asserts (releases) IOCHRDY allowing
the host to complete the write cycle.
8. Peripheral asserts nWAIT, indicating to the
host that any hold time requirements have
been satisfied and acknowledging the
termination of the cycle.
7. Peripheral deasserts nWAIT, indicating that
PData is valid and the chip may begin the
termination phase of the cycle.
8. a) The chip latches the data from the
PData bus for the SData bus and
deasserts nDATASTB or nADDRSTRB.
This marks the beginning of the
termination phase.
9. Chip may modify nWRITE and nPDATA in
preparation for the next cycle.
b) The chip drives the valid data onto the
SData bus and asserts (releases)
IOCHRDY allowing the host to
complete the read cycle.
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:
9. Peripheral tri-states the PData bus and
asserts nWAIT, indicating to the host that
the PData bus is tri-stated.
10. Chip may modify nWRITE, PDIR and
nPDATA in preparation for the next cycle.
EPP 1.7 OPERATION
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.
When the EPP 1.7 mode is selected in the
configuration 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.
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
WRITE or before nDATASTB goes active.
The read can complete once nWAIT is
determined inactive.
Read Sequence of Operation
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 the end of the cycle
1. The host selects an EPP register and drives
nIOR active.
2. The chip drives IOCHRDY inactive (low).
3. If WAIT is not asserted, the chip must wait
until WAIT is asserted.
nIOR or nIOW deasserted).
If a time-out
4. The chip tri-states the PData bus and
deasserts nWRITE.
occurs, the current EPP cycle is aborted and the
time-out condition is indicated in Status bit 0.
5. Chip asserts nDATASTB or nADDRSTRB
indicating that PData bus is tri-stated, PDIR
is set and the nWRITE signal is valid.
6. Peripheral drives PData bus valid.
Software Constraints
Before an EPP cycle is executed, the software
must ensure that the control register bits D0, D1
and D3 are set to zero. Also, bit D5 (PCD) is a
86
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