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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.
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.
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.
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.
Write Sequence of Operation
1.
The host sets PDIR bit in the control register to a logic "0". This asserts nWRITE.
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.
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.
EPP 1.7 Read
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.