Datasheet

PIC12F635/PIC16F636/639

12.10 Context Saving During Interrupts

During an interrupt, only the return PC value is saved

on the stack. Typically, users may wish to save key

registers during an interrupt (e.g., W and STATUS

registers). This must be implemented in software.

Since the lower 16 bytes of all banks are common in the

PIC12F635/PIC16F636/639 (see Figure 2-2), temporary

holding registers, W_TEMP and STATUS_TEMP, should

be placed in here. These 16 locations do not require

banking and therefore, make it easier to context save and

restore. The same code shown in Example 12-1 can be

used to:

• Store the W register.

• Store the STATUS register.

• Execute the ISR code.

• Restore the Status (and Bank Select Bit register).

• Restore the W register.

EXAMPLE 12-1: SAVING STATUS AND W REGISTERS IN RAM

Note: The PIC12F635/PIC16F636/639 normally

does not require saving the PCLATH.

However, if computed GOTO’s are used in

the ISR and the main code, the PCLATH

must be saved and restored in the ISR.

MOVWF W_TEMP ;Copy W to TEMP register

SWAPF STATUS,W ;Swap status to be saved into W

;Swaps are used because they do not affect the status bits

MOVWF STATUS_TEMP ;Save status to bank zero STATUS_TEMP register

:(ISR) ;Insert user code here

SWAPF STATUS_TEMP,W ;Swap STATUS_TEMP register into W

;(sets bank to original state)

MOVWF STATUS ;Move W into STATUS register

SWAPF W_TEMP,F ;Swap W_TEMP

SWAPF W_TEMP,W ;Swap W_TEMP into W