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44-Pin Demo Board Lessons
When the master interrupt line is asserted, the PIC microcontroller finishes the current
instruction, stores the next address on the CALL stack then jumps to the Interrupt Ser-
vice Routine (ISR). It also clears the GIE bit, preventing another interrupt from occur-
ring while servicing the current one.
Save Current Context
The first thing the ISR must do is to save the current context of the processor so it can
be restored before returning to the main program. Certain SFRs that may be changed
in the ISR should be saved, such as the WREG and STATUS registers at the very least.
Make sure to save the WREG first, as moving other SFRs into it will destroy the value
needing to be saved. The last 16 bytes of each PIC16F887 file register page are
unbanked and are good places to save the context, as they may be accessed from any
register page without regard to the RP0 and RP1 bits in the STATUS register. The
location of unbanked registers may vary from part to part. Check the register map to
find the unbanked region for a specific part.
Identify Triggering Event
Next, the ISR has to figure out what triggered the interrupt. It has to check the interrupt
flags to determine what caused the interrupt. When it finds the source, it can then
service the peripheral.
Restore Context
Once the peripheral is serviced, it needs to restore the context and resume the main
program. The “context” is the state of the SFRs when the interrupt occurred. Restoring
the context is a little harder than it might seem at first. The obvious method doesn’t work
because the MOVF W_Temp,w may affect the Z flag, which was restored in the previ-
ous instruction. Instead, a pair of SWAPF instructions can restore WREG without affect-
ing the flags in the STATUS register. SWAPF exchanges the high and low nibbles. The
first SWAPF switches the nibbles in the file register and the second one switches them
back and puts the result in WREG.
EXAMPLE 3-9:
CONTEXT RESTORE
;incorrect context restore
MOVF STATUS_Temp,w
MOVWF STATUS
MOVF W_Temp ;this may change the Z bit
;in the Status register
;good
MOVF
MOVWF
context restore
STATUS_Temp,w
STATUS
SWAPF
SWAPF
W_Temp,f
W_Temp,w
;swap in place
;swap with Wreg destination
Finally, RETFIE transfers control back to the original program and sets the GIE bit,
re-enabling interrupts.
? 2007 Microchip Technology Inc.
DS41296B-page 29
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