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Commit 19b7b8a8 authored by Gregor Boirie's avatar Gregor Boirie Committed by Jonathan Cameron
Browse files

iio:st_pressure: document sampling gains 



Details scaling factors and offsets applied to raw temperature and pressure
samples.

Signed-off-by: default avatarGregor Boirie <gregor.boirie@parrot.com>
Tested-by: default avatarLinus Walleij <linus.walleij@linaro.org>
Signed-off-by: default avatarJonathan Cameron <jic23@kernel.org>
parent c9d5e5b9

drivers/iio/pressure/st_pressure_core.c

+88 −4
Original line number Diff line number Diff line
@@ -28,6 +28,72 @@ 
#include <linux/iio/common/st_sensors.h>

#include "st_pressure.h"



/*

 * About determining pressure scaling factors

 * ------------------------------------------

 *

 * Datasheets specify typical pressure sensitivity so that pressure is computed

 * according to the following equation :

 *     pressure[mBar] = raw / sensitivity

 * where :

 *     raw          the 24 bits long raw sampled pressure

 *     sensitivity  a scaling factor specified by the datasheet in LSB/mBar

 *

 * IIO ABI expects pressure to be expressed as kPascal, hence pressure should be

 * computed according to :

 *     pressure[kPascal] = pressure[mBar] / 10

 *                       = raw / (sensitivity * 10)                          (1)

 *

 * Finally, st_press_read_raw() returns pressure scaling factor as an

 * IIO_VAL_INT_PLUS_NANO with a zero integral part and "gain" as decimal part.

 * Therefore, from (1), "gain" becomes :

 *     gain = 10^9 / (sensitivity * 10)

 *          = 10^8 / sensitivity

 *

 * About determining temperature scaling factors and offsets

 * ---------------------------------------------------------

 *

 * Datasheets specify typical temperature sensitivity and offset so that

 * temperature is computed according to the following equation :

 *     temp[Celsius] = offset[Celsius] + (raw / sensitivity)

 * where :

 *     raw          the 16 bits long raw sampled temperature

 *     offset       a constant specified by the datasheet in degree Celsius

 *                  (sometimes zero)

 *     sensitivity  a scaling factor specified by the datasheet in LSB/Celsius

 *

 * IIO ABI expects temperature to be expressed as milli degree Celsius such as

 * user space should compute temperature according to :

 *     temp[mCelsius] = temp[Celsius] * 10^3

 *                    = (offset[Celsius] + (raw / sensitivity)) * 10^3

 *                    = ((offset[Celsius] * sensitivity) + raw) *

 *                      (10^3 / sensitivity)                                 (2)

 *

 * IIO ABI expects user space to apply offset and scaling factors to raw samples

 * according to :

 *     temp[mCelsius] = (OFFSET + raw) * SCALE

 * where :

 *     OFFSET an arbitrary constant exposed by device

 *     SCALE  an arbitrary scaling factor exposed by device

 *

 * Matching OFFSET and SCALE with members of (2) gives :

 *     OFFSET = offset[Celsius] * sensitivity                                (3)

 *     SCALE  = 10^3 / sensitivity                                           (4)

 *

 * st_press_read_raw() returns temperature scaling factor as an

 * IIO_VAL_FRACTIONAL with a 10^3 numerator and "gain2" as denominator.

 * Therefore, from (3), "gain2" becomes :

 *     gain2 = sensitivity

 *

 * When declared within channel, i.e. for a non zero specified offset,

 * st_press_read_raw() will return the latter as an IIO_VAL_FRACTIONAL such as :

 *     numerator = OFFSET * 10^3

 *     denominator = 10^3

 * giving from (4):

 *     numerator = offset[Celsius] * 10^3 * sensitivity

 *               = offset[mCelsius] * gain2

 */



#define MCELSIUS_PER_CELSIUS			1000



/* Default pressure sensitivity */
@@ -48,7 +114,11 @@ 
#define ST_PRESS_1_OUT_XL_ADDR			0x28

#define ST_TEMP_1_OUT_L_ADDR			0x2b



/* CUSTOM VALUES FOR LPS331AP SENSOR */

/*

 * CUSTOM VALUES FOR LPS331AP SENSOR

 * See LPS331AP datasheet:

 * http://www2.st.com/resource/en/datasheet/lps331ap.pdf

 */

#define ST_PRESS_LPS331AP_WAI_EXP		0xbb

#define ST_PRESS_LPS331AP_ODR_ADDR		0x20

#define ST_PRESS_LPS331AP_ODR_MASK		0x70
@@ -71,7 +141,9 @@ 
#define ST_PRESS_LPS331AP_OD_IRQ_MASK		0x40

#define ST_PRESS_LPS331AP_MULTIREAD_BIT		true



/* CUSTOM VALUES FOR LPS001WP SENSOR */

/*

 * CUSTOM VALUES FOR THE OBSOLETE LPS001WP SENSOR

 */



/* LPS001WP pressure resolution */

#define ST_PRESS_LPS001WP_LSB_PER_MBAR		16UL
@@ -94,7 +166,11 @@ 
#define ST_PRESS_LPS001WP_OUT_L_ADDR		0x28

#define ST_TEMP_LPS001WP_OUT_L_ADDR		0x2a



/* CUSTOM VALUES FOR LPS25H SENSOR */

/*

 * CUSTOM VALUES FOR LPS25H SENSOR

 * See LPS25H datasheet:

 * http://www2.st.com/resource/en/datasheet/lps25h.pdf

 */

#define ST_PRESS_LPS25H_WAI_EXP			0xbd

#define ST_PRESS_LPS25H_ODR_ADDR		0x20

#define ST_PRESS_LPS25H_ODR_MASK		0x70
@@ -117,7 +193,11 @@ 
#define ST_PRESS_LPS25H_OUT_XL_ADDR		0x28

#define ST_TEMP_LPS25H_OUT_L_ADDR		0x2b



/* CUSTOM VALUES FOR LPS22HB SENSOR */

/*

 * CUSTOM VALUES FOR LPS22HB SENSOR

 * See LPS22HB datasheet:

 * http://www2.st.com/resource/en/datasheet/lps22hb.pdf

 */

#define ST_PRESS_LPS22HB_WAI_EXP		0xb1

#define ST_PRESS_LPS22HB_ODR_ADDR		0x10

#define ST_PRESS_LPS22HB_ODR_MASK		0x70
@@ -413,6 +493,10 @@ static const struct st_sensor_settings st_press_sensors_settings[] = { 
		},

		.fs = {

			.fs_avl = {

				/*

				 * Sensitivity values as defined in table 3 of

				 * LPS22HB datasheet.

				 */

				[0] = {

					.num = ST_PRESS_FS_AVL_1260MB,

					.gain = ST_PRESS_KPASCAL_NANO_SCALE,