Re: [bmp280 v1 4/6] iio: pressure: bmp280: enable filter settings for BMx280

[ASSI <Stromeko@nexgo.de>]

Jonathan Cameron writes:
> On Sun,  3 Aug 2025 16:08:00 +0200
> Achim Gratz <Achim.Gratz@Stromeko.DE> wrote:
>
>> These devices were using a hardcoded IIR filter of length 4.  Enable
>> filter_low_pass_3db_frequency settings to control the filter length
>> settings of the device (as done already for the BMx380 and BMx580
>> devices, even though the 3dB corner has an inverse relation to the
>> filter length).  Remove an offset of 1 from the internal handling of
>> the available values.
>
> This confuses me.  Are we saying those other devices have a 3db frequency
> control that is not obeying the ABI? If so that sneaked in past me but
> please don't continue that (and we should fix that broken use of the ABI).

This is correct.  The attribute directly controls the tap length of the
filter and more taps means lower 3dB corner frequency.  Besides, it
isn't easy to even figure out what the corner frequency is, since it
depends both on further settings of the device (provided you want the
actual corner frequency and not one that's normalized to the sampling
frequency) and the internal IIR equation, although you could sort of
reverse engineer it from the step response graph in the datasheet since
it's unlikely to be something complex.  I've had a brief look, but there
doesn't appear to be a more appropriate attribute that could be used
within the IIO framework, like timeconstant / tau maybe?


Regards,
Achim.
-- 
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+