nRF52840: At low temperatures I can't achieve the specified accuracy for the internal temperature sensor

[Frank Viganske]

While testing our custom Hardware with the nRF52840 in a climate chamber in the -40°C to +85°C range i noticed that the value read from the internal temperature sensor was much too low at -40°C. According to the specification the deviation should be within the -5°C to +5°C range over the whole operating temperature range. I am aware of the erratum 66 and have implemented the initialization code for this.

At high temperatures the accuracy is OK, i only see this large deviation at very low temperatures. I tested 12 PCBs simultaneously. Of 12 boards only 3 gave a temperature measurement result of -45°C or higher at -40°C climate chamber temperature. All remaining boards showed values below -45°C The lowest value i have seen on one board was -55,25°C.

I compared the internal CPU temperatures to a TMP1075 I²C temperature sensor mounted on the same board.. The TMP1075 sensors saw a temperature within the range of -39°C to -37°C during the same measurements..

Below is the code used to initialize and read the temperature sensor. I'm not 100% sure about this, as i didn't have any example code available, but the initialization should be ok, i just copied the suggested workaround from the errata sheet..

void init_cpu_temp_sensor(void)
{
  NRF_TEMP->A0 = NRF_FICR->TEMP.A0;
  NRF_TEMP->A1 = NRF_FICR->TEMP.A1;
  NRF_TEMP->A2 = NRF_FICR->TEMP.A2;
  NRF_TEMP->A3 = NRF_FICR->TEMP.A3;
  NRF_TEMP->A4 = NRF_FICR->TEMP.A4;
  NRF_TEMP->A5 = NRF_FICR->TEMP.A5;
  NRF_TEMP->B0 = NRF_FICR->TEMP.B0;
  NRF_TEMP->B1 = NRF_FICR->TEMP.B1;
  NRF_TEMP->B2 = NRF_FICR->TEMP.B2;
  NRF_TEMP->B3 = NRF_FICR->TEMP.B3;
  NRF_TEMP->B4 = NRF_FICR->TEMP.B4;
  NRF_TEMP->B5 = NRF_FICR->TEMP.B5;
  NRF_TEMP->T0 = NRF_FICR->TEMP.T0;
  NRF_TEMP->T1 = NRF_FICR->TEMP.T1;
  NRF_TEMP->T2 = NRF_FICR->TEMP.T2;
  NRF_TEMP->T3 = NRF_FICR->TEMP.T3;
  NRF_TEMP->T4 = NRF_FICR->TEMP.T4;

}

float read_cpu_temp_sensor(void)
{
  int result;
  NRF_TEMP->TASKS_START = 1;
  NRF_TEMP->EVENTS_DATARDY = 0;
  // Read it back to ensure it is written (needed on cortex m4)
  volatile uint32_t dummy = NRF_TEMP->EVENTS_DATARDY;
  (void) dummy;
  while (0 == NRF_TEMP->EVENTS_DATARDY);
  result = NRF_TEMP->TEMP;
  NRF_TEMP->TASKS_STOP = 1;
  return (float)result / 4;
}

Assuming that the code is ok (i hope so), what else could cause this high deviation of the temperature reading?

Best regards

Frank

[Frank Viganske]

Hi kenneth,

I inserted a debug print of the HFCLKSTAT at the beginning of the temperature measurement, and the value was 0x10000 (HFCLK running, but on the internal RC oscillator). So it seems that you're right,  the inaccuracy might be caused by the incorrect clock source.

I'll try to change my firmware to keep the crystal oscillator running during the temperature measurement and see if that improves my accuracy.

Best regards

Frank

[Frank Viganske]

Hi Kenneth,

I changed the Firmware to switch over to the HFXO during the temperature measurement. Now at -40°C i get the result -41.5°C from a board that measured -53.75°C in my previous tests.

Problem solved. Thanks for your help.

Best regards

Frank