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CHAPTER 3 CPU
3.4.4 Interrupt Processing Time
The total time from the generation of an interrupt request until control passes to the
interrupt processing routine is the sum of the time required to complete execution of
the current instruction and the interrupt handling time (the time required to prepare for
interrupt processing). The maximum time for this process is 30 instruction cycles.
■ Interrupt processing time
When an interrupt request occurs, the time until the interrupt is accepted and the interrupt processing
routine is executed includes the interrupt request sampling time and the interrupt handling time.
●
Interrupt request sampling time
Whether or not an interrupt request has occurred is determined by sampling and testing for interrupt
requests during the final cycle of each instruction. Therefore, the CPU is unable to identify interrupt
requests during execution of an instruction. The longest delay occurs when an interrupt request is generated
immediately after starting execution of a DIVU instruction, which has the longest instruction cycles (21
instruction cycles).
●
Interrupt handling time
Nine instruction cycles are required to perform the following preparation for interrupt processing after the
CPU accepts an interrupt request:
• Save the program counter (PC) and program status (PS).
• Set the top address of the interrupt processing routine (the interrupt vector) in the PC.
• Update the interrupt level bits (PS: CCR: IL1, 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
The total interrupt processing time of 21 + 9 = 30 instruction cycles is required if an interrupt request
occurs immediately after starting execution of a DIVU instruction, which has the longest instruction cycles
(21 instruction cycles). If, on the other hand, the program does not use the DIVU or MULU instructions,
the maximum interrupt processing time is 6 + 9 = 15 instruction cycles.
The time of one instruction cycle changes with the clock mode and the main clock frequency as selected by
the "speed-shift" (gear) function. See Section 3.6 "Clocks" for details.
CPU operation
Interrupt waiting time
Execution of a standard instruction
Interrupt request
sampling time
Interrupt request occurs
Interrupt handling
Interrupt handling time
(9 instruction cycles)
Interrupt processing routine
: Final cycle of instruction. Interrupt requests are sampled at this timing.