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CHAPTER 3 CPU
3.4.4 Interrupt Processing Time
From when an interrupt request is generated to when control is transferred to the
interrupt processing routine, both the time to quit the instruction being executed and
the time to manage the interrupt (required to prepare interrupt processing) are required.
The total time must be within 30 instruction cycles.
■ Interrupt Processing Time
From when an interrupt request is generated and accepted to when the interrupt processing routine starts,
sufficient time is required to wait for an interrupt request sample and to manage the interrupt.
● Interrupt request sample wait time
Generation of an interrupt request is checked by sampling an interrupt request at the last cycle of each
instruction. Therefore, the CPU cannot identify an interrupt request while it is executing an instruction. The
wait time becomes maximum when an interrupt request is generated immediately after the CPU executes
the DIVU instruction (21 instruction cycles) with the longest instruction cycle.
● Interrupt handling time
After accepting an interrupt, the CPU needs 9 instruction cycles for interrupt processing preparation to:
• Save the values in the program counter (PC) and program status (PS)
• Set the address at the beginning of the interrupt processing routine (interrupt vector) into the PC
• Update the interrupt level bits (PS: CCR: IL1 and IL0) in the program status (PS).
Figure 3.4-4 shows the interrupt processing time.
Figure 3.4-4 Interrupt Processing Time
When an interrupt request is generated immediately after the DIVU instruction having the longest
instruction cycle (21 instruction cycles), 30 instruction cycles (21 instructions + 9 instructions) are required
for the interrupt processing time. However, if the DIVU instruction and MULU instruction are not used in
the program, a maximum of 15 (6 instructions + 9 instructions) instructions are required for the instruction
processing time.
An instruction cycle is changed by clock speed switching (gears). For details, see Section "3.6 Clock ".
CPU performs
Interrupt wait
time
Execution of
general instruction
Interrupt request
sample wait time
Interrupt handling
Interrupt handling time
(9 instruction cycles)
Interrupt processing
routine
Interrupt request is generated
: Last instruction in which an interrupt is sampled