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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.
s 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.
r 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.
r 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 "Interrupt Processing Time" 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)
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