PIC16F883-ESLASHSO. datasheet(227/338 Pages) MICROCHIP

227 / 338 page

 2006-2012 Microchip Technology Inc.

DS41291G-page 227

PIC16F882/883/884/886/887

14.4

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 upper 16 bytes of all GPR banks are

common in the PIC16F882/883/884/886/887 (see

Figures 2-2 and 2-3), 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 14-1 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 14-1:

SAVING STATUS AND W REGISTERS IN RAM

Note:

The PIC16F882/883/884/886/887 devices

normally do not require saving the

PCLATH. However, if computed GOTOs

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

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