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The timing for a write operation (address or data), with FIFO, is shown in timing diagram EPP 1a and
EPP 1b. IOCHRDY is only driven active low when the system tries to write to one of the EPP registers
when the FIFO is full.
Software Constraints
Before an EPP cycle is executed, the software must ensure that the control register bit PCD is a logic
"0" (i.e., a 04H or 05H should be written to the Control port). If the user leaves PCD as a logic "1", and
attempts to perform an EPP write, the chip is unable to perform the write (because PCD is a logic "1")
and will appear to perform an EPP read on the parallel bus, no error is indicated.
EPP 1.9 Write
The timing for a write operation (address or data) is shown in timing diagram EPP Write Data or
Address cycle. The chip inserts wait states into the LPC I/O write cycle until it has been determined
that the write cycle can complete. The write cycle can complete under the following circumstances:
1. If the EPP bus is not ready (nWAIT is active low) when nDATASTB or nADDRSTB goes active
then the write can complete when nWAIT goes inactive high.
2. If the EPP bus is ready (nWAIT is inactive high) then the chip must wait for it to go active low
before changing the state of nDATASTB, nWRITE or nADDRSTB. The write can complete once
nWAIT is determined inactive.
Write Sequence of operation
1.
The host initiates an I/O write cycle to the selected EPP register.
2.
If WAIT is not asserted, the chip must wait until WAIT is asserted.
3.
The chip places address or data on PData bus, clears PDIR, and asserts nWRITE.
4.
Chip asserts nDATASTB or nADDRSTRB indicating that PData bus contains valid information,
and the WRITE signal is valid.
5.
Peripheral deasserts nWAIT, indicating that any setup requirements have been satisfied and the
chip may begin the termination phase of the cycle.
6.
a)
The chip deasserts nDATASTB or nADDRSTRB, this marks the beginning of the termination
phase. If it has not already done so, the peripheral should latch the information byte now.
b)
The chip latches the data from the internal data bus for the PData bus and drives the sync
that indicates that no more wait states are required followed by the TAR to complete the write
cycle.
7.
Peripheral asserts nWAIT, indicating to the host that any hold time requirements have been
satisfied and acknowledging the termination of the cycle.
8.
Chip may modify nWRITE and nPDATA in preparation for the next cycle.
EPP 1.9 Read
The timing for a read operation (data) is shown in timing diagram EPP Read Data cycle. The chip
inserts wait states into the LPC I/O read cycle until it has been determined that the read cycle can
complete. The read cycle can complete under the following circumstances:
1
If the EPP bus is not ready (nWAIT is active low) when nDATASTB goes active then the read can
complete when nWAIT goes inactive high.
If the EPP bus is ready (nWAIT is inactive high) then the chip must wait for it to go active low
before changing the state of WRITE or before nDATASTB goes active. The read can complete
once nWAIT is determined inactive.
2.