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cles. Typically, a four clock cycle pulse is ge-
nerated. But in the case that a subsequent H1
asserted edge occurs before termination of the
pulse, H2 is negated asynchronously. Thus,
anytime after the leading edge of the H2 pulse,
new data may be transferred to the double-
buffered output latches. The H2S status bit is
always clear. When H12 enable is clear, H2 is
held negated.
The DMAREQ pin may be associated with either in-
put transfers (H3) or output transfers (H1), but not
both. Refer to table 3.4 for a summary of the port B
data register responses in mode 2.
Programmable Options Mode 2 - Port A Submode XX and Port B Submode XX
Table 3.4 :
Mode 2 Port B Data Paths
.
Mode
2
Read PortB Data Register
FIL, D. B.
WritePortB Data Register
IOL/FOL, D. B.
Abbreviations :
IOL - Initial Output Latch
FOL - Final Output Latch
FIL - Final Input Latch
D. B. - Double Buffered
PACR
7 6
X X
Port A Submode
Submode XX.
PACR
5 4 3
X X 0
X X 1
H2 Control
Output pin - interlocked output handshake protocol, H2S is always cleared.
Output pin - pulsed output handshake protocol, H2S is always cleared.
PACR
2
0
1
H2 Interrupt Enable
The H2 interrupt is disabled.
The H2 interrupt is enabled.
PACR
1
0
1
H1 SVCRQ Enable
The H1 interrupt and DMA request are disabled.
The H1 interrupt and DMA request are enabled.
PACR
0
0
is clear when both latches are full and cannot accept new data.
1
The H1S status bit is set when both of the port B output latches are empty. It is clear when at
least one latch is full.
H1 Status Control
The H1 status bit is set when either the port B initial or final output latch can accept new data. It
PBCR
7 6
X X
Port B Submode
Submode XX.
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