ADC on P0.04 doesn't register 1.5v

Question

I am trying to use the ADC on an nRF52840DK.

I have configured the two ADC channels in the overlay

&adc {
	#address-cells = <1>;
	#size-cells = <0>;
	status = "okay";
	channel@0 {
		reg = <0>;
		zephyr,gain = "ADC_GAIN_1_5";
		zephyr,reference = "ADC_REF_INTERNAL";
		zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;
		zephyr,input-positive = <NRF_SAADC_AIN1>;
		zephyr,resolution = <12>;
		zephyr,oversampling = <8>;
	};

	channel@1 {
		reg = <1>;
		zephyr,gain = "ADC_GAIN_1_5";
		zephyr,reference = "ADC_REF_INTERNAL";
		zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;
		zephyr,input-positive = <NRF_SAADC_AIN2>;
		zephyr,resolution = <12>;
		zephyr,oversampling = <8>;
	};
};

I setup the channels like this

#define DT_SPEC_AND_COMMA(node_id, prop, idx) ADC_DT_SPEC_GET_BY_IDX(node_id, idx),

static const struct adc_dt_spec adc_channels[] = {DT_FOREACH_PROP_ELEM(DT_PATH(zephyr_user), io_channels, DT_SPEC_AND_COMMA)};

and take readings using this code

static uint16_t read_temperature(int i)
{
	int err = adc_sequence_init_dt(&adc_channels[i], &temperature_sequence);

	if (err < 0)
	{
		LOG_ERR("Could initialise ADC%d (%d)", i, err);
		return -1;
	}

	err = adc_read(adc_channels[i].dev, &temperature_sequence);

	if (err < 0)
	{
		LOG_ERR("Could not read ADC%d (%d)", i, err);
		return -1;
	}

	int32_t adc_reading = temperature_buf;
	
	...remainder of the function...
}

I have setup a simple voltage divider. Two 10kOhm resisters between GND and VDD. When I check the divider value with a multi-meter, I see 1.5v, as expected.

When I use this divider on P0.03 (AIN1), I get a measurement of just over 2000, which I convert to 1.5v.

When I use this divider on P0.04 (AIN2), I get a measurement of around 30.

If I connect VDD directly to AIN2, I get a measurement over 4000.

If I connect GND to AIN2, I get a measurement around 8.

I am satisfied that I am reading AIN2 based on my VDD & GND tests.

Why, oh, why, doesn't a voltage of 1.5v register on AIN2?

How can I troubleshoot this? Faulty DK?

hmolesworth · Accepted Answer

SW7 will switch P0.04 into the alternate hardware flow control (HWFC) position; this will impact the analogue voltage reading on AIN2 by reducing the impedance. Ensure SW7 is in the "Def" (Default) position. Also must ensure elsewhere in the code HWFC on P0.04 is not enabled.

ADC on P0.04 doesn't register 1.5v

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