until the last byte is transferred out of the FIFO.
The FDC will deactivate the FINT pin and RQM
bit when the FIFO becomes empty.
Execution Phase
All data transfers to or from the FDC occur
during the execution phase, which can proceed
in DMA or non-DMA mode as indicated in the
Specify command.
Non-DMA Mode - Transfers from the Host to the
FIFO
The FINT pin and RQM bit in the Main Status
Register are activated upon entering the
execution phase of data transfer commands.
The host must respond to the request by writing
data into the FIFO. The FINT pin and RQM bit
remain true until the FIFO becomes full. They
are set true again when the FIFO has
<threshold> bytes remaining in the FIFO. The
FINT pin will also be deactivated if TC and
nDACK both go inactive. The FDC enters the
result phase after the last byte is taken by the
FDC from the FIFO (i.e. FIFO empty condition).
After a reset, the FIFO is disabled. Each data
byte is transferred by an FINT or FDRQ
depending on the DMA mode. The Configure
command can enable the FIFO and set the
FIFO threshold value.
The following paragraphs detail the operation of
the FIFO flow control. In these descriptions,
<threshold> is defined as the number of bytes
available to the FDC when service is requested
from the host and ranges from 1 to 16. The
parameter FIFOTHR, which the user programs,
is one less and ranges from 0 to 15.
DMA Mode - Transfers from the FIFO to the
Host
A low threshold value (i.e. 2) results in longer
periods of time between service requests, but
requires faster servicing of the request for both
read and write cases. The host reads (writes)
from (to) the FIFO until empty (full), then the
transfer request goes inactive. The host must
be very responsive to the service request. This
is the desired case for use with a "fast" system.
The FDC activates the DDRQ pin when the
FIFO contains (16 - <threshold>) bytes, or the
last byte of a full sector transfer has been
placed in the FIFO. The DMA controller must
respond to the request by reading data from the
FIFO. The FDC will deactivate the DDRQ pin
when the FIFO becomes empty. FDRQ goes
inactive after nDACK goes active for the last
byte of a data transfer (or on the active edge of
nIOR, on the last byte, if no edge is present on
nDACK). A data underrun may occur if FDRQ
is not removed in time to prevent an unwanted
cycle.
A high value of threshold (i.e. 12) is used with a
"sluggish" system by affording a long latency
period after a service request, but results in
more frequent service requests.
Non-DMA Mode - Transfers from the FIFO to
the Host
DMA Mode - Transfers from the Host to the
FIFO.
The FINT pin and RQM bits in the Main Status
Register are activated when the FIFO contains
(16-<threshold>) bytes or the last bytes of a full
sector have been placed in the FIFO. The FINT
pin can be used for interrupt-driven systems,
and RQM can be used for polled systems. The
host must respond to the request by reading
data from the FIFO. This process is repeated
The FDC activates the FDRQ pin when entering
the execution phase of the data transfer
commands. The DMA controller must respond
by activating the nDACK and nIOW pins and
placing data in the FIFO. FDRQ remains active
until the FIFO becomes full. FDRQ is again set
34
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