Message-Id: <93E1A889-81AE-4DAB-9297-2A74C87E38B3@watter.com>
Date: Sat, 16 Aug 2025 11:33:17 -0400
From: Ben Collins <bcollins@...ter.com>
To: Jonathan Cameron <jic23@...nel.org>
Cc: David Lechner <dlechner@...libre.com>,
 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 Aug 16, 2025, at 11:19 AM, Ben Collins <bcollins@...ter.com> wrote:
> 
>> 
>> On Aug 16, 2025, at 11:08 AM, Jonathan Cameron <jic23@...nel.org> wrote:
>> 
>> On Sat, 16 Aug 2025 09:12:37 -0400
>> Ben Collins <bcollins@...ter.com> wrote:
>> 
>>>> On Aug 16, 2025, at 5:54 AM, Jonathan Cameron <jic23@...nel.org> wrote:
>>>> 
>>>> On Wed, 13 Aug 2025 17:52:04 -0500
>>>> David Lechner <dlechner@...libre.com> wrote:
>>>> 
>>>>> On 8/13/25 10:15 AM, Ben Collins wrote:  
>>>>>> MCP9600 supports an IIR filter with 7 levels. Add IIR attribute
>>>>>> to allow get/set of this value.
>>>>>> 
>>>>>> Signed-off-by: Ben Collins <bcollins@...ter.com>
>>>>>> ---
>>>>>> drivers/iio/temperature/mcp9600.c | 43 +++++++++++++++++++++++++++++++
>>>>>> 1 file changed, 43 insertions(+)
>>>>>> 
>>>>>> diff --git a/drivers/iio/temperature/mcp9600.c b/drivers/iio/temperature/mcp9600.c
>>>>>> index 5ead565f1bd8c..5bed3a35ae65e 100644
>>>>>> --- a/drivers/iio/temperature/mcp9600.c
>>>>>> +++ b/drivers/iio/temperature/mcp9600.c
>>>>>> @@ -31,6 +31,7 @@
>>>>>> #define MCP9600_STATUS_ALERT(x) BIT(x)
>>>>>> #define MCP9600_SENSOR_CFG 0x5
>>>>>> #define MCP9600_SENSOR_TYPE_MASK GENMASK(6, 4)
>>>>>> +#define MCP9600_FILTER_MASK GENMASK(2, 0)
>>>>>> #define MCP9600_ALERT_CFG1 0x8
>>>>>> #define MCP9600_ALERT_CFG(x) (MCP9600_ALERT_CFG1 + (x - 1))
>>>>>> #define MCP9600_ALERT_CFG_ENABLE BIT(0)
>>>>>> @@ -111,6 +112,7 @@ static const struct iio_event_spec mcp9600_events[] = {
>>>>>> .address = MCP9600_HOT_JUNCTION,        \
>>>>>> .info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |        \
>>>>>>     BIT(IIO_CHAN_INFO_SCALE) |       \
>>>>>> +       BIT(IIO_CHAN_INFO_LOW_PASS_FILTER_3DB_FREQUENCY) |  \
>>>>>>     BIT(IIO_CHAN_INFO_THERMOCOUPLE_TYPE), \
>>>>>> .event_spec = &mcp9600_events[hj_ev_spec_off],        \
>>>>>> .num_event_specs = hj_num_ev,        \
>>>>>> @@ -149,6 +151,7 @@ static const struct iio_chan_spec mcp9600_channels[][2] = {
>>>>>> struct mcp9600_data {
>>>>>> struct i2c_client *client;
>>>>>> u32 thermocouple_type;
>>>>>> + u32 filter_level;
>>>>>> };
>>>>>> 
>>>>>> 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.
>>>>> 
>>>>> So we need to do the math to convert from the register value to
>>>>> the required value.
>>>>> 
>>>>> I'm a bit rusty on my discrete time math, so I had chatgpt help me
>>>>> do the transform of the function from the datasheet to a transfer
>>>>> function and use that to find the frequency response.
>>>>> 
>>>>> It seemed to match what my textbook was telling me, so hopefully
>>>>> it got it right.
>>>>> 
>>>>> Then it spit out the following program that can be used to make
>>>>> a table of 3dB points for a given sampling frequency. If I read the
>>>>> datasheet right, the sampling frequency depends on the number of
>>>>> bits being read.
>>>>> 
>>>>> 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?  
>>>> 
>>>> This is always a fun corner case.  Reality is there is always
>>>> some filtering going on due to the analog side of things we
>>>> just have no idea what it is if the nicely defined filter is
>>>> turned off.  I can't remember what we have done in the past,
>>>> but one option would be to just have anything bigger than 0.58774
>>>> defined as being filter off and return a big number. Not elegant
>>>> though.  Or just don't bother supporting it if we think no one
>>>> will ever want to run with not filter at all.
>>>> 
>>>> Hmm. or given this is a digital filter on a sampled signal, can we establish
>>>> an effective frequency that could be detected without aliasing and
>>>> use that?  Not sure - I'm way to rusty on filter theory (and was
>>>> never that good at it!)  
>>> 
>>> I’ve seen another driver use { U64_MAX, U64_MAX } for this case. It
>>> didn’t seem very clean. I thought to use { 999999, 999999 } or even
>>> { 1, 0 }, but anything other than “off” just felt odd.
>> Ah.  Could we use filter_type? (additional attribute)
>> 
>> That already has a 'none' option.  Nothing there yet that works for the 'on'
>> option here.  These are always tricky to name unless they are a very
>> well known class of filter.   The datasheet calls this one an Exponential
>> Moving Average filter. Not a term I'd encountered before, but google did
>> find me some references.  so maybe ema as a filter type?
> 
> In the docs I have, it says:
> 
> In addition, this device integrates a first order recursive
> Infinite Impulse Response (IIR) filter, also known as
> Exponential Moving Average (EMA).
> 
> The EMA formula I’ve used for an adc-attached thermistor was the same
> formula I’ve seen used in IIR, so I think they are generally the same.

Clarification: An EMA is a 1-pole IIR filter, while IIR filters can be
many other types besides 1-pole.

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