WO1993008561A1

WO1993008561A1 - Hybrid sector servo system

Info

Publication number: WO1993008561A1
Application number: PCT/US1992/008871
Authority: WO
Inventors: Rosser S. Wilson; Michael J. Mayo
Original assignee: Maxtor Corporation
Priority date: 1991-10-18
Filing date: 1992-10-16
Publication date: 1993-04-29
Also published as: AU2880492A; JPH06503196A

Abstract

The present invention is a hybrid sector servo for use in a zoned density sector recording scheme. Sector pulses (1002, 1005) and index pulses (1001) are provided having a constant inter-pulse interval, T (1003). This inter-pulse interval (1003) is programmable, and may vary from zone to zone on the data surface. Instead of fragmenting the records, the preferred embodiment interleaves the sector pulses (1002, 1005) with the data records, thus reducing the complexity of the controller. Two types of sector pulses are generated: wide sector pulses (1005) carrying position information, and narrow sector pulses (1005) demarcating the sector boundaries. The preferred embodiment reduces rotational latency that arises from moving the read/write heads from a track in one zone to a track in antoher zone by computing the next occurence of a sector pulse (1002, 1005) in a new zone without having to wait for an intervening index pulse (1001).

Description

HYBRID SECTOR SERVO SYSTEM

BACKGROUND OF THE INVENTION

HELD OF THE INVENTION

The invention relates to the field of mass storage devices, and in particular, to a track-following hybrid servo system.

BACKGROUND ART

In a typical rotating magnetic media storage system, data is stored on magnetic disks in a series of concentric "tracks." These tracks are accessed by a read/write head that detects variations in a magnetic orientation of the disk surface. The read/write head moves back and forth radially on the disk under control of a head positioning servo mechanism so that it can be positioned over a selected track. Once in position over a track, the servo mechanism causes the head to trace a path that follows the center line of the selected track. This maximizes head-to-track registration, and permits accurate recording and reproduction of data on the track.

The tracks are electronically divided into a plurality of smaller fields, or physical records, called "sectors." Because storage disks are used as random access memory in many applications, such as personal computers, related information is not always written consecutively on the individual tracks. In addition, as old data is removed and new data added, it is not always possible to write new data in adjacent sectors or even adjacent tracks. Therefore, it is important for the disk drive to be able to quickly and accurately locate individual sectors of a track. Two functions of a head-positioning servo are (1) to maintain the selected head accurately positioned over the center of the desired data track while reading or writing is in progress, and (2) to provide for rapid movement of the head to any other selected track. Several types of servo are known in the art. Among these are dedicated, hybrid, sector or optical servos. This patent deals with the hybrid servo.

In a dedicated servo scheme, one entire surface of a disk contains the track position information. A servo head accesses the servo surface of the servo disk to read the position information stored therein. The servo pattern is detected by the servo head, and after appropriate signal processing, yields track position information. The servo head is in a fixed relationship relative to the other read/write heads, so that the other heads are expected to follow accurately the position of the dedicated servo head. However, in practice, thermal differential spindle run out and vibration impair this relationship. The impairment becomes particularly troublesome as track densities rise above 2000 tracks per inch. Accordingly, it is known also to record servo information on the date surfaces interleaved with the data records to provide means for sensing and correcting misregistration of data heads in their respective tracks.

Once the dedicated servo system has located a track, it is important that the read/write head be kept on the center line of that track for accurate reading and writing operations. This positioning of the read/ write head on the center line of a track is known as "track following." The servo tracks are encoded so that they provide a signal that varies linearly with head position. Variations from the center line of the track being followed produce position error signals (PES) that are used to generate a corrective input to the head positioning apparatus to move the head back to the center line position. Servo system performance is an integral part of the overall performance of a disk file. For example, data track misregistration is of primary consideration in determining the upper bound of the track density of a disk file. Although servo head misregistration to a servo track can be held to tight tolerance, track misregistration of a data head to a data track can occur as a result of thermal track shift, as well as from high frequency effects, such as spindle tilt, bearing effects, shock and vibration effects, and disk eccentricities. These misregistration effects grow more acute at higher track densities, currently above 2000 tracks per inch.

One prior art solution to the problem of track misregistration in dedicated servo systems is to use a "hybrid" servo scheme. In a hybrid scheme, the position information from a dedicated servo surface is combined with servo position samples from the data surfaces to generate a "hybrid" position error signal (PES). The servo information is read from the dedicated servo surface at a high sample rate, producing a high bandwidth and allowing for compensation of high frequency misregistration effects. The hybrid servo information, located on the data surface, is read at a lower sample rate, providing a lower bandwidth for compensation of low frequency misregistration effects.

A disadvantage of direct application of the prior art hybrid servo scheme occurs when the sector length is variable from track to track, as in a constant density recording scheme. In constant density recording schemes, the disk is divided into a plurality of concentric "zones." Each zone contains a plurality of adjacent concentric tracks. To maximize the storage capacity of each disk, the write frequency is varied for each zone so that the bit/inch density of the data is approximately constant for the entire disk. Where the track circumference is longest and the write frequency is highest, there are a greater number of sectors per track. Contrary to a non-zoned density recording scheme, in a zoned density recording scheme the record boundaries are not radially aligned. This lack of alignment creates problems where attempts are made to intersperse the hybrid information between the data records using conventional techniques.

