The following paragraphs detail the operation of the FIFO flow control. In these descriptions, <threshold> ranges
from 1 to 16. The 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
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.
DMA TRANSFERS
Note:
PDRQ - Currently selected Parallel Port DRQ channel
nPDACK - Currently selected Parallel Port DACK channel
PINTR - Currently selected Parallel Port IRQ channel
Typical DMA Mode 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 and serviceIntr is asserted, disabling DMA. In order to prevent possible blocking of refresh
requests dReq shall not be asserted for more than 32 DMA cycles in a row. The FIFO is enabled directly by asserting
nPDACK and addresses need not be valid. PINTR is generated when a TC is received. PDRQ must not be
asserted for more than 32 DMA cycles in a row. After the 32nd cycle, PDRQ must be kept unasserted until nPDACK
is deasserted for a minimum of 350nsec. (Note: The only way to properly terminate DMA transfers is with a TC).
DMA may be disabled in the middle of a transfer by first disabling the host DMA controller. Then setting serviceIntr
to 1, followed by setting dmaEn to 0, and waiting for the FIFO to become empty or full.
Restarting the DMA is accomplished by enabling DMA in the host, setting dmaEn to 1, followed by setting
serviceIntr to 0.
DMA Mode - 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).
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).
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.
SMSC DS – FDC37N769
Page 90 of 137
Rev. 02-16-07
DATASHEET
SMSC [ SMSC CORPORATION ]
