EPP 1.9 Read
EPP 1.7 OPERATION
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:
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.
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 changing the state of WRITE or
before nDATASTB goes active. The read can
complete once nWAIT is determined inactive.
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 nIOR or nIOW
deasserted). If a time-out occurs, the current EPP
cycle is aborted and the time-out condition is
indicated in Status bit 0.
Read Sequence of Operation
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.
4. The chip tri-states the PData bus and
deasserts nWRITE.
5. Chip asserts nDATASTB or nADDRSTRB
indicating that PData bus is tri-stated, PDIR is
set and the nWRITE signal is 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
logic "0" for an EPP write or a logic "1" for and
EPP read.
6. Peripheral drives PData bus valid.
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.
b) The chip drives the valid data onto the
SData bus and asserts (releases)
IOCHRDY allowing the host to complete
the read cycle.
EPP 1.7 Write
The timing for a write operation (address or data)
is shown in timing diagram EPP 1.7 Write Data or
Address cycle. IOCHRDY is driven active low
when nWAIT is active low during the EPP cycle.
This can be used to extend the cycle time. The
write cycle can complete when nWAIT is inactive
high.
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.
91
SMSC [ SMSC CORPORATION ]
