preferred embodiment has been described for purposes of this disclosure, numerous changes may be made which will readily suggest themselves to those skilled in the art and which are encompassed in the spirit of the invention disclosed and as defined in the appended claims.