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.
DMA Mode - Transfers from the FIFO to the Host
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
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 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.
DMA Mode - Transfers from the Host to the FIFO
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 true when the FIFO has <threshold> bytes
remaining in the FIFO. The FDC will also deactivate the
FDRQ pin when TC becomes true (qualified by nDACK),
indicating that no more data is required. FDRQ goes
inactive after nDACK goes active for the last byte of
a data transfer (or on the active edge of nIOW of the last
byte, if no edge is present on nDACK). A data overrun
may occur if FDRQ is not removed in time to prevent an
unwanted cycle.
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).
Data Transfer Termination
The FDC supports terminal count explicitly through the
TC pin and implicitly through the underrun/overrun and
end-of-track (EOT) functions. For full sector transfers,
the EOT parameter can define the last sector to be
transferred in a single or multi-sector transfer.
39
SMSC [ SMSC CORPORATION ]