One prior art solution to this problem, that being the problem of hybrid information being on radially contiguous lines, is called record fragmentation. In record fragmentation, the servo system artificially aligns the hybrid sectors radially and "fragments," or splits, the records around these artificially aligned sectors. Unfortunately, record fragmentation introduces additional complexity into the controller, and is not suitable for use in a practicable, constant density recording scheme.

SUMMARY OF THE INVENTION

The present invention is a hybrid servo system for use in a zoned density recording scheme. The preferred embodiment of the present invention provides sector and index pulses having a constant inter-pulse interval in a given zone. This inter-pulse period is programmable, and may vary from zone to zone on the data surface. In addition, the interleave factors of hybrids with data records is arbitrarily programmable and the parameters for this program are chosen to maintain a substantially constant hybrid sample rate in all zones.

The preferred embodiment may be used in a zoned density recording scheme where the inter-zone record boundaries are not radially aligned. Instead of fragmenting the records around hybrid information as is done in the prior art, the preferred embodiment interleaves the sector pulses with the data records, thus reducing the data recovery controller's complexity.

The preferred embodiment generates both wide, "hybrid" sector pulses as well as narrow, non-hybrid sector pulses. The servo system provides data track misregistration information based upon the position information recorded in the hybrid servo areas that occur during the wide hybrid sector pulses. The non-hybrid sector pulses act as sector boundaries between which the data records are recorded.

Further, the preferred embodiment reduces rotational latency during zone switching. When the read /write head is moved from a track in one zone to a track in another zone, the preferred embodiment accurately locates the sector and hybrid servo pulses within each zone without the necessity of waiting for the occurrence of an intervening index pulse. BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1 A-IC are a block diagram illustrating the hybrid sector servo system of the preferred embodiment.

Figures 2A and 2B illustrate the hybrid demodulator and zone density sector counter control of the preferred embodiment.

Figures 3A-3D are flowcharts illustrating the operation of the hybrid control logic of the preferred embodiment.

Figure 4 illustrates a block diagram of the hybrid servo demodulator of the preferred embodiment.

Figure 5 illustrates the scheduling of the A/D converter between hybrid bursts and the dedicated servo information.

Figures 6A and 6B illustrate the zone changeover sequence of the preferred embodiment.

Figure 7 illustrates a hybrid servo sample format for use with the preferred embodiment.

Figure 8 illustrates the guard zones of the preferred embodiment.

Figure 9 illustrates example waveforms of the hybrid servo demodulator.

Figures 10(A) and 10(B) illustrate the track composition of the preferred embodiment.

SUBSTITUTE SHEET DETAILED DESCRIPTION OF THE PRESENT INVENTION

A hybrid servo system for providing track following in a multiple recording zone format is described. In the following description, numerous specific details are set forth to provide a more thorough description of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without these specific details. In other instances, well-known features have not been described in detail so as not to obscure the present invention.

The preferred embodiment of the present invention generates sector and index pulses having a constant inter-pulse interval, the length of which is a programmable quantity that can vary from zone to zone. This inter-pulse interval is specified as an integer count of servo PLO clocks. In the dedicated servo pattern, each servo PLO clock corresponds to one cycle of the servo pattern, and is approximately 242 nanoseconds in duration. One data record is recorded in each inter-pulse interval between each sector pulse.

In the preferred embodiment of the present invention, the data surface contains tracks, client records, an index pulse, hybrid sector pulses and non- hybrid sector pulses. The tracks are arranged concentrically on the data surface. Figures 10(A) and 10(B) illustrate two views of a single track of information on the data surface. Each track is divided circumferentially into a number of sectors 1003. A reference point for an angular position of the head with respect to the particular track being accessed is provided by an "index" 1001 that defines a starting location for each track. Index 1001 is typically determined by decoding fiducial data written as part of the servo information used for control of data head positioning. In the preferred embodiment of the invention, index 1001 is considered to be the first hybrid sample. Each sector 1003 contains client records 1006, and is bounded by either a non-hybrid sector pulse 1002, or a hybrid sector pulse 1005. Each sector 1003 is of length T, where T may vary from zone to zone. Each hybrid sector pulse (and index pulse) is of length t.

The dedicated servo surface contains only servo information and no data records. A dedicated servo surface can be formed as a large number of "sections" that are circumferentially equally spaced. We refer to these sections as "segments" to differentiate them from the"sectors" of the hybrid servo surface.

In the preferred embodiment, there are many more dedicated servo surface segments than there are hybrid servo samples. For example, the dedicated servo surface contains 175 servo samples per revolution, while the hybrid servo surface contains approximately 12 - 20 samples per revolution. A dedicated servo demodulator can demodulate the dedicated surface in a known manner.

The preferred embodiment of the present invention uses two kinds of sector pulses in the hybrid sector servo scheme. The preferred embodiment generates both wide, "hybrid" sector pulses as well as narrow, non-hybrid sector pulses. The servo system provides track following based upon the position information contained in the hybrid sector pulses. The non-hybrid sector pulses act as sector boundaries between which the data records are recorded.

