Re: [PATCH v4] iio: adc: ad7124: fix sample rate for multi-channel use

[Jonathan Cameron <jic23@kernel.org>]

On Wed, 10 Sep 2025 12:33:29 -0500
David Lechner <dlechner@baylibre.com> 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 N)
> 
> where N depends on if SINGLE_CYCLE is set, the selected filter type and,
> in some cases, the power mode.
> 
> The FILTER register has a SINGLE_CYCLE bit that can be set to force the
> single channel case to use the same timing as the multi-channel case.
> 
> Before this change, the first formula was always used, so if all of the
> in_voltageY_sampling_frequency attributes were set to 10 Hz, then doing
> a buffered read with 1 channel enabled would result in the requested
> sampling frequency of 10 Hz. But when more than one channel was
> enabled, the actual sampling frequency would be 2.5 Hz per channel,
> which is 1/4 of the requested frequency.
> 
> After this change, the SINGLE_CYCLE flag is now always enabled and the
> multi-channel formula is now always used. This causes the sampling
> frequency to be consistent regardless of the number of channels enabled.
> 
> For now, we are hard-coding N = 4 since the driver doesn't yet support
> other filter types other than the default sinc4 filter.
> 
> The AD7124_FILTER_FS define is moved while we are touching this to
> keep the bit fields in descending order to be consistent with the rest
> of the file.
> 
> Signed-off-by: David Lechner <dlechner@baylibre.com>
Applied to the togreg branch of iio.git and pushed out as testing for all
the normal reasons.