linux-kernel - Re: [PATCH v2] iio: adc: ad7124: fix sample rate for multi-channel use

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Message-ID: <1ae7994c-4943-4a91-aea0-218c5bd331b2@baylibre.com>
Date: Thu, 4 Sep 2025 14:53:38 -0500
From: David Lechner <dlechner@...libre.com>
To: Michael Hennerich <Michael.Hennerich@...log.com>,
 Jonathan Cameron <jic23@...nel.org>, Nuno Sá
 <nuno.sa@...log.com>, Andy Shevchenko <andy@...nel.org>
Cc: linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] iio: adc: ad7124: fix sample rate for multi-channel
 use

On 9/4/25 11:19 AM, David Lechner wrote:
> Change how the FS[10:0] field of the FILTER register is calculated to
> get consistent sample rates when only one channel is enabled vs when
> multiple channels are enabled in a buffered read.
> 
> By default, the AD7124 allows larger sampling frequencies when only one
> channel is enabled. It assumes that you will discard the first sample or
> so to allow for settling time and then no additional settling time is
> needed between samples because there is no multiplexing due to only one
> channel being enabled. The conversion formula to convert between the
> sampling frequency and the FS[10:0] field is:
> 
>     fADC = fCLK / (FS[10:0] x 32)
> 
> which is what the driver has been using.
> 
> On the other hand, when multiple channels are enabled, there is
> additional settling time needed when switching between channels so the
> calculation to convert between becomes:
> 
>     fADC = fCLK / (FS[10:0] x 32 x (4 + AVG - 1))
> 
Additional testing on top of this patch while implementing filter types has
shown that this formula is actually not what we will need in the future.
Rather, we will need a whole series of undocumented formulas that depends
on the filter type.

I will send a v3 that drops all of the AVG stuff.