linux-kernel - Re: [PATCH v2 5/5] iio: mcp9600: Add support for IIR filter

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Message-ID: <2025081408-umber-axolotl-e6c6dd@boujee-and-buff>
Date: Thu, 14 Aug 2025 09:06:39 -0400
From: Ben Collins <bcollins@...nel.org>
To: David Lechner <dlechner@...libre.com>
Cc: Ben Collins <bcollins@...ter.com>, Jonathan Cameron <jic23@...nel.org>, 
	Nuno Sá <nuno.sa@...log.com>, Andy Shevchenko <andy@...nel.org>, linux-iio@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/5] iio: mcp9600: Add support for IIR filter

On Wed, Aug 13, 2025 at 05:52:04PM -0500, David Lechner wrote:
> On 8/13/25 10:15 AM, Ben Collins wrote:
> > MCP9600 supports an IIR filter with 7 levels. Add IIR attribute
... 
> >  static int mcp9600_read(struct mcp9600_data *data,
> > @@ -186,6 +189,9 @@ static int mcp9600_read_raw(struct iio_dev *indio_dev,
> >  	case IIO_CHAN_INFO_THERMOCOUPLE_TYPE:
> >  		*val = mcp9600_tc_types[data->thermocouple_type];
> >  		return IIO_VAL_CHAR;
> > +	case IIO_CHAN_INFO_LOW_PASS_FILTER_3DB_FREQUENCY:
> > +		*val = data->filter_level;
> 
> We can't just pass the raw value through for this. The ABI is defined
> in Documentation/ABI/testing/sysfs-bus-iio and states that the value
> is the frequency in Hz.
...
> For example, for 3 Hz sample rate (18-bit samples), I got:
> 
>   n  f_3dB (Hz)
>   1  0.58774
>   2  0.24939
>   3  0.12063
>   4  0.05984
>   5  0.02986
>   6  0.01492
>   7  0.00746
> 
> I had to skip n=0 though since that is undefined. Not sure how we
> handle that since it means no filter. Maybe Jonathan can advise?

Thanks for notes. If I'm reading for datasheet formula right,

k = 2 / (2^n + 1)

So n=0 would be k=1. I did this formula for n=[0-7] and get:

n	k
0	1.00000
1	0.66667
2	0.40000
3	0.22222
4	0.11765
5	0.06061
6	0.03077
7	0.01550

I'm not versed in filter frequency, but would these be the correct
values to use for the coefficients?

-- 
 Ben Collins
 https://libjwt.io
 https://github.com/benmcollins
 --
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