KEAZN64 Internal temperature sensor

[kiran5]

Hi 

I am using S9KEAZN64AMLC for our project. 

We have planned to use the internal temperature sensor which is inbuilt in the controller. From documents I can get the formula as

Temp = 25 – ((Vtemp – Vtemp@25)/Slope)

The count value from ADCR register is 1100

Vtemp = 1100*5/4095 = 1.3431

Vtemp@25 = 1.396

Temp = 25 – ((Vtemp – Vtemp@25)/hot slope)

              = 25 – ((1.3431- 1.396)/0.003638)

              =39.54

The temperature value calculated was 39.54 Celcius.

We also used an external NTC temperature sensor placed directly on the back of the PCB where the controller is installed. The value we calculated from this is 34 Celcius. 

There is a difference of 5.5 Celcius between the internal temperature sensor and the external(NTC) sensors.

How Can we tell the accuracy?

Please provide a solution for this issue.

[CarlosGarabito]

This type of MCU has a temperature sensor that differs with an external sensor, this temperature accuracy is approximately of  +/- 4 C in some MCU or +/- 8 MCU, depending on some factors or characteristics, causes them to have these variations, that is why the internal temperature sensor should not be taken as a precise measurement, but as a reference only, and that is why you have these variations.

[CarlosGarabito]

You can check this treads which talk the same issue

https://community.nxp.com/t5/Kinetis-Microcontrollers/KEA128-On-Board-Temperature/m-p/1378316

https://community.nxp.com/t5/Kinetis-Microcontrollers/S9KEAZ128-on-chip-temperature-sensor/m-p/57054...

https://www.nxp.com/docs/en/application-note/AN3031.pdf

[kiran5]

Hi Carlos

The value observed from the ADC R register for the Bandgap channel is different in 2 different situations.

Here i am using 12 bit resolution.

When we enable only the bandgap channel(Channel 23) the value of the ADC R can be observed as 385. 

When we enable both the temperature sensor (Channel 22) and the bandgap (Channel 23) the value of

the ADC R register for the bandgap is 1223. Why is there 2 different values according to the number of channels enabled?

We observed ADCRbg as 1223 and ADCRtemp as 1110.

According to calculation from the AN3031 document

Vdd = (4095*1.16*10)/1223 = 38.8

ADCRtemp @ 25 = (4095*1.396*10)/38.8 = 1473

Hot slope = (4095 *0.003638*10*100)/38.8 = 383

Temp = 25 - (1110-1473)/383 = 25.95 ~= 26 Celcius

We have an external NTC temperature sensor mounted on the back side of the PCB below the controller. The value calculated is 31 Celcius. There is a difference in the temperature values.

Please provide with a solution.

Regards

Kiran