Interrupts
7-36
Table 7–8. Interrupt Latency
Cycle Description Fetch Decode Read Execute
1Recognize interrupt in single-cycle fetched
(prog a + 1) instruction prog a + 1 prog a prog a–1 prog a–2
2 Clear GIE bit. Clear interrupt flag interrupt prog a prog a–1
3 Read the interrupt vector table interrupt prog a
4 Store return address to stack interrupt
5 Pipeline begins to fill with ISR instruction isr1
6 Pipeline continues to fill with ISR instruction isr2 isr1
7 Pipeline continues to fill with ISR instruction isr3 isr2 isr1
8Execute first instruction of interrupt service routine isr4 isr3 isr2 isr1

7.6.8 External Interrupts

The four external maskable interrupt pins INT0–INT3 are enabled at the IF reg-
ister (Section 3.1.9,

CPU Interrupt Flag (IF) Register

, on page 3-11) and are syn-
chronized internally. They are sampled on the falling edge of H1 and passed
through a series of H1/H3 latches internally. These latches require the interrupt
signal to be held low for at least one H1/H3 clock cycle to be recognized by the
‘C3x. Once synchronized, the interrupt input sets the corresponding interrupt
flag register (IF) bit if the interrupt is active.
Figure 7–7 shows a functional diagram of the logic used to implement external
interrupt inputs.