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Subject[PATCH 1/2] 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)

Signed-off-by: Maciej Purski <m.purski@samsung.com>
---
drivers/iio/adc/ina2xx-adc.c | 59 ++++++++++++++++++++++++--------------------
1 file changed, 32 insertions(+), 27 deletions(-)

diff --git a/drivers/iio/adc/ina2xx-adc.c b/drivers/iio/adc/ina2xx-adc.c
index 84a4387..0f25087 100644
--- a/drivers/iio/adc/ina2xx-adc.c
+++ b/drivers/iio/adc/ina2xx-adc.c
@@ -110,11 +110,11 @@ enum ina2xx_ids { ina219, ina226 };

struct ina2xx_config {
u16 config_default;
- int calibration_factor;
+ int calibration_value;
int shunt_div;
int bus_voltage_shift;
int bus_voltage_lsb; /* uV */
- int power_lsb; /* uW */
+ int power_lsb_factor;
enum ina2xx_ids chip_id;
};

@@ -124,6 +124,8 @@ struct ina2xx_chip_info {
const struct ina2xx_config *config;
struct mutex state_lock;
unsigned int shunt_resistor_uohm;
+ unsigned int current_lsb_uA;
+ unsigned int power_lsb_uW;
int avg;
int int_time_vbus; /* Bus voltage integration time uS */
int int_time_vshunt; /* Shunt voltage integration time uS */
@@ -133,20 +135,20 @@ struct ina2xx_chip_info {
static const struct ina2xx_config ina2xx_config[] = {
[ina219] = {
.config_default = INA219_CONFIG_DEFAULT,
- .calibration_factor = 40960000,
+ .calibration_value = 4096,
.shunt_div = 100,
.bus_voltage_shift = 3,
.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_div = 400,
.bus_voltage_shift = 0,
.bus_voltage_lsb = 1250,
- .power_lsb = 25000,
+ .power_lsb_factor = 25,
.chip_id = ina226,
},
};
@@ -210,14 +212,15 @@ 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->power_lsb_uW;
*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;
}
}

@@ -434,28 +437,35 @@ 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 (eq. 3) 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);
+ chip->power_lsb_uW = chip->config->power_lsb_factor *
+ chip->current_lsb_uA;

return 0;
}
@@ -485,11 +495,6 @@ 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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