
Parallel Port Interface
9-6
MC68322 USER’S MANUAL
MOTOROLA
can be reprogrammed at any time, but if an ECP cycle is currently in progress, it
completes as normal.
11 = Enable ECP mode hardware handshaking with RLE support during forward data
transfers. The PPI performs the same ECP mode handshaking as above, except
RLE decompression is enabled. RLE decompression enables the PPI to detect
and intercept run-length counts and to automatically perform data
decompression. However, during this mode, only channel addresses cause the
PIER’s CRD bit to be set. The software is responsible for responding to channel
addresses. If MODE is reprogrammed when decompression is occurring (when
the RLD bit is set), the decompression continues unhindered to completion. The
RST bit can be set to immediately abort decompression.
DFE—Digital Filtering Enable
Setting this bit enables digital filtering on all four host control signal inputs—SELECTIN,
STROBE, AUTOFD, and INIT.
RST—Reset
Setting this bit causes the PPI’s handshake control and decompression logic to immediately
terminate the current operation and return to idle. RST clears the RLD and FLL bits. The PPI
state machine BUSY and ACK are negated. If the PPIR’s BSY2 and ACK2 bits are clear,
then BSY1 = 0 and ACK1 = 1. The software should set the MODE field to 00 to disable
handshaking when setting RST to prevent the PPI state machine from starting again. RST
is a write-only bit and setting it causes the reset. Clearing RST has no effect. This bit always
reads as zero.
9.1.3 PPI Interrupt Event Register
The PPI interrupt event register (PIER) contains 11 bits that can be enabled and used to
drive the parallel port using a software driver. Eight of the eleven bits indicate when a rising
or falling edge is seen on any of SELECTIN, INIT, AUTOFD, or STROBE host inputs. The
remaining three bits indicate when a data or command byte is received or when an invalid
termination event is detected. Figure 9-4 illustrates the PIER.
Figure 9-4. PPI Interrupt Event Register
The following bits describe each of the parallel port interrupt events that can be posted by
the PPI. The first eight interrupt events signal level changes that occur at the host control
signal input pins. Note that these events are detected after the host inputs are synchronized,
optionally digitally filtered, and recorded in the PPIR.
= RESERVED
15
14
13
12
11
10
IVD
9
8
7
65432
1
0
CRD DRD
INL
INH
AFL AFH STL STH SNL SNH
15
14
13
12
11
10
9
8
7
65432
1
0
ENABLE
INTERRUPT
LEVEL
00FFF760
00FFF762
00FFF764