Datasheet

PIC16F688

11.4 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

PIC16F688 (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 11-1

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 11-1: SAVING STATUS AND W REGISTERS IN RAM

Note: The PIC16F688 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