register, the standard and bi-directional modes are also
available. If no EPP Read, Write or Address cycle is
currently executing, then the PDx bus is in the standard
or bi-directional mode, and all output signals (STROBE,
AUTOFD, INIT) are as set by the SPP Control Port and
direction is controlled by PCD of the Control port.
changing the state of nDATASTB, nWRITE or
nADDRSTB. The write can complete once nWAIT
is determined inactive.
Write Sequence of operation
1. The host selects an EPP register, places data on the
SData bus and drives nIOW active.
2. The chip drives IOCHRDY inactive (low).
3. If WAIT is not asserted, the chip must wait until
WAIT is asserted.
4. The chip places address or data on PData bus,
clears PDIR, and asserts nWRITE.
5. Chip asserts nDATASTB or nADDRSTRB indicating
that PData bus contains valid information, and the
WRITE signal is valid.
In EPP mode, the system timing is closely coupled to the
EPP timing. For this reason, a watchdog timer is required
to prevent system lockup. The timer indicates if more
than 10usec have elapsed from the start of the EPP cycle
(nIOR or nIOW asserted) to nWAIT being deasserted
(after command). If a time-out occurs, the current EPP
cycle is aborted and the time-out condition is indicated in
Status bit 0.
During an EPP cycle, if STROBE is active, it overrides
the EPP write signal forcing the PDx bus to always be in
a write mode and the nWRITE signal to always be
asserted.
6. Peripheral deasserts nWAIT, indicating that any
setup requirements have been satisfied and the chip
may begin the termination phase of the cycle.
7. 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.
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.
b) The chip latches the data from the SData bus
for the PData bus and asserts (releases)
IOCHRDY allowing the
write cycle.
host to complete the
8. Peripheral asserts nWAIT, indicating to the host that
any hold time requirements have been satisfied and
acknowledging the termination of the cycle.
EPP 1.9 Write
9. Chip
may modify nWRITE and nPDATA
in
preparation for the next cycle.
The timing for a write operation (address or data) is
shown in timing diagram EPP 1.9 Write Data or Address
cycle. IOCHRDY is driven active low at the start of each
EPP write and is released when it has been determined
that the write cycle can complete. The write cycle can
complete under the following circumstances:
EPP 1.9 Read
The timing for a read operation (data) is shown in timing
diagram EPP Read Data cycle. IOCHRDY is driven
active low at the start of each EPP read and is released
when 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 or nADDRSTB goes active then
the write can complete when nWAIT goes inactive
high.
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.
2. If the EPP bus is ready (nWAIT is inactive high)
then the chip must wait for it to go active low before
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