Jinx says:
Like I said I would, I've tried to quantify the WDT timer against temperature. The results I got surprised me, considering the simple tools I used for the experiment, a freezer and a pot of heated water.I used 4 PICF84s (different batches) running at 10MHz. Apart from a4/mclr pullups, no external connections, and a well-filtered 5.00V PSU. All four are on the same board, each WDT pulse coming out of their respective b4 pins, to a 4-way rotary switch to select the PIC to measure. The wiper of the switch goes to another PIC which has a 100us IRQ and a 6-digit LED display. Alternate WDT pulses start/ stop the timer, times are shown here to 1/10000th sec. Temperature was measured using a fairly expensive multimeter thermocouple probe stated as being +/- 2% accurate. As the same equipment was used for all four PICs that should cancel out any errors and make the results comparable. There is no 0.1 digit on the TC, so I had to use my best judgement when to take the time reading. That was done when the display stopped fickering between the current and previous temperature reading
Below are the results (untouched, really), and as you can see, remarkable linearity for what I consider careful (but imperfect) measurement. The probe was inserted under a PIC with some heatsink compound, between the chip and the top of the socket. This held it securely and gave good contact to the PIC body.
I chose three stable temperatures as references. -15C, 20C and 82C and calculated the expected time differences on these temperatures to work out the deviations. One PIC was used to gather these figures, the other three came within 0.1% at spot readings.
As the TC probe is metal and the PIC a comparatively big thermal mass, the whole heating and cooling process was done as slowly as I could stomach.
Once a particular PIC is calibrated at two extreme temperatures I would see no reason why it shouldn't be very good sensor, bearing in mind its thermal mass. Putting the PIC on a heatsink would help the response time. Under more controlled conditions you could reasonably expect 100.00 all the way down these figures
82 2.9816 100.00 81 2.9682 99.99 80 2.9573 99.99 79 2.9450 99.99 78 2.9332 99.96 77 2.9158 100.13 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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