What happens when an HC12 gets in unmasked interrupt:

1.

Completes current instruction

2.

Clears instruction queue

3.

Calculates return address

4.

Stacks return address and contents of CPU registers

5.

Sets I bit of CCR

6.

Sets X bit of CCR if an XIRQ interrupt is pending

7.

Fetches interrupt vector for the highest-priority interrupt which is pending

8.

Executes ISR at the location of the interrupt vector

What happens when an HC12 exits an ISR with the RTI instruction:

1.

If no other interrupt pending,

(a)

HC12 recovers stacked registers

(b)

Execution resumes at the return address

2.

If another interrupt pending

(a)

HC12 recovers stacked registers

(b)

Subtracts 9 from SP

(c)

Sets I bit of CCR

(d)

Sets X bit of CCR if an XIRQ interrupt is pending

(e)

Fetches interrupt vector for the highest-priority interrupt which is pending

(f)

Executes ISR at the location of the interrupt vector

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Bill Rison
2000-02-23