379 lines
9.2 KiB
C
379 lines
9.2 KiB
C
// SPDX-License-Identifier: GPL-2.0+
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/*
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* sgp40.c - Support for Sensirion SGP40 Gas Sensor
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*
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* Copyright (C) 2021 Andreas Klinger <ak@it-klinger.de>
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*
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* I2C slave address: 0x59
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*
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* Datasheet can be found here:
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* https://www.sensirion.com/file/datasheet_sgp40
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*
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* There are two functionalities supported:
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*
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* 1) read raw logarithmic resistance value from sensor
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* --> useful to pass it to the algorithm of the sensor vendor for
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* measuring deteriorations and improvements of air quality.
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*
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* 2) calculate an estimated absolute voc index (0 - 500 index points) for
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* measuring the air quality.
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* For this purpose the value of the resistance for which the voc index
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* will be 250 can be set up using calibbias.
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*
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* Compensation values of relative humidity and temperature can be set up
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* by writing to the out values of temp and humidityrelative.
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*/
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#include <linux/delay.h>
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#include <linux/crc8.h>
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#include <linux/module.h>
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#include <linux/mutex.h>
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#include <linux/i2c.h>
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#include <linux/iio/iio.h>
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/*
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* floating point calculation of voc is done as integer
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* where numbers are multiplied by 1 << SGP40_CALC_POWER
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*/
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#define SGP40_CALC_POWER 14
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#define SGP40_CRC8_POLYNOMIAL 0x31
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#define SGP40_CRC8_INIT 0xff
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DECLARE_CRC8_TABLE(sgp40_crc8_table);
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struct sgp40_data {
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struct device *dev;
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struct i2c_client *client;
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int rht;
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int temp;
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int res_calibbias;
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/* Prevent concurrent access to rht, tmp, calibbias */
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struct mutex lock;
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};
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struct sgp40_tg_measure {
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u8 command[2];
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__be16 rht_ticks;
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u8 rht_crc;
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__be16 temp_ticks;
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u8 temp_crc;
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} __packed;
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struct sgp40_tg_result {
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__be16 res_ticks;
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u8 res_crc;
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} __packed;
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static const struct iio_chan_spec sgp40_channels[] = {
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{
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.type = IIO_CONCENTRATION,
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.channel2 = IIO_MOD_VOC,
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.info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED),
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},
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{
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.type = IIO_RESISTANCE,
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.info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |
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BIT(IIO_CHAN_INFO_CALIBBIAS),
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},
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{
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.type = IIO_TEMP,
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.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
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.output = 1,
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},
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{
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.type = IIO_HUMIDITYRELATIVE,
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.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
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.output = 1,
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},
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};
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/*
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* taylor approximation of e^x:
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* y = 1 + x + x^2 / 2 + x^3 / 6 + x^4 / 24 + ... + x^n / n!
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*
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* Because we are calculating x real value multiplied by 2^power we get
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* an additional 2^power^n to divide for every element. For a reasonable
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* precision this would overflow after a few iterations. Therefore we
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* divide the x^n part whenever its about to overflow (xmax).
