Post by thread:[PIC]: Using WDT reset for temperature sense?

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 -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics