DP8459 datasheet(26/35 Pages) NSC

26 / 35 page

FIGURE 24. DP8459 Static Window Truncation Test Flow Chart

4.3.2 ERROR-BOUND SECTOR/TRACK DATA RECOVERY

A standard technique exists for attempting to recover illegible

data from a sector or track within a disk system which involves

the re-reading of the bad data while shifting the data

synchronizer window a small amount early/late with respect to

the nominal position. A typical early/late strobe value for data

retrieval is in the range from approximately 2% to 3% of the

total window width. The strobe step size produced by the

DP8459 window control circuitry easily allows for this type of

data recovery procedure, and is in fact small enough to

feasibly permit more than one degree of window movement

within the data recovery algorithm.

4.3.3 AUTO WINDOW ALIGNMENT (DE-SKEW ROUTINE)

It is possible to conﬁgure an intelligent drive system to employ

the DP8459 strobe feature in a window auto-calibration

(de-skew) routine implemented to center the detection window

about the mean position of the bit distribution curve. The

de-skew routine would maximize the read channel window

margin and correspondingly minimize the bit error rate (BER).

The auto-calibration routine would be conﬁgured as an

extension of the window margin routine (Section 4.3.1), where

the early and late strobe values determined at the error rate

threshold crossing points would be numerically combined to

determine the window center skew. For example, if at 10 Mb/s

the strobe-until-error value in the “early” direction were found

to be M = −8 and the “late” value M = 4, window skew would

be determined as follows:

skew = 1.8 x τVCO x[Mearly +Mlate]/2

= 0.9 ns x [−8 + 4]/2

= − 1.8 ns

The window has an apparent shift of 1.8 ns in the late

direction. The strobe setting in the DP8459 would then be set

to compensate for the skew, centering the synchronization

window and maximizing the available read channel window

margin. In this case, the strobe setting would be M = −2. This

routine could be executed at system power-up and perhaps on

a regular, speciﬁed time schedule during system operation to

maintain

ﬁne-tuning

the

read

channel

timing

characteristics under varying operating conditions (conceiv-

ably eliminating the need for an error-strobe routine).

TL/F/9322-25

PrintDate=1996/07/31 PrintTime=11:06:12 ds009322 Rev. No. 1

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