nRF51822 TEMP internal temperature sensor characteristic

RE: nRF51822 TEMP internal temperature sensor characteristic

Bouke — Wed, 25 Mar 2015 16:30:24 GMT

I did a repeat experiment, this time with each device sleeping for 20 seconds, then doing a measurement immediately after wakeup, and then communicating that data. The results look similar, but this time (same hardware) the offset is smaller, between 6 and 12 degrees. So perhaps the difference is caused by the die temperature after all...

RE: nRF51822 TEMP internal temperature sensor characteristic

Bouke — Wed, 25 Mar 2015 08:30:47 GMT

QFABB0 - 1405AN; all four of them.

RE: nRF51822 TEMP internal temperature sensor characteristic

Susheel Nuguru — Wed, 25 Mar 2015 08:05:22 GMT

What hardware are you using Bouke? earlier versions had a minor glitch with calibrating the TEMP sensor at factory for very very small number of devices (maybe the reason the 4th device is little off with the the other three), but that has been handled for newer hardware. Hopefully you will not see the difference in offsets with newer builds.

RE: nRF51822 TEMP internal temperature sensor characteristic

Bouke — Wed, 25 Mar 2015 08:00:22 GMT

I mean to say that I would think a very cold die would have a smaller temp difference with its environment (since it gets cooled a lot), and a hot die would have a larger temperature difference since it loses less heat to the environment. There is no such effect in my measurements. EDIT: After talking to a hardware engineer, I understand that this is not how ICs work, but that there is a constant number of degrees per watt between environmental end die. The following point still holds though: Furthermore, if those 13 degrees are caused by the CPU heating the die, how would we explain the fact that out of 4 devices (identical hardware, identical software), 3 have about 13 degrees offset, and 1 has 9 degrees? While all devices respond very similar to external influences like sudden temperature changes?

RE: nRF51822 TEMP internal temperature sensor characteristic

Susheel Nuguru — Wed, 25 Mar 2015 07:19:55 GMT

"what surprises me most is that it is fairly constant over the whole temperature range" what do you mean by this? you mean that the temperature of die remained fairly constant at the whole tested temperature range? looking at your chart it seemed that the die temperature was varying accordingly. Also the thermal conductivity of chip is another charecteristics which neccessarily is not linear with outside temperature.

RE: nRF51822 TEMP internal temperature sensor characteristic

Bouke — Tue, 24 Mar 2015 16:18:45 GMT

Wow, that sounds like a really nice setup! Unfortunately this is not viable in my case...

RE: nRF51822 TEMP internal temperature sensor characteristic

Albert Proust — Tue, 24 Mar 2015 15:50:09 GMT

In fact is not a fixed value, unfortunately! We use our reflow oven to have an exact temperature (24°C) and we set via bluetooth the offset value between the oven's temperature and the internal temp sensor. It's a bit tricky but that's work.

RE: nRF51822 TEMP internal temperature sensor characteristic

Bouke — Tue, 24 Mar 2015 14:37:53 GMT

Hi Aryan, a strategy similar to this was implemented, although some communication takes place right before the measurement. I would expect (and accept) perhaps a slightly higher die temperature, but what surprises me most is that it is fairly constant over the whole temperature range. If the environmental temperature is -15 degrees, the chip would lose its heat very easily, and in a 35 degrees environment it would be much warmer. However, there is no such effect; the difference is very similar over the whole temperature range.

RE: nRF51822 TEMP internal temperature sensor characteristic

Susheel Nuguru — Tue, 24 Mar 2015 12:31:57 GMT

Bouke, we still have to consider that the external temperature sensor measurement is still not the same as the internal sensor measuring die temperature. The specced accuracy calculations was made specific to the die temperature at a very large variations of temperature it is working in. How much of the time is the system kept on? maybe we can get a better result by trying to keep the chip in sleep mode most of the time (like a minute) and just wakeup to read the temperature very fast and go back to sleep. That COULD ensure that the die temperature is much closer to outside temperature and then we can use the external temp sensor reading to compare.

RE: nRF51822 TEMP internal temperature sensor characteristic

Bouke — Tue, 24 Mar 2015 12:16:10 GMT

Thanks for sharing your experience! Is the offset you set of a similar value as the one suggested by my measurements? (-13 degrees)

RE: nRF51822 TEMP internal temperature sensor characteristic

Bouke — Tue, 24 Mar 2015 12:14:46 GMT

I excluded the possibility if the HFCLK not running just yet. Is there any possible cause for this large difference between the specced accuracy and the actual accuracy?

RE: nRF51822 TEMP internal temperature sensor characteristic

Susheel Nuguru — Tue, 24 Mar 2015 11:56:08 GMT

Yes, I agree. It wont make a difference if the anamoly does not apply to the hardware you have.

RE: nRF51822 TEMP internal temperature sensor characteristic

Bouke — Mon, 23 Mar 2015 14:40:13 GMT

Hi Aryan, it is mentioned in the Product Anomaly Notice 31: TEMP: Temperature offset value has to be manually loaded to the TEMP module..

EDIT: On closer inspection I see that this PAN does not apply to the specific build number of my hardware. The TEMP offset register is 0 in any case, so setting it to 0 or not does not make a difference.

RE: nRF51822 TEMP internal temperature sensor characteristic

Susheel Nuguru — Mon, 23 Mar 2015 13:58:08 GMT

Hi Bouke, Which temperature offset register are you talking about? I dont see any offset register in the documentation I have.

RE: nRF51822 TEMP internal temperature sensor characteristic

Susheel Nuguru — Fri, 13 Mar 2015 08:55:52 GMT

reading multiple readings and taking an average might result in less noisy reading but the average will not deviate too much (like +/-4 C) from single reading. Also note that the noise in result is more as the die is getting closer to one end of its operating temperatures, otherwise you will only see negligle noise (about +/-0.25 C deviation if die is operating at 10-60 C). Hope this helps

RE: nRF51822 TEMP internal temperature sensor characteristic

Albert Proust — Thu, 12 Mar 2015 15:14:22 GMT

Hello Bouk,

We use the TEMP sensor and we set an offset. You don't have any another way, I guess.

RE: nRF51822 TEMP internal temperature sensor characteristic

Bouke — Thu, 12 Mar 2015 14:44:03 GMT

From this question, it seems that it is a pure offset:

The error read on the TEMP peripheral should be pure offset error. You can calibrate the temperature value in software, i.e. if you know the temperature in your room, just add or subtract from the measured value to get the correct value.

However, be aware that the die temperature is in general not equal to the ambient temperature.

I did some measurements comparing the internal temperature sensor value with temperature measured by a dedicated temperature sensor (TMP112) on the same PCB, in a controlled environment. Because the application makes very little use of the CPU, I do not expect that it is significantly hotter than the environment.

This confirms previously made statements that the noise is low, and the accuracy is mainly distorted by an offset. To my surprise however, this offset is around 13 degrees! When corrected for the average, the accuracy is nicely within the specced +/-4 degrees. Could this large offset have something to do with having to set the temperature offset register to 0?

RE: nRF51822 TEMP internal temperature sensor characteristic

Bouke — Thu, 12 Mar 2015 12:52:38 GMT

Hi Aryan, thanks for the feedback. I understand that I cannot change the sensor or its behaviour. But I am wondering if you have more details on the accuracy of the sensor. Can I process the data afterwards to get more precise results?

RE: nRF51822 TEMP internal temperature sensor characteristic

Susheel Nuguru — Thu, 12 Mar 2015 12:31:44 GMT

Hi,

The TEMP sensor is calibrated during production and there is nothing much that can be done to change its behaviour thereafter.

/Aryan