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adc: add microvolt conversion functions

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Add a family of functions that convert to microvolts instead of
millivolts. The resolution of an ADC with a 600 mV reference and a 12
bit output (nRF SAADC for example) is an order of magnitude better than
millivolts (0.146 mV), even before considering non-unity gains.

Signed-off-by: Jordan Yates <jordan@embeint.com>
This commit is contained in:
Jordan Yates 2025-02-20 11:33:03 +10:00 • committed by Fabio Baltieri
commit c0ca636692
1 changed files with 97 additions and 32 deletions
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129
include/zephyr/drivers/adc.h
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@ -859,10 +859,10 @@ static inline uint16_t adc_ref_internal(const struct device *dev)
}
/**
* @brief Convert a raw ADC value to millivolts.
* @brief Conversion from raw ADC units to a specific output unit
*
* This function performs the necessary conversion to transform a raw
* ADC measurement to a voltage in millivolts.
* ADC measurement to a physical voltage.
*
* @param ref_mv the reference voltage used for the measurement, in
* millivolts. This may be from adc_ref_internal() or a known
@ -875,15 +875,21 @@ static inline uint16_t adc_ref_internal(const struct device *dev)
* resolution in struct adc_sequence.
*
* @param valp pointer to the raw measurement value on input, and the
* corresponding millivolt value on successful conversion. If
* corresponding output value on successful conversion. If
* conversion fails the stored value is left unchanged.
*
* @retval 0 on successful conversion
* @retval -EINVAL if the gain is not reversible
*/
static inline int adc_raw_to_millivolts(int32_t ref_mv,
enum adc_gain gain,
uint8_t resolution,
typedef int (*adc_raw_to_x_fn)(int32_t ref_mv, enum adc_gain gain, uint8_t resolution,
int32_t *valp);
/**
* @brief Convert a raw ADC value to millivolts.
*
* @see adc_raw_to_x_fn
*/
static inline int adc_raw_to_millivolts(int32_t ref_mv, enum adc_gain gain, uint8_t resolution,
int32_t *valp)
{
int32_t adc_mv = *valp * ref_mv;
@ -896,6 +902,72 @@ static inline int adc_raw_to_millivolts(int32_t ref_mv,
return ret;
}
/**
* @brief Convert a raw ADC value to microvolts.
*
* @see adc_raw_to_x_fn
*/
static inline int adc_raw_to_microvolts(int32_t ref_mv, enum adc_gain gain, uint8_t resolution,
int32_t *valp)
{
int64_t adc_uv = (int64_t)*valp * ref_mv * 1000;
int ret = adc_gain_invert_64(gain, &adc_uv);
if (ret == 0) {
*valp = (int32_t)(adc_uv >> resolution);
}
return ret;
}
/**
* @brief Convert a raw ADC value to an arbitrary output unit
*
* @param[in] conv_func Function that converts to the final output unit.
* @param[in] spec ADC specification from Devicetree.
* @param[in] channel_cfg Channel configuration used for sampling. This can be
* either the configuration from @a spec, or an alternate sampling configuration
* based on @a spec, for example a different gain value.
* @param[in,out] valp Pointer to the raw measurement value on input, and the
* corresponding output value on successful conversion. If conversion fails
* the stored value is left unchanged.
*
* @return A value from adc_raw_to_x_fn or -ENOTSUP if information from
* Devicetree is not valid.
* @see adc_raw_to_x_fn
*/
static inline int adc_raw_to_x_dt_chan(adc_raw_to_x_fn conv_func,
const struct adc_dt_spec *spec,
const struct adc_channel_cfg *channel_cfg,
int32_t *valp)
{
int32_t vref_mv;
uint8_t resolution;
if (!spec->channel_cfg_dt_node_exists) {
return -ENOTSUP;
}
if (channel_cfg->reference == ADC_REF_INTERNAL) {
vref_mv = (int32_t)adc_ref_internal(spec->dev);
} else {
vref_mv = spec->vref_mv;
}
resolution = spec->resolution;
/*
* For differential channels, one bit less needs to be specified
* for resolution to achieve correct conversion.
*/
if (channel_cfg->differential) {
resolution -= 1U;
}
return conv_func(vref_mv, channel_cfg->gain, resolution, valp);
}
/**
* @brief Convert a raw ADC value to millivolts using information stored
* in a struct adc_dt_spec.
@ -909,34 +981,27 @@ static inline int adc_raw_to_millivolts(int32_t ref_mv,
* Devicetree is not valid.
* @see adc_raw_to_millivolts()
*/
static inline int adc_raw_to_millivolts_dt(const struct adc_dt_spec *spec,
int32_t *valp)
static inline int adc_raw_to_millivolts_dt(const struct adc_dt_spec *spec, int32_t *valp)
{
int32_t vref_mv;
uint8_t resolution;
return adc_raw_to_x_dt_chan(adc_raw_to_millivolts, spec, &spec->channel_cfg, valp);
}
if (!spec->channel_cfg_dt_node_exists) {
return -ENOTSUP;
}
if (spec->channel_cfg.reference == ADC_REF_INTERNAL) {
vref_mv = (int32_t)adc_ref_internal(spec->dev);
} else {
vref_mv = spec->vref_mv;
}
resolution = spec->resolution;
/*
* For differential channels, one bit less needs to be specified
* for resolution to achieve correct conversion.
*/
if (spec->channel_cfg.differential) {
resolution -= 1U;
}
return adc_raw_to_millivolts(vref_mv, spec->channel_cfg.gain,
resolution, valp);
/**
* @brief Convert a raw ADC value to microvolts using information stored
* in a struct adc_dt_spec.
*
* @param[in] spec ADC specification from Devicetree.
* @param[in,out] valp Pointer to the raw measurement value on input, and the
* corresponding microvolt value on successful conversion. If conversion fails
* the stored value is left unchanged.
*
* @return A value from adc_raw_to_microvolts() or -ENOTSUP if information from
* Devicetree is not valid.
* @see adc_raw_to_microvolts()
*/
static inline int adc_raw_to_microvolts_dt(const struct adc_dt_spec *spec, int32_t *valp)
{
return adc_raw_to_x_dt_chan(adc_raw_to_microvolts, spec, &spec->channel_cfg, valp);
}
/**