Using WDT as a Temp Sensor?

[Archilochus]

Hi modifyit,
If you check the app note section:

"FIGURE 1: FIRMWARE FLOW DIAGRAM"
there is a flowchart describing how the code works... RESET > TestWDT > Etc

This is from the 12F675 data sheet about resets, and what registers are changed on various resets:

Some registers are not affected in any RESET
condition; their status is unknown on POR and
unchanged in any other RESET. Most other registers
are reset to a RESET state on:
Power-on Reset
MCLR Reset
WDT Reset
WDT Reset during SLEEP
Brown-out Detect (BOD) Reset

They are not affected by a WDT wake-up, since this is
viewed as the resumption of normal operation. TO and
PD bits are set or cleared differently in different RESET
situations as indicated in Table 9-4. These bits are
used in software to determine the nature of the RESET.
See Table 9-7 for a full description of RESET states of
all registers.
A simplified block diagram of the On-Chip Reset Circuit
is shown in Figure 9-4.
The MCLR Reset path has a noise filter to detect and
ignore small pulses. See Table 12-4 in Electrical
Specifications Section for pulse width specification.

Arch

[modifyit]

maybe this is what I am missing: Does the WDT reseting of the PIC clear existing variables? For example will a variable "count" be zeroed out on this reset or will it maintain its value prior to the reset?

[modifyit]

Here is what I've got so far on a 12F675

'defines
DEFINE OSCCAL_1K 1 
define OSC 4
DEFINE NO_CLRWDT
'end defines

'includes
'Include "Elapsed.bas"
'end includes

' Config Fuses
@ __config _INTRC_OSC_NOCLKOUT & _CPD_OFF & _CP_OFF & _BODEN_ON & _MCLRE_ON & _PWRTE_ON & _WDT_ON

' Variable Definitions

LED VAR GPIO.1
TEMPERATURE_CNT VAR WORD
TEMPERATURE VAR WORD

'End Variable Defs

' Initialize

INTCON=%10100000
CMCON=7
TRISIO=%11111101 ' Input 0,2,3,4,5 Output 1
ANSEL=0

'End Initialize

ON INTERRUPT GOTO TMR:

OPTION_REG.3=1 ; set WDT prescaler to 1:8
OPTION_REG.2=0
OPTION_REG.1=1
OPTION_REG.0=1

startup:

LED=0

main:

IF STATUS.4=0 THEN  ;CHECKS TO SEE IF WDT TIMEOUT OCCURED
    TEMPERATURE = TEMPERATURE_CNT ;TEMPERATURE EQUALS COUNT VARIABLE
    TEMPERATURE_CNT=0  ;CLEARS COUNT VARIABLE
@CLRWDT            ;CLEARS STATUS.4 REG BACK TO 1
ENDIF  

IF TEMPERATURE>300 THEN
    LED=1 ;IF TEMPERATURE IS ABOVE A DETERMINED COUNT VARIABLE TURN ON LED
ELSE
    LED=0
ENDIF

GOTO MAIN 


DISABLE INTERRUPT
TMR: ;INTERUPT OCCURS WHEN TMR0 ROLLS OVER
    TEMPERATURE_CNT=TEMPERATURE_CNT+1 ;INCREMENT COUNT VARIABLE
    INTCON=%10100000                  ;CLEAR TMR0 INTERRUPT
resume
ENABLE INTERRUPT

[modifyit]

At this point I can't seem to get the TMR0 timer to work with the code above.