Abstract:
A method and apparatus are provided for implementing improved phase alignment in a direct access storage device (DASD). A reference clock input is received for providing a system clock. Locking to a synchronization field of a readback signal is performed by adjusting the phase of the system clock. A timing mark is detected and then the adjusted phase of the system clock is held. Responsive to the detected timing mark, a reference delay of a predefined number and fraction of system clock periods is identified. At an end of the reference delay, a write circuit accepts data and generates write signals for a write operation. The phase of the system clock is adjusted corresponding to a predefined fractional delay and is used to run a programmable counter that counts the predefined number of system clock periods corresponding to the reference delay.

Description:
CO-PENDING RELATED APPLICATIONS 
   This application is related to U.S. Ser. No. 10/184,343, filed on even date herewith entitled “Improved Self-Servowriting Multislot Timing Pattern” (IBM YOR920000374US1). 

   FIELD OF THE INVENTION 
   The present invention relates to a method and apparatus for implementing improved phase alignment in a direct access storage device (DASD). 
   DESCRIPTION OF THE RELATED ART 
   Direct access storage devices (DASDs) often incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data is recorded in concentric, radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads driven in a path toward and away from the disk axis of rotation write data to the disks and read data from the disks. Typically servo information is provided on one or more disk surfaces for reading by the transducer heads for accurately and reliably positioning transducer heads on the disk surfaces to read and write data. 
   Servo information is used to identify the start of different information fields around the track circumference to read and write data. The accuracy and reliability of head position measurements is very important, since poor tolerance in these measurements will degrade the performance and storage capacity of the DASD. Detection of timing marks is hindered by signal noise and track-to-track timing phase alignment offsets. 
   In the absence of phase alignment issues, an increase in servowriter frequency would simultaneously improve format efficiency and signal processing performance. A fundamental limiting factor in servo format efficiency is the phase alignment system at the servowriter. 
   A need exists for an improved method and apparatus for implementing improved phase alignment in a direct access storage device (DASD). 
   SUMMARY OF THE INVENTION 
   A principal object of the present invention is to provide a method and apparatus for implementing improved phase alignment in a direct access storage device (DASD). Other important objects of the present invention are to provide such method and apparatus for implementing improved phase alignment substantially without negative effect and that overcome many of the disadvantages of prior art arrangements. 
   In brief, a method and apparatus are provided for implementing improved phase alignment in a direct access storage device (DASD). A reference clock input is received for providing a system clock. Locking to a synchronization field of a readback signal is performed by adjusting the phase of the system clock. A timing mark is detected and then the adjusted phase of the system clock is held. Responsive to the detected timing mark, a reference delay of a predefined number and fraction of system clock periods is identified. At an end of the reference delay, a write circuit accepts data and generates write signals for a write operation. 
   In accordance with features of the invention, the phase of the system clock is adjusted corresponding to a predefined fractional delay and runs a programmable counter that counts the predefined number of system clock periods corresponding to the reference delay. Modifying the system clock phase corresponding to the predefined fractional delay enables a higher granularity in delay than one clock period. Measurement of the time between detected consecutive timing marks is provided by a counter that starts counting system clock periods when a timing mark is detected and continues until a next timing mark is detected. The number of system clock periods since detecting a previous timing mark and the current adjusted phase of the system clock are stored. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention together with the above and other objects and advantages may best be understood from the following detailed description of the preferred embodiments of the invention illustrated in the drawings, wherein: 
       FIG. 1  is a block diagram representation illustrating a data channel including apparatus for implementing methods for improved phase alignment in data channels in accordance with the preferred embodiment; 
       FIG. 2  is a flow chart illustrating exemplary functional logic functions for implementing improved phase alignment in data channels in accordance with the preferred embodiment; 
       FIGS. 3 and 4  are graphs illustrating operation of the improved phase alignment apparatus of  FIG. 1  in accordance with the preferred embodiment; and 
       FIG. 5  is a graph illustrating timing mark repeatability versus input noise level in data channel of  FIG. 1  in accordance with the preferred embodiment; 
       FIG. 6  is a graph illustrating open loop drift versus time with no noise and noisy operation of a conventional data channel; 
       FIG. 7  is a graph illustrating absolute write data variability versus relative delay in data channel of  FIG. 1  in accordance with the preferred embodiment; 
       FIG. 8  is a graph illustrating two different phase settings versus relative delay with a minimum change in phase set in data channel of  FIG. 1  in accordance with the preferred embodiment; and 
       FIG. 9  is a graph illustrating read-to-read measurement variability in data channel of  FIG. 1  in accordance with the preferred embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Having reference now to the drawings, in  FIG. 1 , there is shown a data channel generally designated by the reference character  100  including apparatus for implementing methods for improved phase alignment of the preferred embodiment. As shown in  FIG. 1 , data channel  100  includes a variable gain amplifier (VGA)  102  receiving a differential read signal input. A continuous time filter (CTF)  104  receives the output of VGA  102  and provides a filtered input to an analog-to-digital converter (ADC)  106 . ADC  106  converts the filtered read signal to a digital form. The digital read signal is equalized using a finite impulse response (FIR) filter  108  coupled to the output of ADC  106 . The FIR filter  108  applies digital sample values to a timing mark detect logic  110  and a timing loop logic  112  of the preferred embodiment. 
