Details scaling factors and offsets applied to raw temperature and pressure samples. Signed-off-by: Gregor Boirie <gregor.boirie@xxxxxxxxxx> --- drivers/iio/pressure/st_pressure_core.c | 92 +++++++++++++++++++++++++++++++-- 1 file changed, 88 insertions(+), 4 deletions(-) diff --git a/drivers/iio/pressure/st_pressure_core.c b/drivers/iio/pressure/st_pressure_core.c index 8fa3d81..2ab1056 100644 --- a/drivers/iio/pressure/st_pressure_core.c +++ b/drivers/iio/pressure/st_pressure_core.c @@ -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, -- 2.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-iio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
