removed or added from/to the
FIFO does not cross the
threshold.
FIFO Operation
The FIFO threshold is set in the chip
configuration registers. All data transfers to or
from the parallel port can proceed in DMA or
Programmed I/O (non-DMA) mode as indicated
by the selected mode. The FIFO is used by
selecting the Parallel Port FIFO mode or ECP
Parallel Port Mode. (FIFO test mode will be
addressed separately.) After a reset, the FIFO
is disabled. Each data byte is transferred by a
Programmed I/O cycle or PDRQ depending on
the selection of DMA or Programmed I/O mode.
The interrupt generated is ISA friendly in that it
must pulse the interrupt line low, allowing for
interrupt sharing.
After a brief pulse low
following the interrupt event, the interrupt line is
tri-stated so that other interrupts may assert.
An interrupt is generated when:
1. For DMA transfers: When serviceIntr is 0,
dmaEn is 1 and the DMA TC is received.
The following paragraphs detail the operation of
the FIFO flow control. In these descriptions,
2. For Programmed I/O:
a.
When serviceIntr is 0, dmaEn is 0,
direction is and there are
writeIntrThreshold or more free bytes in
the FIFO. Also, an interrupt is
generated when serviceIntr is cleared
to whenever there are
<threshold> ranges from
1
to 16.
The
0
parameter FIFOTHR, which the user programs,
is one less and ranges from 0 to 15.
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 must be very
responsive to the service request. This is the
0
writeIntrThreshold or more free bytes in
the FIFO.
b.(1)
When serviceIntr is 0, dmaEn
is 0, direction is 1 and there
are readIntrThreshold or more
bytes in the FIFO. Also, an
interrupt is generated when
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.
serviceIntr is cleared to
whenever there
0
are
readIntrThreshold or more
bytes in the FIFO.
DMA TRANSFERS
DMA transfers are always to or from the
ecpDFifo, tFifo or CFifo. DMA utilizes the
standard PC DMA services. To use the DMA
transfers, the host first sets up the direction and
state as in the programmed I/O case. Then it
programs the DMA controller in the host with the
desired count and memory address. Lastly it
sets dmaEn to 1 and serviceIntr to 0. The ECP
requests DMA transfers from the host by
activating the PDRQ pin. The DMA will empty
or fill the FIFO using the appropriate direction
and mode. When the terminal count in the DMA
controller is reached, an interrupt is generated
3. When nErrIntrEn is 0 and nFault transitions
from high to low or when nErrIntrEn is set
from 1 to 0 and nFault is asserted.
4. When ackIntEn is 1 and the nAck signal
transitions from a low to a high.
101
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