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*/
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static u32 sgp40_exp(int exp, u32 power, u32 rounds)
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{
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u32 x, y, xp;
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u32 factorial, divider, xmax;
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int sign = 1;
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int i;
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if (exp == 0)
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return 1 << power;
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else if (exp < 0) {
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sign = -1;
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exp *= -1;
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}
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xmax = 0x7FFFFFFF / exp;
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x = exp;
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xp = 1;
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factorial = 1;
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y = 1 << power;
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divider = 0;
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for (i = 1; i <= rounds; i++) {
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xp *= x;
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factorial *= i;
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y += (xp >> divider) / factorial;
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divider += power;
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/* divide when next multiplication would overflow */
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if (xp >= xmax) {
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xp >>= power;
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divider -= power;
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}
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}
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if (sign == -1)
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return (1 << (power * 2)) / y;
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else
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return y;
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}
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static int sgp40_calc_voc(struct sgp40_data *data, u16 resistance_raw, int *voc)
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{
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int x;
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u32 exp = 0;
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/* we calculate as a multiple of 16384 (2^14) */
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mutex_lock(&data->lock);
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x = ((int)resistance_raw - data->res_calibbias) * 106;
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mutex_unlock(&data->lock);
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/* voc = 500 / (1 + e^x) */
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exp = sgp40_exp(x, SGP40_CALC_POWER, 18);
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*voc = 500 * ((1 << (SGP40_CALC_POWER * 2)) / ((1<<SGP40_CALC_POWER) + exp));
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dev_dbg(data->dev, "raw: %d res_calibbias: %d x: %d exp: %d voc: %d\n",
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resistance_raw, data->res_calibbias, x, exp, *voc);
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return 0;
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}
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static int sgp40_measure_resistance_raw(struct sgp40_data *data, u16 *resistance_raw)
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{
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int ret;
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struct i2c_client *client = data->client;
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u32 ticks;
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u16 ticks16;
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u8 crc;
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struct sgp40_tg_measure tg = {.command = {0x26, 0x0F}};
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struct sgp40_tg_result tgres;
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mutex_lock(&data->lock);
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ticks = (data->rht / 10) * 65535 / 10000;
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ticks16 = (u16)clamp(ticks, 0u, 65535u); /* clamp between 0 .. 100 %rH */
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tg.rht_ticks = cpu_to_be16(ticks16);
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tg.rht_crc = crc8(sgp40_crc8_table, (u8 *)&tg.rht_ticks, 2, SGP40_CRC8_INIT);
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ticks = ((data->temp + 45000) / 10 ) * 65535 / 17500;
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ticks16 = (u16)clamp(ticks, 0u, 65535u); /* clamp between -45 .. +130 °C */
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tg.temp_ticks = cpu_to_be16(ticks16);
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tg.temp_crc = crc8(sgp40_crc8_table, (u8 *)&tg.temp_ticks, 2, SGP40_CRC8_INIT);
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mutex_unlock(&data->lock);
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ret = i2c_master_send(client, (const char *)&tg, sizeof(tg));
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if (ret != sizeof(tg)) {
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dev_warn(data->dev, "i2c_master_send ret: %d sizeof: %zu\n", ret, sizeof(tg));
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return -EIO;
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}
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msleep(30);
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ret = i2c_master_recv(client, (u8 *)&tgres, sizeof(tgres));
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if (ret < 0)
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return ret;
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if (ret != sizeof(tgres)) {
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dev_warn(data->dev, "i2c_master_recv ret: %d sizeof: %zu\n", ret, sizeof(tgres));
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return -EIO;
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}
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crc = crc8(sgp40_crc8_table, (u8 *)&tgres.res_ticks, 2, SGP40_CRC8_INIT);
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if (crc != tgres.res_crc) {
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dev_err(data->dev, "CRC error while measure-raw\n");
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return -EIO;
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}
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*resistance_raw = be16_to_cpu(tgres.res_ticks);
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return 0;
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}
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static int sgp40_read_raw(struct iio_dev *indio_dev,
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struct iio_chan_spec const *chan, int *val,
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int *val2, long mask)
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{
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struct sgp40_data *data = iio_priv(indio_dev);
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int ret, voc;
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u16 resistance_raw;
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switch (mask) {
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case IIO_CHAN_INFO_RAW:
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switch (chan->type) {
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case IIO_RESISTANCE:
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ret = sgp40_measure_resistance_raw(data, &resistance_raw);
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if (ret)
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return ret;
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*val = resistance_raw;
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return IIO_VAL_INT;
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case IIO_TEMP:
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mutex_lock(&data->lock);
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*val = data->temp;
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mutex_unlock(&data->lock);
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return IIO_VAL_INT;
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case IIO_HUMIDITYRELATIVE:
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mutex_lock(&data->lock);
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*val = data->rht;
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mutex_unlock(&data->lock);
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return IIO_VAL_INT;
