BIT 5 DIRECTION
If mode=000 or mode=010, this bit has no effect and the direction is always out regardless of the state of this bit. In
all other modes, Direction is valid and a logic 0 means that the printer port is in output mode (write); a logic 1 means
that the printer port is in input mode (read).
Bits 6 and 7
during a read are a low level, and cannot be written.
cFifo (Parallel Port Data FIFO)
ADDRESS OFFSET = 400h
Mode = 010
Bytes written or DMAed from the system to this FIFO are transmitted by a hardware handshake to the peripheral
using the standard parallel port protocol. Transfers to the FIFO are byte aligned. This mode is only defined for the
forward direction.
ecpDFifo (ECP Data FIFO)
ADDRESS OFFSET = 400H
Mode = 011
Bytes written or DMAed from the system to this FIFO, when the direction bit is 0, are transmitted by a hardware
handshake to the peripheral using the ECP parallel port protocol. Transfers to the FIFO are byte aligned.
Data bytes from the peripheral are read under automatic hardware handshake from ECP into this FIFO when the
direction bit is 1. Reads or DMAs from the FIFO will return bytes of ECP data to the system.
tFifo (Test FIFO Mode)
ADDRESS OFFSET = 400H
Mode = 110
Data bytes may be read, written or DMAed to or from the system to this FIFO in any direction.
Data in the tFIFO will not be transmitted to the to the parallel port lines using a hardware protocol handshake.
However, data in the tFIFO may be displayed on the parallel port data lines.
The tFIFO will not stall when overwritten or underrun. If an attempt is made to write data to a full tFIFO, the new data
is not accepted into the tFIFO. If an attempt is made to read data from an empty tFIFO, the last data byte is re-read
again. The full and empty bits must always keep track of the correct FIFO state. The tFIFO will transfer data at the
maximum ISA rate so that software may generate performance metrics.
The FIFO size and interrupt threshold can be determined by writing bytes to the FIFO and checking the full and
serviceIntr bits.
The writeIntrThreshold can be determined by starting with a full tFIFO, setting the direction bit to 0 and emptying it a
byte at a time until serviceIntr is set. This may generate a spurious interrupt, but will indicate that the threshold has
been reached.
The readIntrThreshold can be determined by setting the direction bit to 1 and filling the empty tFIFO a byte at a time
until serviceIntr is set. This may generate a spurious interrupt, but will indicate that the threshold has been reached.
Data bytes are always read from the head of tFIFO regardless of the value of the direction bit. For example if 44h,
33h, 22h is written to the FIFO, then reading the tFIFO will return 44h, 33h, 22h in the same order as was written.
SMSC DS – FDC37N769
Page 85 of 137
Rev. 02-16-07
DATASHEET
SMSC [ SMSC CORPORATION ]
