516CHAPTER 22 INTERRUPT FUNCTIONS22.4.2 Maskable interrupt request acknowledge operationA maskable interrupt request becomes acknowledgeable when an interrupt request flag is set to 1 and theinterrupt mask (MK) flag is cleared to 0. A vectored interrupt request is acknowledged in an interrupt enable state(with IE flag set to 1). However, a low-priority interrupt request is not acknowledged during high-priority interruptservice (with ISP flag reset to 0).Wait times maskable interrupt request generation to interrupt servicing are shown in Figure 22-3.Refer to Figures 22-14 and 22-15 for the interrupt request acknowledge timing.Table 22-3. Times from Maskable Interrupt Request Generation to Interrupt ServiceMinimum Time Maximum TimeNoteWhen xxPRx = 0 7 clocks 32 clocksWhen xxPRx = 1 8 clocks 33 clocksNote If an interrupt request is generated just before a divide instruction, the wait time is maximized.Remark 1 clock: (f CPU : CPU clock)If two or more maskable interrupt requests are generated simultaneously, the request specified for higher prioritywith the priority specify flag is acknowledged first. If two or more requests are specified for the same priority withthe priority specify flag, the interrupt request with the higher default priority is acknowledged first.Any reserved interrupt requests are acknowledged when they become acknowledgeable.Figure 22-13 shows interrupt request acknowledge algorithms.When a maskable interrupt request is acknowledged, the contents of program status word (PSW) and programcounter (PC) are saved to stacks, in this order. Then, the IE flag is reset (to 0), and the value of the acknowledgedinterrupt priority specify flag is transferred to the ISP flag. Further, the vector table data determined for each interruptrequest is loaded into PC and branched.Return from the interrupt is possible with the RETI instruction.f CPU1
