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    Subject[PATCH v4 3/9] iio: adc: mcp3911: use correct formula for AD conversion
    Date
    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: 3a89b289df5d ("iio: adc: add support for mcp3911")
    Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
    Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
    ---
    drivers/iio/adc/mcp3911.c | 17 ++++++++++++-----
    1 file changed, 12 insertions(+), 5 deletions(-)

    diff --git a/drivers/iio/adc/mcp3911.c b/drivers/iio/adc/mcp3911.c
    index f8875076ae80..890af7dca62d 100644
    --- a/drivers/iio/adc/mcp3911.c
    +++ b/drivers/iio/adc/mcp3911.c
    @@ -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;
    }

    --
    2.36.1
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