12F615 datasheet(108/168 Pages) MICROCHIP

108 / 168 page

PIC12F609/615/12HV609/615

DS41302A-page 106

Preliminary

11.5

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.

Temporary

holding

registers

W_TEMP

and

STATUS_TEMP should be placed in the last 16 bytes

of GPR (see Figure 2-2). These 16 locations are

common to all banks and do not require banking. This

makes context save and restore operations simpler.

The 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

11.6

Watchdog Timer (WDT)

The Watchdog Timer is a free running, on-chip RC

oscillator, which requires no external components. This

RC oscillator is separate from the external RC oscillator

of the CLKIN pin and INTOSC. That means that the

WDT will run, even if the clock on the OSC1 and OSC2

pins of the device has been stopped (for example, by

execution of a SLEEP instruction). During normal oper-

ation, a WDT time out generates a device Reset. If the

device is in Sleep mode, a WDT time out causes the

device to wake-up and continue with normal operation.

The WDT can be permanently disabled by program-

ming

the

Configuration

bit,

WDTE,

clear

(Section 11.1 “Configuration Bits”).

11.6.1

WDT PERIOD

The WDT has a nominal time-out period of 18 ms (with

prescaler).

The

time-out

periods

vary

with

temperature, VDD and process variations from part to

part (see DC specs). If longer time-out periods are

desired, a prescaler with a division ratio of up to 1:128

can be assigned to the WDT under software control by

writing to the OPTION register. Thus, time-out periods

up to 2.3 seconds can be realized.

The CLRWDT and SLEEP instructions clear the WDT

and the prescaler, if assigned to the WDT, and prevent

it from timing out and generating a device Reset.

The TO bit in the STATUS register will be cleared upon

a Watchdog Timer time out.

Note:

The PIC12F609/615/12HV609/615 does

not require saving the PCLATH. However,

if computed GOTOs are used in both 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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