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CHAPTER 3 CPU
3.4.3 Multiple Interrupts
Multiple interrupts are allowed by setting different levels into the interrupt level setting
registers (ILR1 to ILR4) for multiple interrupt requests from peripheral functions.
■ Multiple Interrupts
When an interrupt request with a higher interrupt level is generated while the interrupt processing routine is
operating, the current interrupt processing cycle is stopped to accept the higher-level interrupt request.
Interrupt levels 1, 2, and 3 can be specified. Level 3 prohibits the CPU from accepting interrupts.
● Example of multiple interrupts
As an example of multiple-interrupt processing, suppose a case in which a timer interrupt has precedence
using the A/D interrupt, and the A/D interrupt level is set to level 2 and the timer interrupt level is set to
level 1. Figure 3.4-3 shows the sequence performed when an external interrupt is generated while an A/D
interrupt is being processed.
Figure 3.4-3 Example of Multiple Interrupts
• In the A/D interrupt processing, the interrupt level bits in the condition code register (CCR: IL1 and
IL0) are set to the same value as the value in the interrupt level setting register corresponding to the A/
D interrupt (ILR1, 2, 3, or 4) (i.e., 2 in this example). If an interrupt request with a higher interrupt level
specified is generated (1 in this example), processing for the higher interrupt level is effected first.
• To temporarily prohibit multiple interrupts in the A/D interrupt processing, turn off the interrupt enable
flag (CCR: I = 0) in the condition code register, or set 00
B
to the interrupt level bits (IL1 and IL0).
• Executing the return instruction (RETI) after interrupt processing restores the values of the program
counter (PC) and program status (PS) and ensures resumption of the interrupted program.
• The value in the condition code register (CCR) is returned to the value used before interruption when
the program status (PS) value is restored.
Main program
Initializes
peripherals
A/D interrupt
generated
Main
program
restarts
Interrupt
level 2
A/D interrupt
processing
Timer interrupt
generated
Stop
process-
ing
Resumes
processing
A/D interrupt
processed
Return from A/D
interrupt processing
Interrupt
level 1
Timer interrupt
processing
Timer interrupt
processed
Return from
timer interrupt
processing