It is understood that client data is not recorded during a hybrid or non- hybrid sector pulse. These areas are write protected to avoid inadvertant overwirte or disruption of hybrid or non-hybrid sector information. Because not every sector pulse is a hybrid sector pulse in the preferred embodiment, overhead is reduced and more space is available on each data surface to record data records. The preferred embodiment of the present invention allows the interleaving of hybrids to a programmed degree. The amount of interleave varies from zone to zone to keep a roughly constant sample rate.

The preferred embodiment is able to issue either hybrid or non-hybrid sector pulses almost immediately after a zone changeover without having to wait for the index pulse. This is accomplished by loading residual registers with the number of hybrid and non-hybrid sector pulses to be issued until the next index pulse. This allows relaxation of the controller requirements, since the hybrids do not have to lie on radial lines. The hybrid servo system of the preferred embodiment removes static and low-frequency misregistration effects from the data head.

Because the preferred embodiment interleaves hybrid servo position information with data records, record fragmentation is not required. A nominally constant hybrid channel sample rate can be achieved in all zones of the data surface. The interleave ratio of hybrid sectors to non-hybrid sectors is programmable.

The preferred embodiment permits rapid changeover of sector inter- pulse interval and hybrid channel sample rate parameters when seeking into a new zone. This reduces rotational latency by eliminating the need to wait for the next intervening index pulse to accurately modulate the hybrid servo and data pulses. A block diagram illustrating a hybrid servo sector system for use with the present invention is illustrated in Figvires lA-lC. The head disk assembly (HDA) is generally illustrated by that area in Figure 1A enclosed by dashed line 100. The HDA includes one or more disks 149 mounted on a spindle shaft. The spindle shaft is coupled to a spindle motor 101 for rotating the shaft and ultimately the disks. The spindle driver 115 provides drive signal 146 to the spindle motor 101.

Spindle phase detector 116 receives spindle reference pulse 129 and delivers this phase information to DSP bus 142. Spindle pulse-width-modulated digital to analog converter 114 takes this phase information from DSP bus 142 and converts it to control signals for spindle driver 115. Spindle driver 115 converts spindle current samples to high power current flow in spindle motor 101. Spindle driver 115 also provides commutation for a brushless DC spindle motor, as is well known in the art.

The surfaces of each disk 149 are accessed by read/ write heads that are mechanically coupled to voice coil actuator motor 102. Voice coil actuator motor 102 consists of an armature coil moving in two gaps of a permanent magnet stator assembly. The voice coil actuator motor 102 provides radial movement of the heads relative to the disk surface. This permits the heads to be moved from track to track on each disk. The heads detect space division, multiplexed servo and data information from the surface of the disk and provide the combined servo and data signals on lines 130 and 131 to amplifiers 103 and 104.

Data amplifier 103 is coupled to read channel 105 through lines 132. Read channel 105 is coupled to storage director 180 and to hybrid demodulator 106. Hybrid demodulator 106 provides hybrid PES 135 to multiplexer 109, and zero crossing signal 136 to hybrid demodulator and zone sector counter control 122.

SUBSTITUTE SHEET Servo amplifier 104 provides dedicated servo PES information to dedicated servo demodulator 107. Dedicated servo demodulator 107 provides PES signals to multiplexer 109, and digital bit signal 138 to demodulator control 117, index vote logic 118 and coarse position deserializer 119. Dedicated servo demodulator 107 also provides information to servo PLO, (phase locked oscillator) 108. The purpose of the ADC and multiplexer are to convert an analog signal to digital form for use directly by the digital signal processor (DSP). The DSP uses control algorithms that feed back to control the position of the head through the voice coil motor and power amplifier.

Servo PLO 108 provides clock signal 139 to demodulator control 117, coarse position deserializer 119, index vote logic 118, hybrid sequencer 122 and storage director 180.

Multiplexer 109 receives position error signals from dedicated servo demodulator 107 and hybrid demodulator 106, as well as measured current 114 from power amp 113. The multiplexed information is converted by analog to digital converter 110, passed through register file 111, and is made available to DSP bus 142. Power amp 113 is coupled to voice coil motor current digital to analog converter 112 which, in turn, is coupled to DSP bus 142.

The dedicated servo head reads the dedicated servo information from the dedicated servo surface and transmits this information via lines 131 and 133 to dedicated servo demodulator 107. Demodulator control 117 transmits dedicated demodulator control signal 158 to both dedicated servo demodulator 107 and servo PLO 108. Dedicated servo demodulator 107 demodulates the dedicated servo information and transmits dedicated odd and even PES signals to multiplexer 109. The dedicated servo information is multiplexed to analog to digital converter 110, and the digitized dedicated servo information is passed on to DSP bus 142 via register file 111. Segment wrap counter 154 is contained in demodulator control 117. Segment wrap counter 154 divides each servo sample interval into a fixed number of sub-intervals, and provides a master timing reference to control operation of hybrid and dedicated servo demodulator blocks, the ADC, and other blocks as shown in Figures 1 A-IC

The data head reads both data and hybrid servo information from the data surfaces. The hybrid servo information is passed to read channel 105 and then to hybrid demodulator 106. Hybrid demodulator 106 then transmits analog hybrid PES 135 to multiplexer 109, where hybrid PES 135 is converted to digital information by analog to digital converter 110 and passed on to DSP- bus 142 via register file 111. Hybrid demodulator 106 is controlled by hybrid demodulator and zone sector counter control 122, which issues hybrid demodulator control signal 166 to hybrid demodulator 106. Hybrid demodulator and zone sector counter control 122 also receives zero crossing signal 136 from hybrid demodulator 106 as well as clocking signal 139 from servo PLO 108.

