Lessons
2012 Microchip Technology Inc. DS41628B-page 77
3.11.5 New Instructions
3.11.5.1 BOTH
3.11.5.1.1 retfie
The retfie instruction exits the ISR by popping the previous address from the stack
and setting the GIE bit. The PIC18 requires the retfie, fast instruction to restore
the saved context, whereas the enhanced mid-range does not have this distinction.
3.11.6 Assembly
3.11.6.1 BOTH
EXAMPLE 3-37:
By using interrupts, the main loop can spend time doing other things such as crunching
numbers or writing to an LCD. The program no longer needs to wait for the flag to
become set to continue like the previous lesson did. This example code will simply
branch to MainLoop indefinitely, doing nothing while waiting for the interrupt.
3.11.6.2 ENHANCED MID-RANGE
EXAMPLE 3-38:
This jumps to the ISR routine. Notice how the goto statement is directly after the inter-
rupt vector address.
EXAMPLE 3-39:
Inside the ISR, the cause of the interrupt is determined. Once determined, one of the
services that must be completed is clearing the interrupt flag so that the ISR can be
successfully left. The retfie instruction exits the ISR by restoring the saved context,
re-enables the GIE bit and returns to the instruction following the last instruction
executed when the interrupt occurred.
TABLE 3-35: NEW INSTRUCTIONS FOR BOTH DEVICES
Instruction English Purpose
retfie Return from interrupt Return to normal execution
MainLoop:
bra MainLoop ;can spend rest of time doing something critical here
Org 0x0 ;Reset Vector starts at 0x0000
bra Start ;main code execution
ORG 0x0004 ;Interrupt Vector starts at address 0x0004
goto ISR
;Enter here if an interrupt has occurred
;First, check what caused the interrupt by checking the ISR flags
;This lesson only has 2 flags to check
ISR:
banksel IOCAF ;bank7
btfsc IOCAF, 3 ;check the interrupt-on-change flag
bra Service_SW1 ;switch was pressed
bra Service_TMR0 ;Timer0 overflowed
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