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iio: adc: mcp3911: use correct formula for AD conversion

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The ADC conversion is actually not rail-to-rail but with a factor 1.5.
Make use of this factor when calculating actual voltage.

Fixes: 3a89b289df ("iio: adc: add support for mcp3911")
Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220722130726.7627-4-marcus.folkesson@gmail.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
This commit is contained in:
Marcus Folkesson 2022-07-22 15:07:20 +02:00 committed by Jonathan Cameron
parent cfbd76d5c9
commit 9e2238e3ae
1 changed files with 12 additions and 5 deletions
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17
drivers/iio/adc/mcp3911.c
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@ -40,8 +40,8 @@
#define MCP3911_CHANNEL(x) (MCP3911_REG_CHANNEL0 + x * 3)
#define MCP3911_OFFCAL(x) (MCP3911_REG_OFFCAL_CH0 + x * 6)
/* Internal voltage reference in uV */
#define MCP3911_INT_VREF_UV 1200000
/* Internal voltage reference in mV */
#define MCP3911_INT_VREF_MV 1200
#define MCP3911_REG_READ(reg, id) ((((reg) << 1) | ((id) << 5) | (1 << 0)) & 0xff)
#define MCP3911_REG_WRITE(reg, id) ((((reg) << 1) | ((id) << 5) | (0 << 0)) & 0xff)
@ -139,11 +139,18 @@ static int mcp3911_read_raw(struct iio_dev *indio_dev,
*val = ret / 1000;
} else {
*val = MCP3911_INT_VREF_UV;
*val = MCP3911_INT_VREF_MV;
}
*val2 = 24;
ret = IIO_VAL_FRACTIONAL_LOG2;
/*
* For 24bit Conversion
* Raw = ((Voltage)/(Vref) * 2^23 * Gain * 1.5
* Voltage = Raw * (Vref)/(2^23 * Gain * 1.5)
*/
/* val2 = (2^23 * 1.5) */
*val2 = 12582912;
ret = IIO_VAL_FRACTIONAL;
break;
}