PIC16F882-I datasheet(225/328 Pages) MICROCHIP

225 / 328 page

DS41291F-page 223

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 com-

mon

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

nor-

mally

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

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