Possible bug: STM32L4 ADC always reading zeroes

[petarsusac]

I'm using the sensortile_box board (STM32L4R9ZIJ6 MCU) and I'm trying to get some samples from the on-board analog microphone with the MCU's internal ADC. But no matter which channel I try to sample, I'm constantly getting zero values and nothing else. Unfortunately I cannot access the pin directly and measure the voltage, but I think there should at least be some noise and not always stuck at exactly zero. This can be replicated by running the adc_dt sample with the following overlay:

/ {
	zephyr,user {
		io-channels = <&adc1 8>;
	};
};

&adc1 {
	#address-cells = <1>;
        #size-cells = <0>;
	status = "okay";
	pinctrl-0 = <&adc1_in8_pa3>;
	pinctrl-names = "default";
	st,adc-clock-source = "SYNC";
	st,adc-prescaler = <1>;
	channel@8 {
		reg = <8>;
		zephyr,gain = "ADC_GAIN_1";
		zephyr,reference = "ADC_REF_INTERNAL";
		zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;
		zephyr,resolution = <12>;
	};
};

Could this be a bug in the ADC driver? Maybe a clock configuration issue?

Some of the things I tried:

• enabling the debug logs of the ADC driver - everything seems fine, no errors
• setting a higher prescaler value
• using an async clock source derived from SYSCLK
• different ADC channels

[gautierg-st]

Hello @petarsusac,

I don't have a sensortile_box with me right now, but I can tell you that the ADC works on a Disco L475. So normally it should also be working on a L4R9.

First I would advise against having a prescaler of 1 with a synchronous clock. Having a prescaler a 4 should be good enough.
You could also try to increase the acquisition time to ADC_ACQ_TIME_MAX.

By the way did you correctly set the 2V7_EN pin (PF7) ? If not, I'd say this is normal that you see 0 all the time on channel 8 (PA3).

[petarsusac]

The 2V7_EN pin wasn't on... Thanks for the tip!