DRQ can be hi Z), and TC and DENSEL become
active high signals.
RESET
There are three sources of system reset on the
FDC: the RESET pin of the FDC, a reset
generated via a bit in the DOR, and a reset
generated via a bit in the DSR. At power on, a
Power On Reset initializes the FDC. All resets
take the FDC out of the power down state.
PS/2 mode - (IDENT low, MFM high)
This mode supports the PS/2 models 50/60/80
configuration and register set. The DMA bit of the
DOR becomes a "don't care", (FINTR and DRQ
are always valid), TC and DENSEL become active
low.
All operations are terminated upon a RESET, and
the FDC enters an idle state. A reset while a disk
write is in progress will corrupt the data and CRC.
Model 30 mode - (IDENT low, MFM low)
This mode supports PS/2 Model 30 configuration
and register set. The DMA enable bit of the DOR
becomes valid (FINTR and DRQ can be hi Z), TC
is active high and DENSEL is active low.
On exiting the reset state, various internal
registers are cleared, including the Configure
command information, and the FDC waits for a
new command. Drive polling will start unless
disabled by a new Configure command.
DMA TRANSFERS
DMA transfers are enabled with the Specify
command and are initiated by the FDC by
activating the FDRQ pin during a data transfer
command. The FIFO is enabled directly by
asserting nDACK and addresses need not be
valid.
RESET Pin (Hardware Reset)
The RESET pin is a global reset and clears all
registers except those programmed by the Specify
command. The DOR reset bit is enabled and
must be cleared by the host to exit the reset state.
Note that if the DMA controller (i.e. 8237A) is
programmed to function in verify mode, a pseudo
read is performed by the FDC based only on
nDACK. This mode is only available when the
FDC has been configured into byte mode (FIFO
disabled) and is programmed to do a read. With
the FIFO enabled, the FDC can perform the above
operation by using the new Verify command; no
DMA operation is needed.
DOR Reset vs. DSR Reset (Software Reset)
These two resets are functionally the same. Both
will reset the FDC core, which affects drive status
information and the FIFO circuits. The DSR reset
clears itself automatically while the DOR reset
requires the host to manually clear it. DOR reset
has precedence over the DSR reset. The DOR
reset is set automatically upon a pin reset. The
user must manually clear this reset bit in the DOR
to exit the reset state.
Two DMA transfer modes are supported for the
FDC: Single Transfer and Burst Transfer. In the
case of the single transfer, the DMA Req goes
active at the start of the DMA cycle, and the DMA
Req is deasserted after the nDACK. In the case of
the burst transfer, the Req is held active until the
last transfer (independent of nDACK). See timing
diagrams for more information.
MODES OF OPERATION
The FDC has three modes of operation, PC/AT
mode, PS/2 mode and Model 30 mode. These
are determined by the state of the IDENT and
MFM bits 3 and 2 respectively of LD8CRF0.
Burst mode is enabled via Bit[1] of CRF0 in
Logical Device 0. Setting Bit[1]=0 enables burst
mode; the default is Bit[1]=1, for non-burst mode.
PC/AT mode - (IDENT high, MFM a "don't care")
The PC/AT register set is enabled, the DMA
enable bit of the DOR becomes valid (FINTR and
CONTROLLER PHASES
34
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