PIC16F627A datasheet(111/180 Pages) MICROCHIP

111 / 180 page

DS40044D-page 109

PIC16F627A/628A/648A

TABLE 14-8:

SUMMARY OF INTERRUPT REGISTERS

14.6

Context Saving During Interrupts

During an interrupt, only the return PC value is saved

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

registers during an interrupt (e.g., W register and

Status register). This must be implemented in software.

Example 14-1 stores and restores the Status and W

registers. The user register, W_TEMP, must be defined

in a common memory location (i.e., W_TEMP is

defined at 0x70 in Bank 0 and is therefore, accessible

at 0xF0, 0x170 and 0x1F0). The Example 14-1:

• Stores the W register

• Stores the Status register

• Executes the ISR code

• Restores the Status (and bank select bit register)

• Restores the W register

EXAMPLE 14-1:

SAVING THE STATUS

AND W REGISTERS IN

RAM

14.7

Watchdog Timer (WDT)

The Watchdog Timer is a free running on-chip RC

oscillator

which

does

not

require

any

external

components. This RC oscillator is separate from the

RC oscillator of the CLKIN pin. 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

operation, 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

programming the configuration bit WDTE as clear

(Section 14.1 “Configuration Bits”).

14.7.1

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 Specifications, Table 17-7). If longer time-

out periods are desired, a postscaler 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 postscaler, 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.

14.7.2

WDT PROGRAMMING

CONSIDERATIONS

It should also be taken in account that under worst case

conditions (VDD = Min., Temperature = Max., max.

WDT prescaler) it may take several seconds before a

WDT time out occurs.

Address

Name

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Value on

POR Reset

Value on all

other

Resets(1)

0Bh, 8Bh,

10Bh, 18Bh

INTCON

GIE

PEIE

T0IE

INTE

RBIE

T0IF

INTF

RBIF

0000 000x

0000 000u

0Ch

PIR1

EEIF

CMIF

RCIF

TXIF

—

CCP1IF

TMR2IF TMR1IF

0000 -000

8Ch

PIE1

EEIE

CMIE

RCIE

TXIE

—

CCP1IE TMR2IE TMR1IE

0000 -000

Note

1: Other (non Power-up) Resets include MCLR Reset, Brown-out Reset and Watchdog Timer Reset during normal

operation.

MOVWF

W_TEMP

;copy W to temp register,

;could be in any bank

SWAPF

STATUS,W

;swap status to be saved

;into W

BCF

STATUS,RP0 ;change to bank 0

;regardless of current

;bank

MOVWF

STATUS_TEMP ;save status to bank 0

;register

:(ISR)

SWAPF

STATUS_TEMP,W;swap STATUS_TEMP

;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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