BQ76905: looking for clarification on external temperature reading

Hi Dave,

Are you reading the temperature measurements on BQStudio or on an MCU?

Regards,

Max Verboncoeur

Reading from an MCU. I'm able to read other registers on the chip - cell voltages, stack voltage, internal temperature, just don't understand how to read the external thermistor's temperature

Hi Dave,

Have you tried using the example microcontroller code that we have available on the product page?

Regards,

Max Verboncoeur

Max,

Yes I've looked at that code, but I don't believe it has an example of reading the thermistor temperature (it does for the internal temperature).

Dave

Hi Dave,

Do you have logic analyzer captures of the reads? I'm curious what I2C frames you are sending.

We are able to open Saleae Logic files, so you can attach a capture directly. Otherwise, you may need to send screenshots of the sequence.

Regards,

Max Verboncoeur

Hi Dave,

One additional thing to check is that the thermistor is connected to the pin correctly. I quick checked the reading that you said you were getting, and it is near the upper limit. You may want to connect a scope across the thermistor and see if the bias voltage is correctly pulsing.

Regards,

Max Verboncoeur

OK I'll try looking with a scope - I do see it pulsing on a DMM, but need a scope to capture properly.

I believe I'm reading the I2C registers right - I'm able to read other registers in the part OK. I see there are two modes for the TS pin - one for thermistor use, one for generic ADC use. I belive I'm configured for the thermistor use.

The data sheet has this paragraph:

The data is reported using the TS Measurement() command, in units of 16-bit ADC counts. The full-scale digital
value reflects an analog input level of its reference × 5 / 3. So when the TS pin is measuring a thermistor in
ratiometric mode using the 1.8V internal regulator for its reference, the 16-bit LSB is 1.8 V × 5 / 3 / 32768 ≅ 92
μV. When the TS pin is measuring in ADCIN mode using the VREF1 reference, the 16-bit LSB is 1.1962 V ×
5 / 3 / 32768 ≅ 61 μV. The processing performed on the ADC data before reporting is shown below, using the
Calibration:Temperature:TS Offset data memory parameter.
TS Measurement() = (16-bit ADC counts) – (TS offset)

Does the chip convert that voltage reading into deg C - or do I need to do that conversion, given a 20k pull up and the R0 & B factors of the thermistor we're using?  I had originally thought that it would do that conversion - but now am questioning that - I see the overtemp in charge protection uses a setting in ADC counts, not temperature.

Thanks,

Dave

Hi Dave,

Yes, you'll need to do the conversion yourself. The measurement is reported in ADC counts so that there are not the necessary added steps to calibrate the AFE to a specific thermistor curve.

Regards,

Max Verboncoeur