linux-kernel - Re: [PATCH v3 3/9] iio: adc: mcp3911: use correct formula for AD conversion

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Date:   Sat, 16 Jul 2022 18:18:37 +0100
From:   Jonathan Cameron <jic23@...nel.org>
To:     Marcus Folkesson <marcus.folkesson@...il.com>
Cc:     Kent Gustavsson <kent@...oris.se>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/9] iio: adc: mcp3911: use correct formula for AD
 conversion

On Mon,  4 Jul 2022 19:21:10 +0200
Marcus Folkesson <marcus.folkesson@...il.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@...il.com>
> Acked-by: Jonathan Cameron <jic23@...nel.org>
?  I doubt I gave a tag (mostly because I don't give tags to patches
I'll pick up and give a SoB on.

Note that tags have to be given explicitly in full form before you can
add them to a patch.  If in doubt (people often given handwavey responses)
then ask people to give an explicit tag.

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;
>  	}
>