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

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commit 9e2238e3ae40d371a1130226e0e740aa1601efa6 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Marcus Folkesson 2022-07-22 15:07:20 +02:00 committed by Greg Kroah-Hartman
parent d186c65599
commit 0d90ef8749
1 changed files with 12 additions and 5 deletions
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17
drivers/iio/adc/mcp3911.c
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@ -38,8 +38,8 @@
#define MCP3911_CHANNEL(x) (MCP3911_REG_CHANNEL0 + x * 3) #define MCP3911_CHANNEL(x) (MCP3911_REG_CHANNEL0 + x * 3)
#define MCP3911_OFFCAL(x) (MCP3911_REG_OFFCAL_CH0 + x * 6) #define MCP3911_OFFCAL(x) (MCP3911_REG_OFFCAL_CH0 + x * 6)
/* Internal voltage reference in uV */ /* Internal voltage reference in mV */
#define MCP3911_INT_VREF_UV 1200000 #define MCP3911_INT_VREF_MV 1200
#define MCP3911_REG_READ(reg, id) ((((reg) << 1) | ((id) << 5) | (1 << 0)) & 0xff) #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) #define MCP3911_REG_WRITE(reg, id) ((((reg) << 1) | ((id) << 5) | (0 << 0)) & 0xff)
@ -137,11 +137,18 @@ static int mcp3911_read_raw(struct iio_dev *indio_dev,
*val = ret / 1000; *val = ret / 1000;
} else { } 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; break;
} }