Re: [PATCH v5 02/11] iio: adc: Add basic support for AD4170

[David Lechner <dlechner@baylibre.com>]

On 6/10/25 3:31 PM, Marcelo Schmitt wrote:
> From: Ana-Maria Cusco <ana-maria.cusco@analog.com>
> 
> The AD4170 is a multichannel, low noise, 24-bit precision sigma-delta
> analog to digital converter. The AD4170 design offers a flexible data
> acquisition solution with crosspoint multiplexed analog inputs,
> configurable ADC voltage reference inputs, ultra-low noise integrated PGA,
> digital filtering, wide range of configurable output data rates, internal
> oscillator and temperature sensor, four GPIOs, and integrated features for
> interfacing with load cell weigh scales, RTD, and thermocouple sensors.
> 
> Add basic support for the AD4170 ADC with the following features:
> - Single-shot read.
> - Analog front end PGA configuration.
> - Differential and pseudo-differential input configuration.
> 

...

> +static int ad4170_fill_scale_tbl(struct iio_dev *indio_dev,
> +				 struct iio_chan_spec const *chan)
> +{
> +	struct ad4170_state *st = iio_priv(indio_dev);
> +	struct ad4170_chan_info *chan_info = &st->chan_infos[chan->address];
> +	struct device *dev = &st->spi->dev;
> +	int bipolar = chan->scan_type.sign == 's' ? 1 : 0;
> +	int precision_bits = chan->scan_type.realbits;
> +	int pga, ainm_voltage, ret;
> +	unsigned long long offset;
> +
> +	ainm_voltage = 0;
> +	ret = ad4170_get_ain_voltage_uv(st, chan->channel2, &ainm_voltage);
> +	if (ret < 0)
> +		return dev_err_probe(dev, ret, "Failed to fill scale table\n");
> +
> +	for (pga = 0; pga < AD4170_NUM_PGA_OPTIONS; pga++) {

From what I read in the datasheet, it sounds like if adi,reference-buffer is
precharge, then the PGA is bypassed, so there would only be 1 option in that
case (gain = 1).

> +		u64 nv;
> +		unsigned int lshift, rshift;
> +
> +		/*
> +		 * The PGA options are numbered from 0 to 9, with option 0 being
> +		 * a gain of 2^0 (no actual gain), and 7 meaning a gain of 2^7.
> +		 * Option 8, though, sets a gain of 0.5, so the input signal can
> +		 * be attenuated by 2 rather than amplified. Option 9, allows
> +		 * the signal to bypass the PGA circuitry (no gain).
> +		 *
> +		 * The scale factor to get ADC output codes to values in mV
> +		 * units is given by:
> +		 * _scale = (input_range / gain) / 2^precision
> +		 * AD4170 gain is a power of 2 so the above can be written as
> +		 * _scale = input_range / 2^(precision + gain)
> +		 * Keep the input range in µV to avoid truncating the less
> +		 * significan bits when right shifting it so to preserve scale
> +		 * precision.
> +		 */
> +		nv = (u64)chan_info->input_range_uv * NANO;
> +		lshift = !!(pga & BIT(3)); /* handle options 8 and 9 */
> +		rshift = precision_bits - bipolar + (pga & GENMASK(2, 0)) - lshift;
> +		chan_info->scale_tbl[pga][0] = 0;
> +		chan_info->scale_tbl[pga][1] = div_u64(nv >> rshift, MILLI);
> +
> +		/*
> +		 * If the negative input is not at GND, the conversion result
> +		 * (which is relative to IN-) will be offset by the level at IN-.
> +		 * Use the scale factor the other way around to go from a known
> +		 * voltage to the corresponding ADC output code.
> +		 * With that, we are able to get to what would be the output
> +		 * code for the voltage at the negative input.
> +		 * If the negative input is not fixed, there is no offset.
> +		 */
> +		offset = ((unsigned long long)abs(ainm_voltage)) * MICRO;
> +		offset = DIV_ROUND_CLOSEST_ULL(offset, chan_info->scale_tbl[pga][1]);
> +
> +		/*
> +		 * After divided by the scale, offset will always fit into 31
> +		 * bits. For _raw + _offset to be relative to GND, the value
> +		 * provided as _offset is of opposite sign than the real offset.
> +		 */
> +		if (ainm_voltage > 0)
> +			chan_info->offset_tbl[pga] = -(int)(offset);
> +		else
> +			chan_info->offset_tbl[pga] = (int)(offset);
> +	}
> +	return 0;
> +}
> +
> +static int ad4170_read_avail(struct iio_dev *indio_dev,
> +			     struct iio_chan_spec const *chan,
> +			     const int **vals, int *type, int *length,
> +			     long info)
> +{
> +	struct ad4170_state *st = iio_priv(indio_dev);
> +	struct ad4170_chan_info *chan_info = &st->chan_infos[chan->address];
> +
> +	switch (info) {
> +	case IIO_CHAN_INFO_SCALE:
> +		*vals = (int *)chan_info->scale_tbl;
> +		*length = ARRAY_SIZE(chan_info->scale_tbl) * 2;
> +		*type = IIO_VAL_INT_PLUS_NANO;
> +		return IIO_AVAIL_LIST;
> +	default:
> +		return -EINVAL;
> +	}
> +}
> +
> +static int ad4170_set_pga(struct ad4170_state *st,
> +			  struct iio_chan_spec const *chan, int val, int val2)
> +{
> +	struct ad4170_chan_info *chan_info = &st->chan_infos[chan->address];
> +	struct ad4170_setup *setup = &chan_info->setup;
> +	unsigned int pga;

Likewise, here would also need to check for precharge and only allow
gain = 1 in in that case.

> +
> +	for (pga = 0; pga < AD4170_NUM_PGA_OPTIONS; pga++) {
> +		if (val == chan_info->scale_tbl[pga][0] &&
> +		    val2 == chan_info->scale_tbl[pga][1])
> +			break;
> +	}
> +
> +	if (pga == AD4170_NUM_PGA_OPTIONS)
> +		return -EINVAL;
> +
> +	guard(mutex)(&st->lock);
> +	setup->afe &= ~AD4170_AFE_PGA_GAIN_MSK;
> +	setup->afe |= FIELD_PREP(AD4170_AFE_PGA_GAIN_MSK, pga);
> +
> +	return ad4170_write_channel_setup(st, chan->address, false);
> +}
> +