Demodulator control 117 is coupled to analog to digital converter 110, register file 111, index vote logic 118, coarse position deserializer 119, servo PLO 108, dedicated servo demodulator 107, control and status register 123 and DSP 124.

Hybrid write switch 120 is coupled to data amplifier 103, and receives hybrid write date signal 165 from control block 122 as well as write data signal 175 from storage director 180. Hybrid write switch 120 transmits write toggle signal 161 to data amplifier 103 based on whether data or servo information is to be written on the hybrid servo surface.

Fault logic 121 receives hybrid protect signal 167 and hybrid write signal 168 from control block 122, as well as read/ write gate signal 176 from storage director 180. Fault logic 121 transmits write and signal 162 to data amplifier, and fault signal 164 to storage director 180. Fault logic 121 is also coupled to registers 123.

Microwire serial port 128 is coupled to DSP bus 142 and transmits 5 orders/status signal 174 to storage director 180. Storage director 180 also receives index and sector signals 169 and 170 from control block 122.

In the preferred embodiment, DSP 124 contains ROM 125, RAM 126 and timer 127. DSP 124 is coupled to DSP bus 142, and demodulator control 117. DSP

10 124 monitors the information on DSP bus 142, most notably, the contents of the register file which contains the digitized analog information for the ADC and uses that to correct head position to the voice coil motor. DSP 124 controls the spindle motor and voice coil motor current, and loads the registers resident in hybrid demodulator and zoned sector counter control 122 with information necessary to 5 effect sequencing of the demodualtion and recovery of hybrid information required for track following.

Figures 10(A) and 10(B) illustrate one possible arrangement of hybrid and non-hybrid sector pulses on the hybrid servo surface. Each hybrid sector pulse 1001 0 has period 1004 of duration t. The inter-sector spacing 1003 between each non- hybrid sector pulse 1002, and between each hybrid 1001 and non-hybrid sector pulse 1002, has duration T. In Figures 10(A) and 10(B), each hybrid sector pulse 1001 is separated by three equally spaced non-hybrid sector pulses 1002. In a zoned density recording scheme, the actual number of non-hybrid sector pulses 1002 between 5 each hybrid sector pulse 1001 may vary from zone to zone. Data records are written during inter-sector period 1003. In the preferred embodiment, the hybrid sector pulse width 1004 is made wide enough to accommodate the track following servo information. Each non-hybrid sector pulse 1002 has a period equal to one cycle of servo PLO clock 108, and is used to

SUBSTITUTE SHEET demarcate the sector boundaries. The inter-sector period 1003 between each sector pulse is programmable.

Each hybrid sector pulse 1001 contains a servo pattern format that enables the servo system to perform track following operations. A hybrid servo sample format is illustrated in Figure 7. The hybrid servo pattern is encoded in a single phase differential burst format. The pattern utilizes two (termed odd and even) radially offset constant frequency bursts to encode the position information. These two bursts are prefaced by an AGC field written in the full on-track position. AGC burst 703, odd burst 704 and even burst 705 are illustrated in Figure 7.

During demodulation of a hybrid sample, the read channel AGC is set to acquisition mode in AGC field 703, after which AGC gain is set for the remainder of the sample. Detection of the PES is accomplished using two area (integrating) detectors. These detectors implement full-wave rectification and integration of the servo burst information. The "difference" detector integrates positively during odd burst 704, then is switched to integrate negatively during even burst 705. The "sum" detector integrates positively for both bursts. A fixed count of twenty transitions is integrated in each burst. Because coarse sequencing of hybrid demodulator 106 is timed by clocks obtained from main servo PLO 108, the timing signals involved in hybrid demodulation are re- synchronized to the hybrid burst data by a zero-crossing clock obtained by hard- limiting the analog burst data. Re-synchronization eliminates phase jitter caused by relative vibration between servo and data heads.

The hybrid servo burst information is written over three adjacent tracks K-l, K and K+l, each of pitch equal to the data track pitch. The central track of the area is written with an arbitrary number of AGC burst-, odd burst- even- burst triplets. The juxtaposition of these three fields is termed a hybrid burst. All bursts are recorded with a constant frequency tone. AGC burst 703 is written with the recording head centered on track K. Odd burst 704 and even burst 705 position bursts are written with the recording head offset from track K center by nominally one-half of track toward the inner and outer diameter of the disk, respectively. Head offset is obtained by issuing an appropriate command to the servo system. AGC burst 703 provides a gain reference that is held constant through the adjacent odd burst 704 and even burst 705.

The format area is DC erased by the interface controller prior to recording of the triplets. Complete erasure is ensured by performing erasure with the head in both offset and on-track positions of tracks K-l, K and K+l.

