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8272
RECALIBRATE
This
command causes the read/write head
within
the
FDD
to
retract
to
the Track 0 position. The FDC clears
the
contents
of
the
PCN
counter, and checks the
status
of
the Track 0 signal from the FDD. As long as the Track
o signal
is
low, the Direction signal remains 1 (high) and
Step Pulses are issued. When the Track 0 signal goes
high, the
SE
(SEEK END) flag in Status Register 0
is
set
to
a 1 (high) and the command
is
terminated. If the Track
o signal is
still
low
after
77
Step Pulses have been·
issued, the FDC sets the
SE
(SEEK END) and
EC
(EQUIP·
MENT CHECK) flags
of
Status Register 0
to
both
1s
(highs), and terminates the command.
The
ability
to overlap RECALIBRATE Commands to
multiple
FDDs, and the loss
of
the READY signal, as
described in the
SEEK Command,
also
applies
to
the
RECALIBRATE Command.
SENSE INTERRUPT STATUS
An Interrupt signal is generated by the
FOC
for one
of
the
following
reasons:
1.
Upon entering the Result Phase of:
a.
Read Data Command
b.
Read a Track Command
c. Read
10
Command
d. Read
Deleted Oata Command
e.
Write Data Command
f.
Format a Cylinder Command
g.
Write
Deleted Data Command
h.
Scan Commands
2.
Ready Line
of
FOO
changes state
3.
End
of
Seek
or
Recalibrate Command
4.
During Execution Phase in the NON·OMA Mode
Interrupts
caused by reasons 1 and 4 above
occur
during
normal command operations and are easily
discernible
by
the
processor. However, interrupts caused by
reasons 2 and 3 above may be uniquely
identified
with
the
aid
of
the Sense Interrupt Status Command. This
command when issued resets the interrupt signal and
via
bits
5,
6,
and 7
of
Status Register 0
identifies
the
cause
of
the interrupt.
TABLE 7. SEEK, INTERRUPT CODES
SEEK END
INTERRUPT CODE
BIT
5
BIT6
BIT7
CAUSE
0 1 1
Ready
Line
changed
state, either polarity
1 0 0
Normal
Termination
of
Seek
or Recalibrate
Command
1 1
0
Abnormal
Termination of
Seek
or Recalibrate
Command
9-61
Neither the Seek
or
Recalibrate Command have a Result
Phase. Therefore, it is mandatory
to
use the Sense Inter·
rupt Status Command
after
these commands
to
effec,
tively terminate them and to provide verification
of
the
head
position
(PCN).
SPECIFY
The Specify Command sets the
initial
values
for
each
of
the three internal timers. The HUT (Head Unload Time)
defines the
time
from
the
end
of
the Execution Phase
of
one
of
the Read/Write Commands
to
the head unload
state. This
timer
is programmable from 16
to
240 ms in
increments
of
16 ms
(01
=
16
ms,
02
=
32
ms
....
OF =
240
ms). The SRT (Step Rate Time) defines the
time
in·
terval
between adjacent step pulses. This
timer
is pro·
grammable
from 1
to
16
ms
in increments
of
1 ms (F = 1
ms, E = 2 ms, 0 = 3 ms, etc.). The HLT (Head Load Time)
defines the
time
between when
the
Head
load
signal
goes high and when the Read/Write operation starts.
This
timer
is programmable from 2
to
254 ms in in·
crements
of
2 ms
(01
= 2 ms,
02
= 4 ms,
03
= 6 ms
....
FE=254
ms).
The
time
intervals mentioned above are a
direct
function
of
the
clock
(ClK
on pin
19).
Times indicated above are
for
an
8 MHz clock,
if
the
clock
was reduced
to
4 MHz
(mini·floppy application) then all
time
intervals are in·
creased by a
factor
of
2.
The choice
of
OMA
or
NON·DMA operation
is
made by
the
NO
(NON·OMA) bit. When
this
bit
is high
(NO
=
1)
the
NON·OMA mode is selected, and when
NO
= 0 the DMA
mode is
selected.
SENSE DRIVE STATUS
This command may be used by the processor whenever
it wishes
to
obtain the
status
of
the FDOs. Status
Register 3
contains
the Drive Status information.
INVALID
If
an
invalid command is sent
to
the
FOC
(a
command
not defined above), then the FDC
will
terminate the com·
mand. No interrupt is generated by the 8272 during
this
condition.
Bit
6 and
bit
7 (010 and RQM) in the Main
Status Register are both high ("1")
indicating
to
the
processor that the 8272 is in the
Result Phase and the
contents
of
Status Register 0
(STO)
must
be read. When
the processor reads Status Register 0 it will find a 80H
indicating
an
invalid command was received.
A
Sense Interrupt Status Command
must
be sent
after
a
Seek
or
Recalibrate interrupt, otherwise the FDC
will
consider the next command
to
be an Invalid Command.
In
some applications the user may wish
to
use
this
com·
mand as a No·Op command,
to
place the FOC
in
a stand·
by
or
no operation state.