TM4C123GH6PGE: Internal temperature sensor reads 10 degrees low

Such has been reported in the past (here) - and has been noted upon (other) ARM MCU forum sites - as well.

We note that you employ ADC_1 for your measure - might a quick switch to ADC_0 - at minimum - prove of interest?

Further - does other code run while this temperature sensing is performed?   Perhaps limiting running code to "MCU Temperature Sense only" provides insight.

Lastly - what is the condition of the MCU's (driven) ADC inputs?   Are they active - do buffer circuits drive - are all analog input signals w/in spec - at all times?

And "supra last" - have you included the appropriate MCU power bypass caps - closely placed - and followed "high-frequency pcb layout guidelines?"   (some provided by this vendor)

You have local capacitors, some of significant capacitance, on the reference pin?

Richard Bland said:

On-chip noise would most likely cause random changes in the temperature reading rather than a fixed offset I would have thought.

Noise can be diode coupled leading to a fixed offset.

Also have you checked the errata on the temperature sensor?

Robert

Richard Bland said:

Yes we have a separate regulator (LM1117MPX-3.3) generating VDDA and I have measured it as 3.294V at the processor pins on this particular board

Have you checked it with an oscilloscope?

Robert

Greetings,

For giggles - we attempted similarly - but used LX4F. (also ran test upon 3 LM3S) And - the results w/these MCUs - was w/in or just outside the  ±5°C spec. (we note that LM3S had (only) a 10 bit ADC)

As to Amit's results under the thermostream - might this indicate that, "Increased warm-up duration (MCU under bias - NOT sitting unpowered) would raise the die temperature to meet spec?"

Note too - as new device revisions are released - there may be new and/or further deviations from said spec.

As poster Robert noted - a "designed to purpose, external temperature sensor" usually proves ideal for those who (really) "need to know!"

cb1 said:

As poster Robert noted - a "designed to purpose, external temperature sensor" usually proves ideal for those who (really) "need to know!"

I had made the decision to use an external sensor initially based on the fact that there was no documentation on the thermal resistance from the sensor to the ambient. The temperature sensor errata provided further reason to use an external sensor as did the large inherent error (which would be compounded by reference and A/D errors).

I came to the conclusion that the only reason for using the on-board sensor would be, perhaps, to measure dies temperature to warn/protect against exceeding T_JMax. However, although the power consumption was not documented it did not seem likely that there would be a significant temperature rise and the degree of coupling of the sensor to the junction temperature was also undocumented. On the actual board there was little temperature rise for the package and there was no obvious corrective action to take even if T_JMax was exceeded so there's never been a serious temptation to use it.

Robert

May I leave a post here, to make it easier for me to find any future considerations on this matter - as I also faced the same problem before.

My boards also typically report ~8 oC less than what expected. We do have external temperature sensors available, so the onboard temp sensor is not that relevant. Still we discussed that in the past and the conclusion back then was that "it should have worked"... It didn't.

Waste of resources? Nope! They added the internal sensor so that developers on their initial learning stages can see something out of the ADC! (And of course, get biased that they are doing something wrong, for the number shown is off...) - Or maybe for random number generation?  :)

Cheers!