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THEORY OF OPERATION
The WDT on all PICmicro microcontrollers has a nominal time-out period of 18 ms. The WDT time-out period varies with temperature, VDD and part-to-part process variations. For a given microcontroller, the WDT exhibits a nearly linear correlation between the time-out period and temperature, assuming that VDD is constant. Figure 1 shows the time-out count asa function of temperature for four different devices. Note that while each device differs in counts for a given temperature, the slope of the line for each device is essentially constant, and is similar for all devices. The only real difference is the offset (or y-intercept) for each device. In practical terms, this means that the thermometer circuit must be calibrated with the offset value forits controller. For this application, two temperatures at opposite ends of the expected temperature range are used to derive both slope and y-intercept. The design of the digital WDT thermometer is based on this principle. Without using a separate temperature sensor, it is possible to calculate the temperature with reasonable accuracy using the WDT time-out period.

Measuring Temperature with thePIC16F84A Watchdog Timer

Leena Chaudhari Microchip Technology Inc.

INTRODUCTION
Almost all temperature sensor circuits use some form of discrete component (such as a thermistor or a solidstate sensor) to actually measure the environment’s temperature. It is left to the microcontroller to interpret the reading into a human-friendly form for the user’s benefit. It is possible, however, todesign a digital thermometer without an external sensor, by using a temperature sensitive property of the microcontroller itself. This Application Note shows how to use the Watchdog Timer (WDT) of a PICmicro® microcontroller for temperature measurement.

FIGURE 1:
5500

WATCHDOG TIMER COUNT VS. TEMPERATURE FOR FOUR PIC16F84A DEVICES

5000 Device1 4500

WDT Count

4000 Device2

3500Device3

3000

Device4

2500

2000 -20 -10 0 10 20 Temperature (C) 30 40 50 60

 2002 Microchip Technology Inc.

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To translate the environment temperature into an actual reading, the system must be able to do the following: • Provide a method for establishing time-out to temperature calibration • Count the number of WDT time-outs for a given period of time •Equate the number of time-outs to a temperature The flow charts showing the firmware implementation of all these steps are presented in Figure 2 and Figure 3. For the sake of brevity, we will only discuss the method for counting WDT time-outs and calculating temperature, in detail. The overall system design also includes wake-on-interrupt key scanning and temperature display, which may not be neededby some users. Those who may be interested in examining these other components are encouraged to download the source code and examine it at their leisure.

FIGURE 2:

MAIN FIRMWARE ROUTINE FOR THE WDT THERMOMETER
START POR, BOR, Wake from SLEEP A YES SET key pressed? NO Clear WDTCOUNT and execute CLRWDT YES STATUS = ‘1’ ? NO

WDT Time-out

DEFAULT = ‘1’ ? NO

YES

Load new calibrationtemps and WDT counts from EEPROM

YES

WDT Time-out? NO

Load default temps and WDT counts

Increment
WDTCOUNT

Calculate temp from current WDTCOUNT

Enable PORTB Interrupt-onchange

TEMP key pressed? NO

YES

Display Temperature

NO SLEEP mode

TEMP key pressed within 5 seconds?

YES

END (Return to START on RESETS)

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 2002 Microchip Technology Inc.AN828
FIGURE 3: CALIBRATION ROUTINE FOR THE WDT THERMOMETER

A

10-minute key press time-out? NO

YES

B

C

2-minute key press time-out? NO Display current high calibration temp and “HI” (alternate)

YES

B

Display current low calibration temp and “LO” (alternating) C UP key pressed? YES Increment low calibration temperature

C

C UP key pressed? NO YES Increment...