
SMSC DS – FDC37N958FR
Page 106
Rev. 09/01/99
EPP 1.9 OPERATION
When the EPP 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 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.
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.
EPP 1.9 Write
The timing for a write operation (address or data)
is shown in timing diagram EPP 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:
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.
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
nDATASTB, nWRITE or nADDRSTB. The
write
can
complete
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.
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.
b)
The chip latches the data from the SData
bus for the PData bus and 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.
9.
Chip may modify nWRITE and nPDATA in
preparation for the next cycle.
once
nWAIT
is