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adc: provide API to help with conversions

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Gain values are specified with enumeration values that can't be used
to reverse the effects of scaling the input signal.  Provide a
function that reverses the effect of the gain by scaling a measured
value.

Also provide a function that converts a raw measurement captured with
a reference voltage and specific gain and resolution to the
corresponding voltage in millivolts.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
This commit is contained in:
Peter A. Bigot 2019-12-26 13:16:21 -06:00 • committed by Carles Cufí
commit 4420c5ed40
3 changed files with 101 additions and 0 deletions
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1
drivers/adc/CMakeLists.txt
View file

@ -2,6 +2,7 @@
zephyr_library()
zephyr_library_sources_ifdef(CONFIG_ADC adc_common.c)
zephyr_library_sources_ifdef(CONFIG_ADC_SHELL adc_shell.c)
zephyr_library_sources_ifdef(CONFIG_ADC_MCUX_ADC12 adc_mcux_adc12.c)
zephyr_library_sources_ifdef(CONFIG_ADC_MCUX_ADC16 adc_mcux_adc16.c)

44
drivers/adc/adc_common.c Normal file
View file

@ -0,0 +1,44 @@
/*
* Copyright (c) 2019 Peter Bigot Consulting, LLC
*
* SPDX-License-Identifier: Apache-2.0
*/
#include <drivers/adc.h>
int adc_gain_invert(enum adc_gain gain,
s32_t *value)
{
struct gain_desc {
u8_t mul;
u8_t div;
};
static const struct gain_desc gains[] = {
[ADC_GAIN_1_6] = {.mul = 6, .div = 1},
[ADC_GAIN_1_5] = {.mul = 5, .div = 1},
[ADC_GAIN_1_4] = {.mul = 4, .div = 1},
[ADC_GAIN_1_3] = {.mul = 3, .div = 1},
[ADC_GAIN_1_2] = {.mul = 2, .div = 1},
[ADC_GAIN_2_3] = {.mul = 3, .div = 2},
[ADC_GAIN_1] = {.mul = 1, .div = 1},
[ADC_GAIN_2] = {.mul = 1, .div = 2},
[ADC_GAIN_3] = {.mul = 1, .div = 3},
[ADC_GAIN_4] = {.mul = 1, .div = 4},
[ADC_GAIN_8] = {.mul = 1, .div = 8},
[ADC_GAIN_16] = {.mul = 1, .div = 16},
[ADC_GAIN_32] = {.mul = 1, .div = 32},
[ADC_GAIN_64] = {.mul = 1, .div = 64},
};
int rv = -EINVAL;
if ((u8_t)gain < ARRAY_SIZE(gains)) {
const struct gain_desc *gdp = &gains[gain];
if ((gdp->mul != 0) && (gdp->div != 0)) {
*value = (gdp->mul * *value) / gdp->div;
rv = 0;
}
}
return rv;
}

56
include/drivers/adc.h
View file

@ -45,6 +45,25 @@ enum adc_gain {
ADC_GAIN_128, /**< x 128. */
};
/**
* @brief Invert the application of gain to a measurement value.
*
* For example, if the gain passed in is ADC_GAIN_1_6 and the
* referenced value is 10, the value after the function returns is 60.
*
* @param gain the gain used to amplify the input signal.
*
* @param value a pointer to a value that initially has the effect of
* the applied gain but has that effect removed when this function
* successfully returns. If the gain cannot be reversed the value
* remains unchanged.
*
* @retval 0 if the gain was successfully reversed
* @retval -EINVAL if the gain could not be interpreted
*/
int adc_gain_invert(enum adc_gain gain,
s32_t *value);
/** @brief ADC references. */
enum adc_reference {
ADC_REF_VDD_1, /**< VDD. */
@ -135,6 +154,43 @@ struct adc_channel_cfg {
#endif /* CONFIG_ADC_CONFIGURABLE_INPUTS */
};
/**
* @brief Convert a raw ADC value to millivolts.
*
* This function performs the necessary conversion to transform a raw
* ADC measurement to a voltage in millivolts.
*
* @param ref_mv the reference voltage used for the measurement, in
* millivolts. This may be from adc_ref_internal() or a known
* external reference.
*
* @param gain the ADC gain configuration used to sample the input
*
* @param resolution the number of bits in the absolute value of the
* sample. For differential sampling this may be one less than the
* resolution in struct adc_sequence.
*
* @param valp pointer to the raw measurement value on input, and the
* corresponding millivolt 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(s32_t ref_mv,
enum adc_gain gain,
u8_t resolution,
s32_t *valp)
{
s32_t adc_mv = *valp * ref_mv;
int ret = adc_gain_invert(gain, &adc_mv);
if (ret == 0) {
*valp = (adc_mv >> resolution);
}
return ret;
}
/* Forward declaration of the adc_sequence structure. */
struct adc_sequence;