iio:st_pressure: document sampling gains
Details scaling factors and offsets applied to raw temperature and pressure samples. Signed-off-by: Gregor Boirie <gregor.boirie@parrot.com> Tested-by: Linus Walleij <linus.walleij@linaro.org> Signed-off-by: Jonathan Cameron <jic23@kernel.org>hifive-unleashed-5.1
Gregor Boirie
Jonathan Cameron
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c9d5e5b97e
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19b7b8a89b
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@ -28,6 +28,72 @@
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#include <linux/iio/common/st_sensors.h>
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#include "st_pressure.h"
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/*
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* About determining pressure scaling factors
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* ------------------------------------------
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*
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* Datasheets specify typical pressure sensitivity so that pressure is computed
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* according to the following equation :
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* pressure[mBar] = raw / sensitivity
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* where :
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* raw the 24 bits long raw sampled pressure
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* sensitivity a scaling factor specified by the datasheet in LSB/mBar
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*
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* IIO ABI expects pressure to be expressed as kPascal, hence pressure should be
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* computed according to :
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* pressure[kPascal] = pressure[mBar] / 10
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* = raw / (sensitivity * 10) (1)
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*
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* Finally, st_press_read_raw() returns pressure scaling factor as an
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* IIO_VAL_INT_PLUS_NANO with a zero integral part and "gain" as decimal part.
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* Therefore, from (1), "gain" becomes :
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* gain = 10^9 / (sensitivity * 10)
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* = 10^8 / sensitivity
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*
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* About determining temperature scaling factors and offsets
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* ---------------------------------------------------------
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*
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* Datasheets specify typical temperature sensitivity and offset so that
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* temperature is computed according to the following equation :
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* temp[Celsius] = offset[Celsius] + (raw / sensitivity)
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* where :
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* raw the 16 bits long raw sampled temperature
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* offset a constant specified by the datasheet in degree Celsius
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* (sometimes zero)
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* sensitivity a scaling factor specified by the datasheet in LSB/Celsius
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*
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* IIO ABI expects temperature to be expressed as milli degree Celsius such as
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* user space should compute temperature according to :
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* temp[mCelsius] = temp[Celsius] * 10^3
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* = (offset[Celsius] + (raw / sensitivity)) * 10^3
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* = ((offset[Celsius] * sensitivity) + raw) *
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* (10^3 / sensitivity) (2)
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*
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* IIO ABI expects user space to apply offset and scaling factors to raw samples
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* according to :
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* temp[mCelsius] = (OFFSET + raw) * SCALE
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* where :
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* OFFSET an arbitrary constant exposed by device
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* SCALE an arbitrary scaling factor exposed by device
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*
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* Matching OFFSET and SCALE with members of (2) gives :
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* OFFSET = offset[Celsius] * sensitivity (3)
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* SCALE = 10^3 / sensitivity (4)
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*
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* st_press_read_raw() returns temperature scaling factor as an
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* IIO_VAL_FRACTIONAL with a 10^3 numerator and "gain2" as denominator.
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* Therefore, from (3), "gain2" becomes :
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* gain2 = sensitivity
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*
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* When declared within channel, i.e. for a non zero specified offset,
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* st_press_read_raw() will return the latter as an IIO_VAL_FRACTIONAL such as :
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* numerator = OFFSET * 10^3
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* denominator = 10^3
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* giving from (4):
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* numerator = offset[Celsius] * 10^3 * sensitivity
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* = offset[mCelsius] * gain2
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*/
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#define MCELSIUS_PER_CELSIUS 1000
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/* Default pressure sensitivity */
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@ -48,7 +114,11 @@
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#define ST_PRESS_1_OUT_XL_ADDR 0x28
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#define ST_TEMP_1_OUT_L_ADDR 0x2b
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/* CUSTOM VALUES FOR LPS331AP SENSOR */
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/*
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* CUSTOM VALUES FOR LPS331AP SENSOR
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* See LPS331AP datasheet:
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* http://www2.st.com/resource/en/datasheet/lps331ap.pdf
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*/
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#define ST_PRESS_LPS331AP_WAI_EXP 0xbb
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#define ST_PRESS_LPS331AP_ODR_ADDR 0x20
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#define ST_PRESS_LPS331AP_ODR_MASK 0x70
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@ -71,7 +141,9 @@
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#define ST_PRESS_LPS331AP_OD_IRQ_MASK 0x40
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#define ST_PRESS_LPS331AP_MULTIREAD_BIT true
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/* CUSTOM VALUES FOR LPS001WP SENSOR */
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/*
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* CUSTOM VALUES FOR THE OBSOLETE LPS001WP SENSOR
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*/
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/* LPS001WP pressure resolution */
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#define ST_PRESS_LPS001WP_LSB_PER_MBAR 16UL
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@ -94,7 +166,11 @@
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#define ST_PRESS_LPS001WP_OUT_L_ADDR 0x28
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#define ST_TEMP_LPS001WP_OUT_L_ADDR 0x2a
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/* CUSTOM VALUES FOR LPS25H SENSOR */
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/*
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* CUSTOM VALUES FOR LPS25H SENSOR
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* See LPS25H datasheet:
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* http://www2.st.com/resource/en/datasheet/lps25h.pdf
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*/
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#define ST_PRESS_LPS25H_WAI_EXP 0xbd
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#define ST_PRESS_LPS25H_ODR_ADDR 0x20
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#define ST_PRESS_LPS25H_ODR_MASK 0x70
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@ -117,7 +193,11 @@
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#define ST_PRESS_LPS25H_OUT_XL_ADDR 0x28
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#define ST_TEMP_LPS25H_OUT_L_ADDR 0x2b
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/* CUSTOM VALUES FOR LPS22HB SENSOR */
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/*
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* CUSTOM VALUES FOR LPS22HB SENSOR
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* See LPS22HB datasheet:
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* http://www2.st.com/resource/en/datasheet/lps22hb.pdf
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*/
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#define ST_PRESS_LPS22HB_WAI_EXP 0xb1
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#define ST_PRESS_LPS22HB_ODR_ADDR 0x10
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#define ST_PRESS_LPS22HB_ODR_MASK 0x70
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@ -413,6 +493,10 @@ static const struct st_sensor_settings st_press_sensors_settings[] = {
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},
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.fs = {
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.fs_avl = {
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/*
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* Sensitivity values as defined in table 3 of
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* LPS22HB datasheet.
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*/
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[0] = {
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.num = ST_PRESS_FS_AVL_1260MB,
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.gain = ST_PRESS_KPASCAL_NANO_SCALE,
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