Parallel Port Interface
9-2
MC68322 USER’S MANUAL
MOTOROLA
9.1 PPI REGISTERS
There are three memory-mapped registers that control the PPI:
PPI interface register
PPI control register
PPI interrupt event register
9.1.1 Parallel Port Interface Register
The parallel port interface register (PPIR) is a read/write register that contains two 8-bit
fields—one that controls the ACK pulse width and another that contains the latched parallel
data transmitted from the host to the printer. The PPIR also contains eleven bits that control
the parallel port interface signals. These eleven bits consist of four read-only bits that are
used to read the logic level of the host input pins, two read-only bits to read the logic level
on the BUSY and ACK printer output pins, and five read/write bits control the logic levels on
the printer output pins. Figure 9-2 illustrates the parallel port interface register.
Figure 9-2. Parallel Port Interface Register
ACKW—ACK Width
This field defines the ACK pulse width when compatibility mode is enabled (PPCR MODE
field = 01). The ACK pulse width is selectable from 0 to 255 CLK1 periods wide. At 16 MHz,
the software can set pulse widths anywhere in the range of 0 to 16
s. If the field is clear,
no ACK pulse is issued. Otherwise, the cycle proceeds as normal.
ACKW can be changed at any time and with any PPCR MODE encoding selected, but it can
only be used during a compatibility mode handshaking cycle. If ACKW is changed near the
end of a data transfer (when an ACK is already low), then the new pulse width value does
not affect the current cycle. The new value of ACKW is used when the next cycle occurs.
CMD—Command
When read, this bit provides the logic level of AUTOFD when STROBE transitioned from
high to low with the PPCR’s PDE bit clear. If set, AUTOFD was latched high and if clear,
AUTOFD was latched low. This is a read-only bit, so writing CMD has no effect.
STR*
AFD*
INT*
SIN* ACK1* BSY1* ACK2
00FFF302
00FFF304
00FFF306
BSY2 PER SEL
FLT
15
14
13
12
11
10
9
8
7
65432
1
0
15
14
13
12
11
10
9
8
7
65432
1
0
*These bits are read-only bits.
CMD
DATA
ACKW
= RESERVED