22
DATA RATE SELECT REGISTER (DSR)
Address 3F4 WRITE ONLY
This register is write only. It is used to program
the data rate, amount of write precompensation,
power down status, and software reset.
The
data
rate
is
programmed
using
the
Configuration Control Register (CCR) not the
DSR,
for
PC/AT
and
PS/2
Model
30
and
Microchannel
applications.
Other
applications can set the data rate in the DSR.
The data rate of the floppy controller is the most
recent write of either the DSR or CCR. The DSR
is unaffected by a software reset. A hardware
reset
will
set
the
DSR
to
02H,
which
corresponds to the default precompensation
setting and 250 Kbps.
BIT 0 and 1 DATA RATE SELECT
These bits control the data rate of the floppy
controller.
See Table 11 for the settings
corresponding to the individual data rates. The
data rate select bits are unaffected by a
software reset, and are set to 250 Kbps after a
hardware reset.
BIT
2
through
4
PRECOMPENSATION
SELECT
These three bits select the value of write
precompensation that will be applied to the
WDATA output signal.
Table 10 shows the
precompensation values for the combination of
these bits settings.
Track 0 is the default
starting track number to start precompensation.
this starting track number can be changed by
the configure command.
BIT 5 UNDEFINED
Should be written as a logic "0".
BIT 6 LOW POWER
A logic "1" written to this bit will put the floppy
controller into manual low power mode. The
floppy controller
clock
and data mode after a
software reset or access to the Data Register or
Main Status Register.
BIT 7 SOFTWARE RESET
This active high bit has the same function as the
DOR RESET (DOR bit 2) except that this bit is
self clearing.
Note: The DSR is Shadowed in the Floppy Data
Rate Select Shadow Register, LD8:CRC2[7:0].
separator circuits will be turned off.
The
controller will come out of manual low power
Table 10 - Precompensation Delays
*2Mbps data rate is only available if VCC = 5V.
7
6
5
4
3
2
1
0
S/W
RESET
POWER
DOWN
0
PRE-
COMP2
PRE-
COMP1
PRE-
COMP0
DRATE
SEL1
DRATE
SEL0
RESET
COND.
0
1
0
PRECOMP
432
PRECOMPENSATION
DELAY (nsec)
<2Mbps
2Mbps*
111
001
010
011
100
101
110
000
0.00
41.67
83.34
125.00
166.67
208.33
250.00
Default
0
20.8
41.7
62.5
83.3
104.2
125
Default
Default: See Table 12