Although the hybrid sector bursts 1001 are aligned radially within each zone on the recording medium, the hybrid bursts are not necessarily aligned from zone to zone. Because odd burst 704 and even burst 705 overlap adjacent tracks, if the zones were placed along common boundaries hybrid servo and data information from different zones would overlap, possibly causing PES errors. To compensate for this, the preferred embodiment places one or more tracks devoid of customer information, called "guard zones," between each zone. These guard tracks are needed because the hybrid bursts in each zone are written at different circumferential positions.

Figure 8 illustrates two zones 802 on the data surface of the disk. Index pulse 801 is aligned radially in all zones. Each zone 802 contains narrow, non- hybrid sector pulses 1002 and wide, hybrid sector pulses 1001. Data records are written between each sector pulse, in sector 805. Guard zone 806 is placed between adjacent zones 802. Each guard zone 806 contains two tracks onto which no data is written. These blank tracks allow the servo system to accurately read hybrid information 1001 from the boundary tracks without any interference from neighboring tracks.

A block diagram of hybrid servo demodulator 106 is illustrated in Figure 4. Inputs 404 and 405 are coupled to VGA 419, as is automatic gain control 406. Output 413 of VGA 419 is coupled to carrier zero-crossing comparator 421, area detector 424 and area detector 425. The output of carrier zero-crossing comparator 421 is coupled to logic block 423, area detectors 424 and 425, re- synchronizing flip-flop 420, and summing block 422. The output of logic block 423 is provided to zero-crossing output 412.

Re-synchronizing flip-flop 420 also receives as input gate input 407 and reset input 409. The output of flip-flop 420 is coupled to area detectors 424 and 425. Reset input 408 is coupled to area detectors 424 and 425. Carrier INV/SEL sum input 410 is coupled to summing block 422 and AND gates 426 and 427. Select multiplex input 411 is coupled to AND gates 426 and 427. The output of summing block 422 is coupled to area detector 425. The purpose or the resynchronizing flip-flop is to ensure that an integer number of hybrid cycles are integrated, avoiding timing errors.

The output of AND gate 426 is coupled to the enable input of comparator 428, and the output of AND gate 427 is coupled to the enable input of comparator 429. The output 414 is area detector 424 is coupled to the input of comparator 429, and the output 415 of area detector 425 is coupled to the input of comparator 428. The output of comparators 428 and 429 are multiplexed through multiplex amp 430. Output 416 of amplifier 430 is coupled through inverters to the inputs of comparators 428 and 429. Hybrid servo demodulator example waveforms are illustrated in Figure 9. During AGC burst 703, gate 407, carr inv/sel sum 410, and sel mux 411 inputs are held low, while reset input 408 is asserted. When odd burst 704 is read, first reset 408 is de-asserted, then gate input 407 is asserted for a pre- determined number of cycles. When even burst 705 is read, input 410 is asserted, and gate input 407 is again asserted for a pre-determined number of cycles. Next, two area (integrating) detectors are alternated. Input 410 is held low so that the "difference" detector 425 and "sum" detector integrate positively during odd burst 704. Input 410 is then asserted so that "difference" detector 425 integrates negatively, and "sum" detector integrates positively for even burst 705. A fixed count of twenty transitions is integrated in each burst.

Multiplexer 109 receives position error signals from both the hybrid demodulator 106 and dedicated servo demodulator 107. Access to analog to digital converter 110 is controlled by multiplexer 109. That is, multiplexer 109 controls the scheduling of ADC 110 between hybrid bursts from hybrid demodulator 106 and dedicated servo PES from dedicated servo demodulator 107. Figure 5 illustrates the scheduling of ADC between hybrid bursts and the dedicated servo surface. The dedicated servo surface is divided into a specific number of uniform servo sample segments 508. Each servo segment 508 is further divided into four smaller areas: frame sync 504, digital 505, analog PES 506 and fill 507. As described in greater detail above, each data surface contains hybrid sector pulses 501, narrow, non-hybrid sector pulses 502 and data records 503. The number of narrow pulses 502 between each hybrid sector pulse 501 varies depending on which zone the hybrid servo data is written.

Multiplexer 109 schedules three separate time slots for access to ADC 110. These three time slots are illustrated in Figure 5 as schedules 520, 521 and 522. Separate time slots are allocated to the dedicated servo PES (509), the hybrid servo PES (510) and the voice coil motor current conversion (512). In each servo segment time, every slot allocated to the dedicated PES and to the motor coil current is used. However, slots allocated to the hybrid servo information may or may not be used depending on whether the hybrid burst immediately preceded the time slot. Multiplexer 109 reserves time slot 509 for ADC conversion of the analog PES 506 for each servo segment 508. That is, the same time slot is allocated for ADC conversion of the dedicated servo surface for each servo segment 508. Multiplexer 109 allocates time slots 510 and 511 for conversion of the hybrid PES. However, in a hybrid servo system, there are far fewer hybrid sector pulses 501 per revolution of the data surface than there are servo segments 508 on the dedicated servo surface. Consequently, although a time slot is allocated for hybrid ADC cycles during each servo segment, the preferred embodiment does not require that every one of these time slots be occupied by a hybrid sector pulse request. Thus, as shown in Figure 5, time slots 511 are occupied by a hybrid ADC request, while time slots 510 are allocated but not occupied. Because the exact location of each hybrid sector pulse varies in relation to the servo segments 508 located on the dedicated servo surface depending on the current zone, hybrid ADC requests for ADC 110 are queued until the beginning of the next available hybrid ADC time slot 511. Multiplexer 109 also allocates a time slot 512 for voice coil motor current conversion.

