i.MX 8M Mini TMU temperature readout problem

[ralfgoebel]

Hello,

We have a new custom hardware using the i.MX 8M Mini SoC (commercial grade).

The temperature values reported by the TMU seem to be incorrect. At case temperatures of about 20°C ... 35 °C, I see the following values reported by U-Boot: 16°C, 18°C, 24°C, 28°C, 32°C, 36°C, 64°C, 72°C, 112°C.

Above 35°C, the temperature reading fails.

The same problem occurs within Linux.

Is there some temperature calibration required for the TMU?

Thanks,

Ralf

[ralfgoebel]

After comparing the circuit diagram of the i.MX8M Mini Reference Board with our implementation, we found that there is a 100k resistor to ground at pin TSENSOR_RES_EXT (J24). This pin is open on our board. Could this be the reason?
I found no documentation about this pin.

Thanks,
Ralf

[b36401]

The calibration values are in the efuses at:
0x4F0[7:0] TSENSOR_VALUE_RT[7:0] (room temp)
0x4F0[15:8] TSENSOR_VALUE_HT[7:0] (high temp)

[ralfgoebel]

The efuse values for our boards are within the following ranges:

TSENSOR_VALUE_RT: 0x34 ... 0x39
TSENSOR_VALUE_HT: 0x69 ... 0x6f

Are they reasonable?