a8259 Programmable Interrupt Controller Data Sheet
Table 3. ICW 1 Register Format (Part 2 of 2)
Bit
3
Mnemonic
LTIM
Description
Level-sensitive or edge-triggered input mode. When
high, the
ir[7..0]
pins are level-sensitive inputs;
otherwise, they are positive-edge-triggered.
This bit is used in conjunction with the
a0
signal to select
other command registers (see
“Interrupt Registers” on
page 69).
These bits set the interrupt vector address (bits 5
through 7) in a 3-byte interrupt sequence (see
“Interrupt
Sequencing” on page 70).
4
1
5
6
7
A5
A6
A7
ICW 2
ICW 2 is selected after the
a0
signal has been high. Input data for ICW 2
is sent via the
din[7..0]
bus, and data is clocked by the rising edge of
clk.
ICW 2 is deselected with the next falling edge of the
signal.
Table 4
describes the ICW 2 register format.
Table 4. ICW 2 Register Format
Bit
0
1
2
3
4
5
6
7
Mnemonic
A8
A9
A10
A11 / T3
A12 / T4
A13 / T4
A14 / T6
A15 / T7
Description
These bits set the interrupt vector address. For bits 8
through 15, the interrupt vector address is set in a single-
byte interrupt sequence mode. For bits 3 through 7, the
interrupt vector address is set in the same mode. See
“Operating Modes & Sequence of Events” on page 76
for more information.
If SINGLE (bit 1 of ICW 1) is low, ICW 3 is the next register selected (see
“ICW 3” on page 64).
If SINGLE is high, ICW 3 is skipped.
The next register considered is ICW 4 (see
Figure 3
on
page 61).
If bit 0 of
ICW 1 is high, then ICW 4 is the next register selected; if it is low, ICW 4
is skipped. When a write transaction is completed for ICW 4—or if it is
skipped—the initialization sequence is finished, and the
a8259
is now
ready to accept interrupts.
Altera Corporation
63
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