Datasheet

PIC16F870/871

DS30569C-page 98  2000-2013 Microchip Technology Inc.

11.11 Context Saving During Interrupts

During an interrupt, only the return PC value is saved

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

isters during an interrupt, (i.e., W register and STATUS

For the PIC16F870/871 devices, the register W_TEMP

must be defined in both banks 0 and 1 and must be

defined at the same offset from the bank base address

(i.e., If W_TEMP is defined at 0x20 in bank 0, it must

also be defined at 0xA0 in bank 1). The registers,

PCLATH_TEMP and STATUS_TEMP, are only defined

in bank 0.

Since the upper 16 bytes of each bank are common in

the PIC16F870/871 devices, temporary holding regis-

ters W_TEMP, STATUS_TEMP, and PCLATH_TEMP

should be placed in here. These 16 locations don’t

require banking and therefore, make it easier for con-

text save and restore. The same code shown in

Example 11-1 can be used.

EXAMPLE 11-1: SAVING STATUS, W, AND PCLATH REGISTERS IN RAM

MOVWF W_TEMP ;Copy W to TEMP register

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

CLRF STATUS ;bank 0, regardless of current bank, Clears IRP,RP1,RP0

MOVWF STATUS_TEMP ;Save status to bank zero STATUS_TEMP register

MOVF PCLATH, W ;Only required if using pages 1, 2 and/or 3

MOVWF PCLATH_TEMP ;Save PCLATH into W

CLRF PCLATH ;Page zero, regardless of current page

:(ISR) ;(Insert user code here)

MOVF PCLATH_TEMP, W ;Restore PCLATH

MOVWF PCLATH ;Move W into PCLATH

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