PIC12F609_10 Datasheet, PDF (121/212 Pages) Microchip Technology – 8-Pin, Flash-Based 8-Bit CMOS Microcontrollers

PIC12F609/615/617/12HV609/615
12.5 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 2-3). 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 12-1 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
Note:
The PIC12F609/615/617/12HV609/615
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.
EXAMPLE 12-1: SAVING STATUS AND W REGISTERS IN RAM
MOVWF W_TEMP
SWAPF STATUS,W
MOVWF
:
:(ISR)
:
SWAPF
STATUS_TEMP
STATUS_TEMP,W
MOVWF
SWAPF
SWAPF
STATUS
W_TEMP,F
W_TEMP,W
;Copy W to TEMP register
;Swap status to be saved into W
;Swaps are used because they do not affect the status bits
;Save status to bank zero STATUS_TEMP register
;Insert user code here
;Swap STATUS_TEMP register into W
;(sets bank to original state)
;Move W into STATUS register
;Swap W_TEMP
;Swap W_TEMP into W
12.6 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, as clear
(Section 12.1 “Configuration Bits”).
12.6.1 WDT PERIOD
The WDT has a nominal time-out period of 18 ms (with
no 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.
 2010 Microchip Technology Inc.
DS41302D-page 121