2. The host selects an EPP register, places
data on the SData bus and drives nIOW
active.
3. The chip places address or data on PData
bus.
4. Chip asserts nDATASTB or nADDRSTRB
indicating that PData bus contains valid
information, and the WRITE signal is valid.
5. If nWAIT is asserted, IOCHRDY is
deasserted until the peripheral deasserts
nWAIT or a time-out occurs.
EPP 1.7 OPERATION
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.
6. When the host deasserts nIOW the chip
deasserts nDATASTB or nADDRSTRB and
latches the data from the SData bus for the
PData bus.
7. Chip may modify nWRITE, PDIR and
nPDATA in preparation of the next cycle.
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 10msec
have elapsed from the start of the EPP cycle
(nIOR or nIOW asserted) to the end of the cycle
EPP 1.7 Read
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.
The timing for a read operation (data) is shown
in timing diagram EPP 1.7 Read Data 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 read cycle
can complete when nWAIT is inactive high.
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.
Read Sequence of Operation
1. The host sets PDIR bit in the control
register to a logic "1". This deasserts
nWRITE and tri-states the PData bus.
2. The host selects an EPP register and drives
nIOR active.
3. Chip asserts nDATASTB or nADDRSTRB
indicating that PData bus is tri-stated, PDIR
is set and the nWRITE signal is valid.
4. If nWAIT is asserted, IOCHRDY is
deasserted until the peripheral deasserts
nWAIT or a time-out occurs.
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.
5. The Peripheral drives PData bus valid.
Write Sequence of Operation
1. The host sets PDIR bit in the control
register to a logic "0".
nWRITE.
This asserts
93
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