DMA Mode - Transfers from the FIFO to the
Host
Programmed I/O - Transfers from the FIFO to
the Host
(Note: In the reverse mode, the peripheral may not
continue to fill the FIFO if it runs out of data to
transfer, even if the chip continues to request more
data from the peripheral.)
In the reverse direction an interrupt occurs when
serviceIntr is 0 and readIntrThreshold bytes are
available in the FIFO. If at this time the FIFO is full
it can be emptied completely in a single burst,
otherwise readIntrThreshold bytes may be read
from the FIFO in a single burst.
The ECP activates the PDRQ pin whenever there
is data in the FIFO. The DMA controller must
respond to the request by reading data from the
FIFO. The ECP will deactivate the PDRQ pin
when the FIFO becomes empty or when the TC
becomes true (qualified by nPDACK), indicating
that no more data is required. PDRQ goes
inactive after nPDACK 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
nPDACK). If PDRQ goes inactive due to the FIFO
going empty, then PDRQ is active again as soon
as there is one byte in the FIFO. If PDRQ goes
inactive due to the TC, then PDRQ is active again
when there is one byte in the FIFO, and serviceIntr
has been re-enabled. (Note: A data underrun may
occur if PDRQ is not removed in time to prevent
an unwanted cycle).
readIntrThreshold =(16-<threshold>) data bytes in
FIFO
An interrupt is generated when serviceIntr is 0 and
the number of bytes in the FIFO is greater than or
equal to (16-<threshold>). (If the threshold = 12,
then the interrupt is set whenever there are 4-16
bytes in the FIFO). The PINT pin can be used for
interrupt-driven 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. If at this time the
FIFO is full, it can be completely emptied in a
single burst, otherwise
a minimum of (16-
<threshold>) bytes may be read from the FIFO in
a single burst.
Programmed I/O Mode or Non-DMA Mode
The ECP or parallel port FIFOs may also be
operated using interrupt driven programmed I/O.
Software can determine the writeIntrThreshold,
readIntrThreshold, and FIFO depth by accessing
the FIFO in Test Mode.
Programmed I/O transfers are to the ecpDFifo at
400H and ecpAFifo at 000H or from the ecpDFifo
located at 400H, or to/from the tFifo at 400H. To
use the programmed I/O transfers, the host first
sets up the direction and state, sets dmaEn to 0
and serviceIntr to 0.
The ECP requests programmed I/O transfers from
the host by activating the PINTR pin. The
programmed I/O will empty or fill the FIFO using
the appropriate direction and mode.
Note: A threshold of 16 is equivalent to a threshold
of 15. These two cases are treated the same.
105
SMSC [ SMSC CORPORATION ]
