519CHAPTER 22 INTERRUPT FUNCTIONS22.4.4 Multiple interrupt servicingA multiple interrupt consists in acknowledging another interrupt during the execution of the interrupt.A multiple interrupt is generated only in the interrupt request acknowledge enable state (IE = 1) (except non-maskable interrupt). As soon as an interrupt request is acknowledged, it enters the acknowledge disable state (IE= 0). Therefore, in order to enable a multiple interrupt, it is necessary to set the interrupt enable state by settingthe IE flag (1) with the EI instruction during interrupt servicing.Even in an interrupt enabled state, a multiple interrupt may not be enabled. However, it is controlled accordingto the interrupt priority. There are two priorities, the default priority and the programmable priority. The multipleinterrupt is controlled by the programmable priority control.If an interrupt request with the same or higher priority than that of the interrupt being serviced is generated, itis acknowledged as a multiple interrupt. In the case of an interrupt with a priority lower than that of the interruptbeing processed, it is not acknowledged as a multiple interrupt.Interrupt request not acknowledged as a multiple interrupt due to interrupt disable or a low priority is reservedand acknowledged following one instruction execution of the main processing after the completion of the interruptbeing serviced.During non-maskable interrupt servicing, multiple interrupts are not enabled.Table 22-4 shows an interrupt request enabled for multiple interrupt during interrupt servicing, and Figure 22-16 shows multiple interrupt examples.Table 22-4. Interrupt Request Enabled for Multiple Interrupt during Interrupt ServicingMaskable Interrupt RequestxxPR = 0 xxPR = 1IE = 1 IE = 0 IE = 1 IE = 0Non-maskable interrupt D D D D DISP = 0 E E D D DISP = 1 E E D E DSoftware interrupt E E D E DRemarks 1. E : Multiple interrupt enable2. D : Multiple interrupt disable3. ISP and IE are the flags contained in PSWISP = 0 : An interrupt with higher priority is being servicedISP = 1 : An interrupt request is not accepted or an interrupt with lower priority isbeing servicedIE = 0 : Interrupt request acknowledge is disabledIE = 1 : Interrupt request acknowledge is enabled4. xxPR is a flag contained in PR0L, PR0H, and PRILxxPR = 0 : Higher priority levelxxPR = 1 : Lower priority levelNon-maskableInterruptRequestMultiple InterruptRequestInterrupt being ServicedMaskable interrupt