The state of wrap counter 154 is shown on line 513. Wrap counter state 513 further divides each servo segment 508 into a number of uniform periods. These smaller sub-intervals are governed by the clock signals of servo PLO clock 108. Further, servo segment interrupt 514 is asserted immediately at the end of time slot 509 and prior to fill section 507. The timing of this interrupt is controlled by wrap counter 154. The purpose of the interrupt is to inform the microcode running on the DSP that hybrid servo , dedicated servo, and motor current information is present in the register file for interrogation by DSP for use in the control algorithm.

The zone changeover sequence of the preferred embodiment of the present invention is illustrated in Figures 6 A and 6B. Dedicated surface 600 is subdivided into uniform intervals 508. Each segment number is contained and counted by microcode resident in DSP 124. During each index segment 601, an index pulse 603 is generated on the hybrid or the data surface. In between index pulses 603, a programmable number of sector pulses 604 are generated on the data surface. These sector pulses 604 consist of hybrid sector pulses 501 separated by a programmable number of narrow non-hybrid sector pulses 502. The inter-pulse period 605 between consecutive sector pulses 604 is both uniform and programmable.

In order to understand how the zone changeover sequence works, an example changeover is now described. Assume that the data heads are located in old zone 620 and that changeover to new zone 625 is required. While in old zone 620, both dedicated and hybrid servo information is fed into multiplexer 109. DSP 124 uses this information to perform track following operations. To commence a zone change, first sector changeover bit 607 is de-asserted. Next, sector changeover interlock status bit 608 is also de-asserted. DSP 124 then computes location of first sector pulse to be delivered in the new zone. The first sector pulse will be delivered at a specific location within servo segment S_start- Next, the DSP loads control registers with values computed in the previous step. DSP 124 then asserts sector changeover bit 607 in segment S start - - Finally, DSP 124 awaits the assertion of sector changeover interlock bit 108, signifying delivery of first sector in new zone. At this point, the zone change is complete. Hybrid servo information is read in new zone 625 from the data surface.

SUBSTITUTE SHEET Figures 2A and 2B illustrate hybrid demodulator and zone density sector counter control 122. DSP bus 142 is connected to a series of backing registers. These backing registers are coupled to several working counters, comparators, hybrid control logic 215 and hybrid format sequencer 216.

Sector length register 201 contains the two's complement of the number of servo PLO clock cycles in the inter-sector pulse interval. The resolution of this register is one servo PLO clock cycle. Hybrid suppress register 202 blocks issuance of truncated last hybrid sample intervals. Such intervals can occur when the hybrid period is not an integer multiple of the count of numbers of sectors on a track. Values in the hybrid suppress register of 0 and 1 specify, respectively, that no suppression is to occur, and that a potential hybrid sample on the last sector pulse prior to index is to be suppressed, and that sector pulse is to be issued as a non- hybrid sector pulse.

Residual sector count register 203 contains a value identifying the number of sector pulses to be delivered between the sector pulse changeover point of a zone change operation, and the first subsequent index point. For example, a value of 1 in this register prescribes that the first sector pulse is the only sector pulse to be delivered until index intervenes. Similarly, a value of 2 specifies that the initial sector pulse and a second subsequent sector will issue prior to index.

Sector count register 204 contains a value defining the number of sector pulses to be dehvered between index pulses. For example, values of 0 and 1 in this register specify, respectively, that no sector pulses intervene between index pulses, and that one sector pulse intervenes between index pulses. Note that the contents of register 204 pertain only after issuance of the first index pulse subsequent to a zone change operation, and remain in force until a new zone change operation is commanded. The sector count register suppresses potential shortened last sector intervals.

Residual hybrid period register 205 contains a value defining the count of the number of non-hybrid sector pulses in a new zone that intervene between the first sector pulse delivered and the next hybrid sample. For example, a value in this register of 0 indicates that the first sector pulse delivered after a zone changeover is a hybrid sector pulse. Similarly, a value of 1 indicates that the first sector pulse delivered is a non-hybrid sector pulse, and the second sector pulse delivered is a hybrid sector pulse. Note that once the first hybrid sector pulse has been delivered in a new zone, the hybrid period reverts to the period specified in hybrid period register 206.

Hybrid period register 206 contains a value defining the period at which hybrid samples occur, expressed as a count of sector pulses. For example, values in this register of 0 and 4 mark, respectively, that every sector pulse and every fourth sector pulse are hybrid samples. The index pulse marks sector zero and is always a hybrid sample.

Sector fractional position register 207 contains a value identifying the angular position at which the first sector pulse is delivered following a zone changeover operation. Note that the first sector pulse issues in the servo segment following the segment in which a zone change is ordered, when the wrap counter state equals a number stored in sector fractional position register 207.

