Re: [PATCH 2/2] iio: adc: ti_am335x_tscadc: Improve accuracy of measurement

[Jonathan Cameron]

On Mon, 19 Nov 2018 12:12:36 +0530
Vignesh R <vigneshr@xxxxxx> wrote:

> When performing single ended measurements with TSCADC, its recommended
> to set negative input (SEL_INM_SWC_3_0) of ADC step to ADC's VREFN in the
> corresponding STEP_CONFIGx register.
> Also, the positive(SEL_RFP_SWC_2_0) and negative(SEL_RFM_SWC_1_0)
> reference voltage for ADC step needs to be set to VREFP and VREFN
> respectively in STEP_CONFIGx register.
> Without these changes, there may be variation of as much as ~2% in the
> ADC's digital output which is bad for precise measurement.
> 
> Signed-off-by: Vignesh R <vigneshr@xxxxxx>
Given the header changes in mfd and the fact not much is going on with this
driver in IIO at the moment, I'm happy for this to go through the mfd tree.

Acked-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>

Thanks,

Jonathan

> ---
>  drivers/iio/adc/ti_am335x_adc.c      | 5 ++++-
>  include/linux/mfd/ti_am335x_tscadc.h | 4 ++++
>  2 files changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iio/adc/ti_am335x_adc.c b/drivers/iio/adc/ti_am335x_adc.c
> index cafb1dcadc48..9d984f2a8ba7 100644
> --- a/drivers/iio/adc/ti_am335x_adc.c
> +++ b/drivers/iio/adc/ti_am335x_adc.c
> @@ -142,7 +142,10 @@ static void tiadc_step_config(struct iio_dev *indio_dev)
>  			stepconfig |= STEPCONFIG_MODE_SWCNT;
>  
>  		tiadc_writel(adc_dev, REG_STEPCONFIG(steps),
> -				stepconfig | STEPCONFIG_INP(chan));
> +				stepconfig | STEPCONFIG_INP(chan) |
> +				STEPCONFIG_INM_ADCREFM |
> +				STEPCONFIG_RFP_VREFP |
> +				STEPCONFIG_RFM_VREFN);
>  
>  		if (adc_dev->open_delay[i] > STEPDELAY_OPEN_MASK) {
>  			dev_warn(dev, "chan %d open delay truncating to 0x3FFFF\n",
> diff --git a/include/linux/mfd/ti_am335x_tscadc.h b/include/linux/mfd/ti_am335x_tscadc.h
> index b9a53e013bff..483168403ae5 100644
> --- a/include/linux/mfd/ti_am335x_tscadc.h
> +++ b/include/linux/mfd/ti_am335x_tscadc.h
> @@ -78,6 +78,8 @@
>  #define STEPCONFIG_YNN		BIT(8)
>  #define STEPCONFIG_XNP		BIT(9)
>  #define STEPCONFIG_YPN		BIT(10)
> +#define STEPCONFIG_RFP(val)	((val) << 12)
> +#define STEPCONFIG_RFP_VREFP	(0x3 << 12)
>  #define STEPCONFIG_INM_MASK	(0xF << 15)
>  #define STEPCONFIG_INM(val)	((val) << 15)
>  #define STEPCONFIG_INM_ADCREFM	STEPCONFIG_INM(8)
> @@ -86,6 +88,8 @@
>  #define STEPCONFIG_INP_AN4	STEPCONFIG_INP(4)
>  #define STEPCONFIG_INP_ADCREFM	STEPCONFIG_INP(8)
>  #define STEPCONFIG_FIFO1	BIT(26)
> +#define STEPCONFIG_RFM(val)	((val) << 23)
> +#define STEPCONFIG_RFM_VREFN	(0x3 << 23)
>  
>  /* Delay register */
>  #define STEPDELAY_OPEN_MASK	(0x3FFFF << 0)