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default:
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return -EINVAL;
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}
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case IIO_CHAN_INFO_PROCESSED:
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ret = sgp40_measure_resistance_raw(data, &resistance_raw);
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if (ret)
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return ret;
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ret = sgp40_calc_voc(data, resistance_raw, &voc);
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if (ret)
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return ret;
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*val = voc / (1 << SGP40_CALC_POWER);
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/*
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* calculation should fit into integer, where:
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* voc <= (500 * 2^SGP40_CALC_POWER) = 8192000
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* (with SGP40_CALC_POWER = 14)
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*/
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*val2 = ((voc % (1 << SGP40_CALC_POWER)) * 244) / (1 << (SGP40_CALC_POWER - 12));
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dev_dbg(data->dev, "voc: %d val: %d.%06d\n", voc, *val, *val2);
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return IIO_VAL_INT_PLUS_MICRO;
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case IIO_CHAN_INFO_CALIBBIAS:
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mutex_lock(&data->lock);
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*val = data->res_calibbias;
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mutex_unlock(&data->lock);
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return IIO_VAL_INT;
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default:
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return -EINVAL;
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}
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}
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static int sgp40_write_raw(struct iio_dev *indio_dev,
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struct iio_chan_spec const *chan, int val,
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int val2, long mask)
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{
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struct sgp40_data *data = iio_priv(indio_dev);
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switch (mask) {
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case IIO_CHAN_INFO_RAW:
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switch (chan->type) {
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case IIO_TEMP:
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if ((val < -45000) || (val > 130000))
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return -EINVAL;
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mutex_lock(&data->lock);
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data->temp = val;
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mutex_unlock(&data->lock);
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return 0;
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case IIO_HUMIDITYRELATIVE:
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if ((val < 0) || (val > 100000))
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return -EINVAL;
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mutex_lock(&data->lock);
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data->rht = val;
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mutex_unlock(&data->lock);
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return 0;
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default:
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return -EINVAL;
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}
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case IIO_CHAN_INFO_CALIBBIAS:
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if ((val < 20000) || (val > 52768))
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return -EINVAL;
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mutex_lock(&data->lock);
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data->res_calibbias = val;
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mutex_unlock(&data->lock);
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return 0;
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}
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return -EINVAL;
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}
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static const struct iio_info sgp40_info = {
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.read_raw = sgp40_read_raw,
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.write_raw = sgp40_write_raw,
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};
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static int sgp40_probe(struct i2c_client *client)
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{
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const struct i2c_device_id *id = i2c_client_get_device_id(client);
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struct device *dev = &client->dev;
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struct iio_dev *indio_dev;
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struct sgp40_data *data;
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int ret;
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indio_dev = devm_iio_device_alloc(dev, sizeof(*data));
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if (!indio_dev)
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return -ENOMEM;
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data = iio_priv(indio_dev);
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data->client = client;
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data->dev = dev;
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crc8_populate_msb(sgp40_crc8_table, SGP40_CRC8_POLYNOMIAL);
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mutex_init(&data->lock);
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/* set default values */
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data->rht = 50000; /* 50 % */
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data->temp = 25000; /* 25 °C */
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data->res_calibbias = 30000; /* resistance raw value for voc index of 250 */
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indio_dev->info = &sgp40_info;
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indio_dev->name = id->name;
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indio_dev->modes = INDIO_DIRECT_MODE;
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indio_dev->channels = sgp40_channels;
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indio_dev->num_channels = ARRAY_SIZE(sgp40_channels);
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ret = devm_iio_device_register(dev, indio_dev);
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if (ret)
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dev_err(dev, "failed to register iio device\n");
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return ret;
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}
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static const struct i2c_device_id sgp40_id[] = {
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{ "sgp40" },
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{ }
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};
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MODULE_DEVICE_TABLE(i2c, sgp40_id);
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static const struct of_device_id sgp40_dt_ids[] = {
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{ .compatible = "sensirion,sgp40" },
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{ }
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};
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MODULE_DEVICE_TABLE(of, sgp40_dt_ids);
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static struct i2c_driver sgp40_driver = {
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.driver = {
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.name = "sgp40",
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.of_match_table = sgp40_dt_ids,
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},
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.probe_new = sgp40_probe,
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.id_table = sgp40_id,
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};
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module_i2c_driver(sgp40_driver);
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MODULE_AUTHOR("Andreas Klinger <ak@it-klinger.de>");
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MODULE_DESCRIPTION("Sensirion SGP40 gas sensor");
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MODULE_LICENSE("GPL v2");
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