One possible embodiment of sector fractional position register 207 is described in U. S. Patent No. 4,999,720 entitled, "Zone Density Sector Counter" and assigned to the Assignee of the present application. This patent is incorporated herein by this reference. One embodiment of the invention described in U. S. Pat No. 4,999,720 computes an offset value equal to the difference between each reference zone segment boundary, and the occurrence of the next consecutive sector in each of the recording zones. This permits rapid validation of the sector signal after a zone changeover without the necessity of waiting for the index.

Sector changeover bit 607 is pulled low to stop sector pulse issuance, and is interlocked with sector changeover interlock status bit 608. When sector changeover bit 607 and the interlock status bit 608 are de-asserted, all above- listed registers can be modified to reflect the parameters of the new zone. Then, sector changeover bit 607 is asserted in the servo segment just prior to the segment in which the first sector pulse in the new zone is to be delivered.

Sector changeover interlock status bit 608 is interlocked with sector changeover bit 607. When sector changeover interlock bit 608 is de-asserted, no further sector pulses are delivered. Sector interlock bit 608 is asserted when the first sector in the new zone has issued.

Sector length register 201 is coupled to sector length counter 210. Counter 210 receives load signal 231, count signal 232 and synchronous reset signal 233 from hybrid control logic 215. The overflow of counter 210 is coupled to hybrid control logic 215, and its counter output 250 is coupled to hybrid format sequencer 216.

Sectors per track counter 211 is coupled to switch 208. Switch 208 is either connected to residual sector counter register 203 or sector counter register 204, depending on the current status of select signal 239. Immediately after a zone changeover, switch 208 is coupled to residual sector counter register 203. Once an index pulse has been issued, switch 208 is then coupled to sector counter register 204. Counter 211 will either receive the current register value of register 203 or register 204, depending on the status of switch 208. Counter 211 receives load signal 236 and count signal 237 from hybrid control logic 215. Its underflow counter 238 is also coupled to hybrid control logic 215. The output of counter 211 is coupled to comparator 214 as is the output of hybrid suppress register 202.

Comparator 214 determines when a potential hybrid sample on the last sector pulse prior to index is to be suppressed. The output of comparator 214 is coupled to hybrid control logic 215. Hybrid period counter 212 is coupled to switch 209, which, in turn, is coupled to either register 205 or 206, depending on the current status of select signal 239. Immediately after a zone changeover, switch 209 is coupled to residual hybrid period register 205. However, once the first hybrid has been issued in the new zone, switch 209 is connected to hybrid period register 206. The output of switch 209 is coupled to counter 212 as is load signal 240 and count signal 241. Underflow flag 242 of counter 212 is coupled to hybrid control logic 215.

Comparator 213 is coupled to fractional position register 207 as well as wrap counter 154. Output 243 of comparator 213 is coupled to hybrid control logic 215. Comparator 213 issues a pulse whenever the wrap counter is equal to the fractional position register.

Hybrid control logic 215 is coupled to servo PLO clock 108 and servo control register 123. Hybrid control logic 215 is coupled to fault logic 121 and storage directory 180, as well as to DSP status registers via line 151. Hybrid control logic 215 is also coupled to the enable input of hybrid format sequencer 216. Hybrid format sequencer 216 is also coupled to servo PLO dock 108. Sequencer 216 receives a zero cross dock signal from the hybrid demodulator 106, and issues hybrid write data signal 165, hybrid demodulator control tags 166, hybrid mode signal and request hybrid ADC cyde signal.

The present invention permits the hybrid servo format to be written by the drive itself. The hybrid demodulator and zoned sector counter control block 122 can be used to write hybrid servo information during a formatting operation. A hybrid write data signal 165 is provided to hybrid write switch 120 to enable the hybrid write mode.

The operation of hybrid control logic 215 is illustrated in the flow diagrams of Figures 3A-3D. Figures 3A and 3B illustrate the logic from the zone changeover point to the first subsequent index. First, the request sector changeover bit 607 is asserted. Control bit 607 is de-asserted to stop sector pulse issuance, and is interlocked with sector changeover interlock status bit 608.

Once sector changeover bit 607 is asserted, the first step 301 is to wait for the next segment interrupt. In step 302, the current value in residual sector counter register 203 is loaded into sector/track counter 211. In step 303, the current value in residual hybrid period register 205 is loaded into hybrid period counter 212. In step 304, the sequencer waits for wrap counter 154 to equal the value in fractional position register 207. Once this occurs, hybrid control logic 215 checks to see if underflow 238 is asserted.

If underflow 238 is not asserted, step 306 determines whether the number of sectors per track counter 211 is less than hybrid suppress register 202. If hybrid suppress signal 235 is asserted, control jumps to step 309, where a narrow (non- hybrid) sector pulse is issued, and flow continues at step 314. If,

SUBSTITUTE SHEET however, hybrid suppress 235 is not asserted, then in step 307, hybrid period counter underflow 242 is checked. If underflow 242 is not asserted, hybrid period counter 212 is decremented in step 308, a narrow (non-hybrid) sector pulse is issued at step 309, and flow continues at step 314. If underflow 242 is asserted, then in step 5 310, hybrid period counter 212 is loaded from hybrid period register 206.

