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INTERRUPT CONTROL UNIT
8-30
External interrupt acknowledge cycles must be run for every maskable interrupt. Therefore, the
interrupt response time for every interrupt will be 55 clocks, as shown in Figure 8-21.
Figure 8-21. Interrupt Response Time in Slave Mode
8.5.3 Initializing the Interrupt Control Unit for Master Mode
Follow these steps to initialize the Interrupt Control Unit for Master mode.
1. Determine which interrupt sources you want to use.
2. Determine whether to use the default priority scheme or devise your own.
3. Program the Interrupt Control register for each interrupt source.
— For external interrupt pins, select edge or level triggering.
— For INT0 or INT1, enable cascade mode, special fully nested mode, or both, if you
wish to use them.
— If you are using a custom priority scheme, program the priority level for each interrupt
source.
4. Program the Priority Mask with a priority mask level, if you wish to mask interrupts based
on priority. (The default is level seven, which enables all interrupt levels.)
Clocks
5
4
2
4
5
4
3
4
4
4
3
4
4
5
Total 55
First instruction fetch
from interrupt routine
INTA
IDLE
INTA
IDLE
READ IP
IDLE
READ CS
IDLE
PUSH FLAGS
IDLE
PUSH CS
PUSH IP
IDLE
Interrupt presented to Interrupt Control Unit
Interrupt presented to external 82C59A
A1200-A0