   In accordance with features of the invention, data channel  100  includes a new clocking architecture including a precise reference clock. As shown in  FIG. 1 , the reference clock is applied to a phase mixer (fractional delay) function  114  of the preferred embodiment and the timing loop logic  112  that is coupled to the phase mixer fractional delay function  114 . The phase mixer fractional delay function  114  and the timing mark detect logic  110  are coupled to an integer delay function  116  of the preferred embodiment. A write logic function  118  is coupled to the integer delay function  116 . A write strobe and parallel write data are applied to inputs of the write logic function  118 . The write logic function  118  applies an output write signal to a write driver  120 . 
   In accordance with features of the invention, the new clocking architecture of data channel  100  accepts the reference clock input as a timing reference. Different phases of the reference clock are used internally in data channel  100  to achieve synchronization or sync up, read the timing mark, delay a fractional bit, count out a delay, and write data. Different phases of the reference clock are used to sync up with timing loop logic  112 , read the timing mark with timing mark detect logic  110 , delay a fractional bit with the phase mixer fractional delay function  114 , count out a delay with integer delay function  116 , and write data with write logic  118 . 
   In accordance with features of the invention, data channel  100  fully supports multisync servo options with timing mark detection, variable delay, writing circuits, and time measurements. Additionally automark generation of the programmable delay values, or any component thereof, which are constrained in a given implementation and which can simplify or speed up processor intervention can be simply implemented. Another feature of the present invention is the ability to measure the time interval between timing marks. This may be used as velocity noise feedback, the sequence of which may determine subsequent delay values in the write operation. The timing mark detect logic  110  performs timing mark detection and time measurement by counting and storing a number of clocks since a previous timing mark detect, using one or more counters whose reference also begins at when a timing mark is detected. The counter continues its operation until another sync timing mark is achieved. The raw count value from mark to mark represents a coarse time value in clock units. Fractional clock accuracy is achieved by comparing the acquisition phase Φ M  and Φ M+1  of the two address or timing marks. A resulting time interval in clock units is defined by:
 
Time interval=raw count+((Φ M+1 −Φ M )/phase units per clock unit)
 
   Having reference now to  FIGS. 2 ,  3  and  4 , improved phase alignment operation of the data channel  100  is illustrated. In  FIG. 2  there are shown exemplary functional logic blocks for implementing improved phase alignment for servo detection and writing data in data channel  100  in accordance with the preferred embodiment starting at a block  200 . The timing loop logic  112  locks to a synchronization field by adjusting the phase of the system clock as indicated in a block  202 . Data channel  100  accepts the input reference clock, and the timing loop logic  112  controls the phase of the system clock using the phase mixer  114  at block  202 . 
   In  FIG. 3 , the magnetic waveform shown at a line labeled READBACK includes a pattern designed to give timing information to data channel  100  known as the synchronization field. A pair of sync field/timing marks  302  are shown in the READBACK line. A line labeled MIXER PHASE represents the operation of timing loop logic  112 . A wavy portion of the MIXER PHASE line under the sync field/timing mark  302  represents the timing loop logic  112  locking to the synchronization field  302  by adjusting the phase of the system clock. At a given signal of sync field/timing mark  302 , the timing loop logic  112  aligns the phase of the reference clock to the magnetic waveform by standard control techniques. 
   After synchronization is achieved, timing mark detect logic  110  examines the waveform to determine an absolute timing mark. The timing mark is detected by the timing mark detect logic  110  and then the phase of the reference clock provided by the timing loop logic  112  is held constant as indicated in a block  204 . The output of the phase mixer  114  provides an accurate timepiece as good as the input reference clock. Next, a delay reference begins when the timing mark is detected, and a programmable counter provides a delay of an integral number of clock periods. The current phase of the system clock and the number of clocks since the previous timing mart detect are stored by the timing mark detect logic  110  as indicated in a block  206 . 
   A variable delay indicated in the READBACK line in  FIG. 3  begins when the timing mark is detected, and a programmable counter provided by the integer delay function  116  provides a delay of an integral number of clock periods. To simply achieve a much higher granularity in delay than one clock unit, the system clock, that is the phase mixer output which is running the programmable counter of the integer delay function  116  is modified by a programmable fraction of a clock period with the phase mixer block  114 . 
   The phase of the system clock is adjusted corresponding to the desired fractional delay as indicated in a block  208 . The slew rate of the phase change must be limited so the clocked digital circuits continue to operate properly. By combining these methods, an accurate delay, programmable to fractional clock units, is achieved. System clocks corresponding to the desired integer delay are counted as indicated in a block  210 . At the end of this delay, the write logic circuit  118  accepts data and generates write signals applied to the write driver  120  as indicated in a block  212 . 
