Radio Shack TRS-80 Service Manual page 35

5-Meg Hard Disk Service Manual
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TRS-SO
®
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=—
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MFM write data
for the
disk The crystal
(YD
frequency
is
20,000 MHz for compatible drives
.
Clock Gating
The gating
of the
reference and MFM data into the data
separator
is
dependent upon the condition of the
Read Gate
signal (RGATE) and the spacing of the data on the serial
stream after RGATE
is
brought true* Due to the
techniques
which are employed to
separate data from clocks
,
it is
necessary
to
run the VCO at
a
rate twice the data clock
(RCLK) rate. The VCO
is
therefore set to an open-loop
frequency
of
2
times RCLK* Any variations
in
this rate due
to
variations
in
disk rotational speed must be
compensated
for
by the VCO, but instantaneous shifts in
data due
to
the
effects of adjacent bit cells on the disk and minor noise
must be ignored* Also, the
response
of the
VCO must be
adjusted to
effectively ride over missing clock bits which
occur
as
a
result
of
MFM recoding technique.
The resultant
compromise between response and reject requirements of the
VCO cause the VCO to have
a
tendency
to
become locked onto
harmonics
of the
data rate rather easily* This
is
likely
to
occur
if
the VCO
is
connected
to
a
data stream over
a
field
of
data which has data bits spaced at one and
a
half or two
times the actual RCLK time intervals.
To provide protection against this undesirable condition,
the VCO
is
always held locked onto
a
stable clock running
at
two times the RCLK frequency whenever the controller
is
not
actually reading
data.
Furthermore, great care
is
taken
to
switch
in
read data
to
the VCO error detector only when
it
is
known that the data stream frequency
is
equal to the RCLK
frequency. This can occur only when the data
is a
solid
stream of all ones or all zeros.
High Frequency Detector
The switching function
is
initiated immediately after RGATE
goes true and will only switch read data into the VCO after
16
consecutive ones
or zeros (high frequency) are detected
by a
one-shot
(Ul)
and counter
(U2)
connected directly
to
the
raw MFM data. The one-shot
is
adjusted for
a
pulse width
of
one and one-fourth times the RCLK period. This
is
250
nanoseconds, +/-
10 ns.
These adjustments of the DRUN
one-shot
(Ul)
provide tolerances of up to
one-fourth the
RCLK period
in
jitter on the
MFM data bits while still being
able to
distinguish MFM
zeros or ones from other data
patterns
.
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