linux-kernel - Re: [PATCH] hwmon: (ads7828) Enable internal reference

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Date:	Tue, 19 Apr 2016 06:34:23 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Akshay Bhat <akshay.bhat@...esys.com>
Cc:	jdelvare@...e.com, linux-hwmon@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hwmon: (ads7828) Enable internal reference

On 04/18/2016 12:47 PM, Akshay Bhat wrote:
> On ads7828 the internal reference defaults to off upon power up. When
> using internal reference, it needs to be turned on and the voltage needs
> to settle before normal conversion cycle can be started. Hence perform a
> dummy read in the probe to enable the internal reference allowing the
> voltage to settle before performing a normal read.
>
> Without this fix, the first read from the ADC when using internal
> reference always returns incorrect data.
>
> Signed-off-by: Akshay Bhat <akshay.bhat@...esys.com>

Applied (after fixing 'volatage' in the comment below).

Thanks,
Guenter

> ---
>   drivers/hwmon/ads7828.c | 10 ++++++++++
>   1 file changed, 10 insertions(+)
>
> diff --git a/drivers/hwmon/ads7828.c b/drivers/hwmon/ads7828.c
> index 6c99ee7..b15c9bd 100644
> --- a/drivers/hwmon/ads7828.c
> +++ b/drivers/hwmon/ads7828.c
> @@ -120,6 +120,7 @@ static int ads7828_probe(struct i2c_client *client,
>   	unsigned int vref_mv = ADS7828_INT_VREF_MV;
>   	bool diff_input = false;
>   	bool ext_vref = false;
> +	unsigned int regval;
>
>   	data = devm_kzalloc(dev, sizeof(struct ads7828_data), GFP_KERNEL);
>   	if (!data)
> @@ -154,6 +155,15 @@ static int ads7828_probe(struct i2c_client *client,
>   	if (!diff_input)
>   		data->cmd_byte |= ADS7828_CMD_SD_SE;
>
> +	/*
> +	 * Datasheet specifies internal reference volatage is disabled by
> +	 * default. The internal reference voltage needs to be enabled and
> +	 * voltage needs to settle before getting valid ADC data. So perform a
> +	 * dummy read to enable the internal reference voltage.
> +	 */
> +	if (!ext_vref)
> +		regmap_read(data->regmap, data->cmd_byte, &regval);
> +
>   	hwmon_dev = devm_hwmon_device_register_with_groups(dev, client->name,
>   							   data,
>   							   ads7828_groups);
>