Re: [PATCH v4 3/9] iio: adc: mcp3911: use correct formula for AD conversion

[Jonathan Cameron <jic23@kernel.org>]

On Fri, 22 Jul 2022 15:07:20 +0200
Marcus Folkesson <marcus.folkesson@gmail.com> wrote:

> 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>
Patches 1-3 now applied to the fixes-togreg branch of iio.git
which is currently based on char-misc-linus which is at
Linus' tree just after he took the char-misc pull requests for
the merge window.  I might rebase this on rc1 once available
or I might just use the base as is.  Either way, it should be
fine in linux-next

Thanks,

Jonathan

> ---
>  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;
>  	}
>