DP8459 datasheet(25/35 Pages) NSC

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4.3 WINDOW STROBE

The DP8459 incorporates a window strobe function capable of

shifting the synchronization window either early or late with

respect to its nominal position in small, speciﬁed steps. The

strobe step t

S is deﬁned as the controlled time displacement of

the DP8459 synchronization window from its nominal (strobe

centered) position and is typically

S = M x [1.8% x τVCO]

where M is the value of the strobe control word (−15 through

+15; see

Figure 4 ) set by the ﬁrst 5 bits within the Control

the ﬁve strobe control bits where bits 0 through 3 are the LSB

through MSB and bit 4 is the sign bit.)

The changing of the strobe value t

S is not an instantaneous

event following the changing of the control word in the Control

any change in strobe setting is a function of the timing

elements connected to the TIMING EXTRACTOR FILTER pin

and the data rate at which the device is being operated. A ﬁnite

settling time must be allowed for the delay circuitry to respond

following the loading and latching of the new control word

(latching occurs and strobe changes begin at de-assertion of

CONTROL REGISTER ENABLE, i.e., at transition to logical

ONE). It is recommended that any changes to the strobe

setting be done with READ GATE deasserted and with a

sufficient allowance for settling time prior to the initiation of a

subsequent read operation. Approximate settling times are

given in

Figure 10 for various TEF component values at

speciﬁc data rates. (Please refer to AN-578 Window Strobe

Function.)

4.3.1 MARGIN TESTING

The read channel window margin of a disk/tape memory

system is the portion of the synchronization window remaining

after the subtraction of all possible sources of degradation

such as media bit shift, head-ampliﬁer anomalies, pulse

detector anomalies, cable-induced skew, synchronizer losses,

and extraneous noise. The remaining margin must be

sufficient to allow the system to perform with an acceptable

media error rate under all operating conditions. Acceptable

media error rates will vary between systems depending on

ECC codes, data redundancy, and other factors. The

measured value of the synchronization window margin is often

used as a performance criteria for HDA (head-disk assembly)

and read channel qualiﬁcation, and for gauging the probability

of encountering data errors on the media.

The DP8459 strobe function can be readily used to measure

the window margin within a drive system. Margin tests have

been most frequently employed only during outgoing factory

tests of storage media systems with specialized and costly test

apparatus employed for the purpose; however, the DP8459

allows media/system qualiﬁcation at any time in the factory or

the ﬁeld during the system’s operational life, given the

incorporation of an appropriate margin test algorithm within the

disk system controller. The algorithm may be conﬁgured ﬁrst to

record

the

most

bit-interactive

(shift-producing)

pattern

possible with the recording code being employed (eg., a

repeating hex 6D B6 pattern in MFM) in an area of the media

where recording density is its highest (inner-most track in

constant-angular velocity or constant data rate disk systems),

and secondly to read the track repeatedly while incremently

advancing the degree of window “strobe” (controlled shift) ﬁrst

in the early direction until the data error rate crosses a

pre-determined threshold and then in the late direction until the

same threshold is again crossed. The smaller of the two

DP8459 window strobe measurements (either the early or the

late value) determined at the error rate threshold crossing

points is then equal to the read channel window margin.

http:\\www.national.com

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

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