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(12) Software Operation (ECP)
Before ECP operation can begin, it is first necessary for the Host to switch the mode to 000 to
negotiate with the parallel port. Host determines whether peripheral supports ECP protocol during
the process.
After the negotiation is completed, the mode is set to 011 (ECP). To enable the drivers, direction
must be set to "0". Both strobe and autoFd are set to "0", causing the nStrobe and nAutoFd signals
to be deasserted.
All FIFO data transfers are Pword wide and Pword aligned. Permitted only in the forward
direction, address/RLE transfers are byte-wide. ECP address/RLE bytes may be sent automatically
by writing the ecpAFifo. Similauly, data Pwords may be sent automatically via ecpDFifo.
To change directions, Host switches mode to 001. It then negotiates either the forward or reverse
channel, sets direction to one or zero, and finally switches mode to 001. If the direction is set to
"1", the hardware performs a handshaking form each ECP data byte READ, and tries to fill the
FIFO. At this time, Pwords may be read from the expDFifo while it retains data. It is also possible
for the hardware to performs ECP transfers by handshaking with individual bytes under program
control in mode=001 or 000, even though this is a comparatively time-consuming approach.
(13) Interrupts
When any of the following states are reached, it is necessary to generate an interrupt.
1. serviceIntr = 0, direction = 0, and the number of Pwords in FIFO is greater than or equal to
writeIntrThreshold.
2. serviceIntr = 0, direction = 1, and the number full Pwords in the FIFO is greater than or equal
to READIntrThreshold.
3. nErrIntrEn = 0 and nFault goes from high to low or when nErrIntrEnor is set from "1" to "0"
and nFault is asserted.
4. ackIntEn = 1. In current implementations using existing parallel ports, the generated interrupt
may be either edge or level type, making it "ISA-friendly".
Rev. 3.0, 03/01, page 179 of 390
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