FDC37C669 datasheet(39/164 Pages) SMSC

39 / 164 page

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.

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

(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.

Non-DMA Mode - Transfers from the Host to the

FIFO

The FINT pin and RQM bit in the Main Status

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).

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.

DMA Mode - Transfers from the Host to the

FIFO

The FDC activates the FDRQ pin when entering

the

execution

phase

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.

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.

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