PIC16C71 datasheet(139/337 Pages) MICROCHIP

139 / 337 page

DS30234D-page 139

PIC16C6X

13.6

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

Example 13-1 stores and restores the STATUS and W

registers. Example 13-2 stores and restores the

STATUS, W, and PCLATH registers (Devices with

paged program memory). For all PIC16C6X devices

with greater than 1K of program memory (all devices

except PIC16C61), the register, W_TEMP, must be

deﬁned in all banks 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, 0x120 in bank 2, and 0x1A0 in bank 3).

The examples:

Stores the W register

Stores the STATUS register in bank 0

Stores PCLATH

Executes ISR code

Restores PCLATH

Restores STATUS register (and bank select bit)

Restores W register

Applicable Devices

61 62 62A R62 63 R63 64 64A R64 65 65A R65 66 67

EXAMPLE 13-1: SAVING STATUS AND W REGISTERS IN RAM (PIC16C61)

MOVWF

W_TEMP

;Copy W to TEMP register, could be bank one or zero

SWAPF

STATUS,W

;Swap status to be saved into W

MOVWF

STATUS_TEMP

;Save status to bank zero STATUS_TEMP register

:(ISR)

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

EXAMPLE 13-2: SAVING STATUS, W, AND PCLATH REGISTERS IN RAM

(ALL OTHER PIC16C6X DEVICES)

MOVWF

W_TEMP

;Copy W to TEMP register, could be bank one or zero

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

BCF

STATUS, IRP

;Return to Bank 0

MOVF

FSR, W

;Copy FSR to W

MOVWF

FSR_TEMP

;Copy FSR from W to FSR_TEMP

:(ISR)

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

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