piclist 2002\08\21\183026a >
Thread: Using WDT reset for temperature sense?
www.piclist.com/techref/microchip/ios.htm?key=temperature
BY : Jinx email (remove spam text)

Because WDT timing is resistor-based, each PIC's WDT
is slightly different, but they all fall within a fairly narrow range.
Also, the body of a PIC is not optimised as a sensor and there's
some thermal inertia. You could improve this by putting a metal
fin on it

The easiest way to use WDT as a temp sensor is to set up a timer
IRQ and use it to measure the time between WDTs. The longer
the better, ie set the WDT pre-scaler to maximum, which will give
you a WDT time-out of around 2 seconds. Then look up the measured
time in a calibration table. For example, if you count 22,146 timer IRQs
that are 100us each thats 22,146 x 100us = 2.2146 seconds, which
you may find for THAT PARTICULAR PIC equates to 21.5 degC

In this table here, the third column shows that the temp vs time is
quite linear and extrapolation within a table should give reasonably
accurate results. These figures are from my own experiments with
a number of PICs

Temp Time % of predicted

82  2.9716  100.00      80  2.9573   99.99
78  2.9332   99.96      76  2.9050  100.08
74  2.8769  100.19      72  2.8489  100.31
70  2.8201  100.45      68  2.8081   99.99
66  2.7818  100.05      64  2.7624   99.86
62  2.7382   99.84      60  2.7210   99.56
58  2.6830  100.04      56  2.6635   99.84
54  2.6436   99.65      52  2.6232   99.48
50  2.5922   99.71      48  2.5618   99.93
46  2.5346   99.98      44  2.5103  100.12
42  2.4942   99.65      40  2.4675   99.72
38  2.4395   99.85      36  2.4136   99.89
34  2.3882   99.70      32  2.3522  100.39
30  2.3324  100.18      28  2.3029  100.38
26  2.2767  100.45      24  2.2562  100.26
22  2.2356  100.08      20  2.2125  100.00
18  2.1902   99.89      16  2.1592  100.17
14  2.1432  100.24      12  2.1133  100.00
10  2.0846   99.82       5  2.0311  100.77
0  1.9601   99.86      -3  1.9284  100.04
-7  1.8809  100.12     -10  1.8581  100.34
-13 1.8067  100.13     -15  1.7782  100.00

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