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Subject[PATCH v2] iio: adc: ina2xx: Make calibration register value fixed
Date
Calibration register is used for calculating current register in
hardware according to datasheet:
current = shunt_volt * calib_register / 2048 (ina 226)
current = shunt_volt * calib_register / 4096 (ina 219)

Fix calib_register value to 2048 for ina226 and 4096 for ina 219 in
order to avoid truncation error and provide best precision allowed
by shunt_voltage measurement. Make current scale value follow changes
of shunt_resistor from sysfs as calib_register value is now fixed.

Power_lsb value should also follow shunt_resistor changes as stated in
datasheet:
power_lsb = 25 * current_lsb (ina 226)
power_lsb = 20 * current_lsb (ina 219)

This is a part of the patchset: https://lkml.org/lkml/2017/11/22/394

Signed-off-by: Maciej Purski <m.purski@samsung.com>

---
Changes in v2:
- rebase on top of the latest next
- remove a redundant variable - power_lsb_uW
- fix comments
---
drivers/iio/adc/ina2xx-adc.c | 58 +++++++++++++++++++++++---------------------
1 file changed, 31 insertions(+), 27 deletions(-)

diff --git a/drivers/iio/adc/ina2xx-adc.c b/drivers/iio/adc/ina2xx-adc.c
index ddf8781..3e4972f 100644
--- a/drivers/iio/adc/ina2xx-adc.c
+++ b/drivers/iio/adc/ina2xx-adc.c
@@ -124,11 +124,11 @@ enum ina2xx_ids { ina219, ina226 };

struct ina2xx_config {
u16 config_default;
- int calibration_factor;
+ int calibration_value;
int shunt_voltage_lsb; /* nV */
int bus_voltage_shift; /* position of lsb */
int bus_voltage_lsb; /* uV */
- int power_lsb; /* uW */
+ int power_lsb_factor;
enum ina2xx_ids chip_id;
};

@@ -138,6 +138,7 @@ struct ina2xx_chip_info {
const struct ina2xx_config *config;
struct mutex state_lock;
unsigned int shunt_resistor_uohm;
+ unsigned int current_lsb_uA;
int avg;
int int_time_vbus; /* Bus voltage integration time uS */
int int_time_vshunt; /* Shunt voltage integration time uS */
@@ -149,20 +150,20 @@ struct ina2xx_chip_info {
static const struct ina2xx_config ina2xx_config[] = {
[ina219] = {
.config_default = INA219_CONFIG_DEFAULT,
- .calibration_factor = 40960000,
+ .calibration_value = 4096,
.shunt_voltage_lsb = 10000,
.bus_voltage_shift = INA219_BUS_VOLTAGE_SHIFT,
.bus_voltage_lsb = 4000,
- .power_lsb = 20000,
+ .power_lsb_factor = 20,
.chip_id = ina219,
},
[ina226] = {
.config_default = INA226_CONFIG_DEFAULT,
- .calibration_factor = 5120000,
+ .calibration_value = 2048,
.shunt_voltage_lsb = 2500,
.bus_voltage_shift = 0,
.bus_voltage_lsb = 1250,
- .power_lsb = 25000,
+ .power_lsb_factor = 25,
.chip_id = ina226,
},
};
@@ -229,14 +230,16 @@ static int ina2xx_read_raw(struct iio_dev *indio_dev,

case INA2XX_POWER:
/* processed (mW) = raw*lsb (uW) / 1000 */
- *val = chip->config->power_lsb;
+ *val = chip->config->power_lsb_factor *
+ chip->current_lsb_uA;
*val2 = 1000;
return IIO_VAL_FRACTIONAL;

case INA2XX_CURRENT:
- /* processed (mA) = raw (mA) */
- *val = 1;
- return IIO_VAL_INT;
+ /* processed (mA) = raw*lsb (uA) / 1000 */
+ *val = chip->current_lsb_uA;
+ *val2 = 1000;
+ return IIO_VAL_FRACTIONAL;
}

case IIO_CHAN_INFO_HARDWAREGAIN:
@@ -541,28 +544,34 @@ static ssize_t ina2xx_allow_async_readout_store(struct device *dev,
}

/*
- * Set current LSB to 1mA, shunt is in uOhms
- * (equation 13 in datasheet). We hardcode a Current_LSB
- * of 1.0 x10-3. The only remaining parameter is RShunt.
- * There is no need to expose the CALIBRATION register
- * to the user for now. But we need to reset this register
- * if the user updates RShunt after driver init, e.g upon
- * reading an EEPROM/Probe-type value.
+ * Calibration register is set to the best value, which eliminates
+ * truncation errors on calculating current register in hardware.
+ * According to datasheet (INA 226: eq. 3, INA219: eq. 4) the best values
+ * are 2048 for ina226 and 4096 for ina219. They are hardcoded as
+ * calibration_value.
*/
static int ina2xx_set_calibration(struct ina2xx_chip_info *chip)
{
- u16 regval = DIV_ROUND_CLOSEST(chip->config->calibration_factor,
- chip->shunt_resistor_uohm);
-
- return regmap_write(chip->regmap, INA2XX_CALIBRATION, regval);
+ return regmap_write(chip->regmap, INA2XX_CALIBRATION,
+ chip->config->calibration_value);
}

+/*
+ * In order to keep calibration register value fixed, the product
+ * of current_lsb and shunt_resistor should also be fixed and equal
+ * to shunt_voltage_lsb = 1 / shunt_div multiplied by 10^9 in order
+ * to keep the scale.
+ */
static int set_shunt_resistor(struct ina2xx_chip_info *chip, unsigned int val)
{
- if (val <= 0 || val > chip->config->calibration_factor)
+ unsigned int dividend = DIV_ROUND_CLOSEST(1000000000,
+ chip->config->shunt_div);
+
+ if (val <= 0 || val > dividend)
return -EINVAL;

chip->shunt_resistor_uohm = val;
+ chip->current_lsb_uA = DIV_ROUND_CLOSEST(dividend, val);

return 0;
}
@@ -592,11 +601,6 @@ static ssize_t ina2xx_shunt_resistor_store(struct device *dev,
if (ret)
return ret;

- /* Update the Calibration register */
- ret = ina2xx_set_calibration(chip);
- if (ret)
- return ret;
-
return len;
}

--
2.7.4
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