PIC16F676-I Datasheet, PDF (68/132 Pages) Microchip Technology – 14-Pin, Flash-Based 8-Bit CMOS Microcontrollers

PIC16F630/676
TABLE 9-8: 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, BOD
Value on all
other
Resets
0Bh, 8Bh INTCON GIE
PEIE
T0IE
INTE RAIE T0IF
INTF RAIF 0000 0000
0Ch
PIR1
EEIF ADIF
—
—
CMIF
—
— TMR1IF 00-- 0--0
8Ch
PIE1
EEIE ADIE
—
—
CMIE
—
— TMR1IE 00-- 0--0
Legend: x = unknown, u = unchanged, - = unimplemented read as ‘0’, q = value depends upon condition.
Shaded cells are not used by the Interrupt module.
0000 000u
00-- 0--0
00-- 0--0
9.5 Context Saving During Interrupts
9.6 Watchdog Timer (WDT)
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 9-2 stores and restores the STATUS and W
registers. The user register, W_TEMP, must be defined
in both banks and must be defined at the same offset
from the bank base address (i.e., W_TEMP is defined
at 0x20 in Bank 0 and it must also be defined at 0xA0
in Bank 1). The user register, STATUS_TEMP, must be
defined in Bank 0. The Example 9-2:
• Stores the W register
• Stores the STATUS register in Bank 0
• Executes the ISR code
• Restores the Status (and bank select bit register)
• Restores the W register
EXAMPLE 9-2:
SAVING THE STATUS AND
W REGISTERS IN RAM
MOVWF W_TEMP
;copy W to temp register,
could be in either 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
oscillator, which requires no external components. This
RC oscillator is separate from the external 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 9.1 “Configura-
tion Bits”).
9.6.1 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 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.
9.6.2
WDT PROGRAMMING
CONSIDERATIONS
It should also be taken in account that under worst-
case conditions (i.e., VDD = Min., Temperature = Max.,
Max. WDT prescaler) it may take several seconds
before a WDT time-out occurs.
DS40039F-page 68
 2010 Microchip Technology Inc.