PIC16F627A Datasheet, PDF (108/168 Pages) Microchip Technology – FLASH-Based 8-Bit CMOS Microcontrollers

PIC16F627A/628A/648A
TABLE 14-8: SUMMARY OF INTERRUPT REGISTERS
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, INTCON
10Bh, 18Bh
0Ch
PIR1
8Ch
PIE1
GIE
EEIF
EEIE
PEIE
CMIF
CMIE
T0IE
RCIF
RCIE
INTE
TXIF
TXIE
RBIE T0IF INTF RBIF 0000 000x 0000 000u
— CCP1IF TMR2IF TMR1IF 0000 -000 0000 -000
— CCP1IE TMR2IE TMR1IE 0000 -000 0000 -000
Note 1: Other (non Power-up) Resets include MCLR Reset, Brown-out Reset and Watchdog Timer Reset during normal oper-
ation.
14.6 Context Saving During Interrupts
14.7 Watchdog Timer (WDT)
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 (e.g., W register and STATUS
register). This must be implemented in software.
Example 14-2 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 14-2:
• Stores the W register
• Stores the STATUS register
• Executes the ISR code
• Restores the STATUS (and bank select bit regis-
ter)
• Restores the W register
EXAMPLE 14-2:
SAVING THE STATUS
AND W REGISTERS IN
RAM
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 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
The watchdog timer is a free running on-chip RC oscil-
lator 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 con-
figuration bit WDTE as clear (Section 14.1).
14.7.1 WDT PERIOD
The WDT has a nominal time out period of 18 ms (with
no prescaler). The time out periods vary with tempera-
ture, VDD and process variations from part to part (see
DC Specifications, Table 17-7). If longer time out peri-
ods 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.
DS40044A-page 106
Preliminary
 2002 Microchip Technology Inc.