TI ADS1X1X ADC Driver Usage with NTC Thermistor Generic Senor

[lsd-he360]

Description

We have a custom board on which four NTC thermistors feed into the TI ADS1115 ADC chip. The devicetree for the board defines four ntc-thermistor-generic devices.

When the four ntc thermistor generic devices are initialized, which only happens once, the ntc thermistor driver calls adc_channel_setup_dt. The channel_setup implementation in the ads1x1x driver configures the CONFIG register of the TI ADS1115 chip. Among other things, this configuration involves configuring the input multiplexer. This means the MUX bits in the CONFIG register of the ADS1115 chip are set for whichever NTC thermistor device was last initialized. When you go to fetch a sample for any of the four thermistors with a call to sensor_sample_fetch, it will always return value for whatever MUX bits were set to during the last ntc thermistor init. This seems not ideal.

[lsd-he360]

@anangl @MaureenHelm