In step 311, sector length counter 210 is reset. In step 312, a wide index pulse spanning hybrid burst is issued. Additionally, hybrid control logic executes hybrid sequence, and increments sector length counter 210 at servo PLO clock rate. The 0 sequence states are decoded from sector length counter 210's least significant bits. If hybrid read mode is asserted, hybrid format sequencer 216 requests a hybrid ADC sequence. Otherwise, in step 314, sectors per track counter 211 is decremented. At step 315, sector length counter 210 is loaded from sector length register 201. At step 316, sectors per track counter underflow signal 238 is checked. If this bit is not asserted, control continues to step 317 where sector length counter 210 is incremented at servo PLO dock 108 rate until overflow 234 is asserted, at which point control passes to step 306.

If, however, sedors per track counter underflow is asserted either at step 305 or at step 316, control passes to step 318.

Figures 3C and 3D illustrate the control flow when hybrid control logic 215 is in its "normal" mode. That is, after an index pulse has been detected. At step 318, the control logic waits for the detection of an index signal. Index vote logic 118 asserts index vote signal 159 when this has occurred. In step 319, sectors per track counter 211 is loaded from sector count register 204. At step 320, sector period counter 212 is loaded from hybrid period register 206. At step 321, sector length counter 210 is reset. At step 322, a wide index pulse that spans the hybrid

SUBSTITUTE SHEET sector pulse is issued. Additionally, the hybrid sequence is executed, and sector length counter 210 is incremented at servo PLO dock rate. If the hybrid read mode is requested, format sequencer 216 requests an ADC sequence.

At step 324, sector length counter 210 is loaded from sector length register 201. At step 325, underflow 238 is once again examined. If it is asserted, control returns to step 318. If it is not asserted, control passes onto step 326, where sector length counter 210 is incremented at the servo PLO clock rate until overflow 234 is asserted.

Control then passes to step 327. At step 327, if sectors per track counter 211 is less than hybrid suppress register 202, then control passes to step 328 where a narrow (non-hybrid) sector pulse is issued. From step 328, control then passes to step 335. If, however, hybrid suppress signal 235 is not asserted, control passes to step 329. At step 329, hybrid period counter underflow 242 is examined. If underflow 242 is not asserted, then at step 330 hybrid period counter 212 is decremented and control passes to step 328. If underflow 242 is asserted, then control passes to step 331, where hybrid period counter 212 is loaded from hybrid period register 206. At step 332, sector length counter 210 is reset. At step 333, a wide sector pulse is issued that spans the hybrid.

Additionally, the hybrid sequence is executed and sector length counter 210 is incremented at the servo PLO clock rate. If the hybrid read mode is asserted, hybrid format sequencer 216 requests an ADC sequence. At step 335, sectors per track counter 211 is decremented and control passes to step 324. Control will continue on in this manner until request sector changeover bit is asserted.

Thus, a hybrid sector servo system is described.

Claims

1. A drcuit for providing sequencing of hybrid information in a storage system having a plurality of recording zones comprising: first storage means for storing a variable inter-sector period; first counting means coupled to first storage means for providing a sector length output signal after occurrence of said inter-sector period; second storage means for storing the number of residual sector pulses between a zone changeover point and a first index signal; second counting means coupled to said second storage means for providing a residual sectors per track output signal after occurrence of said number of residual sector pulses; third storage means for storing the number of residual non-hybrid sector pulses between a first sector pulse issued in said new zone and a first hybrid sector pulse; third counting means coupled to said third storage means for providing a residual hybrid period output signal after occurrence of said number of residual non-hybrid sector pulses.

2. The circuit of claim 1 further including: fourth storage means coupled to said second counting means, said fourth storage means for storing the number of sector pulses between successive index signals in a new zone; said second counting means providing a sectors per track output signal after occurrence of said number of sector pulses.

3. The circuit of daim 2 further including: fifth storage means coupled to said third counting means, said fifth storage means for storing the number of non-hybrid sector pulses between successive hybrid sector pulses in said new zone; said third counting means providing a hybrid period output signal after occurrence of said number of non-hybrid sector pulses.

4. The circuit of daim 3 further induding a first switching means coupled to said second counting means and said second and fourth storage means, said first switching means for switching said second counting means from said second to said fourth storage means when said first index signal is detected.

5. The rcuit of daim 4 further including a second switching means coupled to said third counting means and to said third and fifth storage means, said second switching means for switching said third counting means from said third to said fifth storage means when said first hybrid sector pulse is detected.

6. A method for providing sequencing of hybrid information in a storage system having a plurality of recording zones, said method comprising the steps of: storing a variable inter-sector period; providing a sector length output signal based on said inter-sector period; storing the number of residual sector pulses between a zone changeover point and a first index signal; providing a residual sectors per track output signal based on said number of residual sector pulses; storing the number of residual non-hybrid sector pulses between a first sector pulse issued in said new zone and a first hybrid sector pulse; providing a residual hybrid period output signal based on said number of residual non-hybrid sector pulses.

7. The method of daim 6 having the additional steps of: storing the number of sector pulses between successive index signals in a new zone; providing a sectors per track output signal based on said number of sector pulses when an index signal is detected in one of said zones.

8. The method of claim 7 having the additional steps of: storing the number of non-hybrid sector pulses between successive hybrid sector pulses in said new zone; providing a hybrid period output signal based on said number of non- hybrid sector pulses when a hybrid sector pulse is detected in one of said zones.