   Checking for more writes is performed as indicated in a decision block  214 . When more writes are identified, then the phase of the system clock is adjusted corresponding to the desired fractional delay at block  208 , the system clocks corresponding to the desired integer delay are counted at block  210 , and a next write is performed at block  212 . When no more writes are identified, then the operations return to block  202  where the timing loop logic  112  locks to a synchronization field by adjusting the phase of the system clock and continue. 
   It is to be understood by those well versed in the art, that any combination or repetition of these operations, such as multiple writes at multiple delay times after the timing mark, may be implemented and are covered by the present invention. One example variation known as write while read multislot is shown in FIG.  4 . 
   Referring to  FIG. 4 , the graph illustrates the write while read multislot operation of the improved phase alignment apparatus of data channel  100  in accordance with the preferred embodiment. In the upper portion of  FIG. 4 , two sectors, sector N and sector N+1, are illustrated. A write operation is illustrated at the top of the upper portion and a read operation is illustrated at the bottom of the upper portion. Each of the sector N and sector N+1 includes a trigger pattern  402  and a servo pattern  404 . Trigger and servo patterns  402  and  404  are shown as a shaded area and are indicated that the head is writing. Previously written patterns are indicated by vertical cross-hatched areas and patterns that the head is reading are indicated by slanted cross-hatched areas. 
   A corresponding phase operation of the improved phase alignment apparatus of data channel  100  is shown at a lower portion of FIG.  4 . An initial phase from a previous trigger is indicated as Φ N−1 , followed by a measure phase Φ M  of the first sector N, then the phase returns to the initial value. The phase is set for the servo pattern  404  write of the first sector N as indicated by phase Φ N . Then the phase Φ N  as the initial phase from previous trigger precedes a measure phase Φ M+1  of the next sector N+1. Then the phase is set for the servo pattern  404  write of the sector N+1 as indicated by phase Φ N+1 . 
     FIG. 5  illustrates timing mark repeatability versus input noise level in a data channel  100 . At noise levels which support reasonable error rates, for example, about 25 dB or higher attenuation, the variation in timing mark accuracy is less than 50 pS 1 sigma, or so small as to be difficult to measure. 
     FIG. 6  illustrates open loop drift versus time with no noise and noisy operation of a conventional data channel. The timing jitter or increase in timing uncertainty results as the trigger delay is extended, without the precise reference clock of the data channel  100 . Important delays are on the order of tens of microseconds. The timing jitter of a conventional data channel has too much variability. 
   Having reference now to  FIGS. 7 ,  8  and  9 , improved phase alignment operation of the data channel  100  is illustrated. The new fundamental operation of data channel  100  is to read a data timing mark using the new analog capability, delay an integral and fractional delay, and begin a write operation. To measure the success of data channel  100 , an arbitrary waveform generator was used to simulate head signals with the added benefit of very accurate and independent timing marks. A particular data pattern was arranged to appear as write data, and focused on one transition of the write data. The variability in time of this transition with respect to the known timing mark is a figure of merit. 
     FIG. 7  illustrating absolute write data variability versus relative delay in data channel  100  in accordance with the preferred embodiment. After a delay of 20 microseconds following a detected timing mark, the variability in a particular transition on the channel&#39;s write data lines is shown in FIG.  7 . This variability includes variability due to sync mark detection, delay, and write circuits, as well as measurement errors. This example data was taken at a 2X reference clock corresponding to a 4.2 nS channel bit time, that is, for example, about 475 MHz as input as the 2X clock. 
     FIG. 8  illustrates two different phase settings versus relative delay with a minimum change in phase set in data channel  100  in accordance with the preferred embodiment. As shown, a first phase is set to 40 units and a second phase is set to 39 units for a 1 LSB change in the delay value. The average delays show monotonic and expected behavior with different phase settings. 
     FIG. 9  illustrates read-to-read measurement variability in data channel  100  in accordance with the preferred embodiment. The read-to-read measurement variability in data channel  100  shown in  FIG. 9  illustrates the channel&#39;s performance in measuring the time interval between consecutive timing mark detections. To evaluate this measurement, an arbitrary waveform generator was used to simulate head signals. The read data consisted of two data timing marks space apart by 30 microseconds. After each read-to-read operation, information from the coarse measurement and fractional measurement were used to calculate the read-to-read time interval. The distribution of measured times in shown in FIG.  9 . This example data was taken at a 2X reference clock corresponding to a 4.2 nS channel bit time, that is, for example, about 475 MHz as input as the 2X clock. The read-to-read variation is about 60 pS 1 sigma. 
   While the present invention has been described with reference to the details of the embodiments of the invention shown in the drawing, these details are not intended to limit the scope of the invention as claimed in the appended claims.