On Tue, 17 Dec 2024 16:47:29 -0500 Trevor Gamblin <tgamblin@xxxxxxxxxxxx> wrote: > Add a section to the ad4695 documentation describing how to use the > oversampling feature. Also add some clarification on how the > oversampling ratio influences effective sample rate in the offload > section. > > Signed-off-by: Trevor Gamblin <tgamblin@xxxxxxxxxxxx> This describes what I was expecting so all looks good to me. Obviously need to wait for the spi offload series anyway so plenty of time for others to take a look. Thanks Jonathan > --- > Documentation/iio/ad4695.rst | 36 +++++++++++++++++++++++++++++++++++- > 1 file changed, 35 insertions(+), 1 deletion(-) > > diff --git a/Documentation/iio/ad4695.rst b/Documentation/iio/ad4695.rst > index ead0faadff4b..f40593bcc37d 100644 > --- a/Documentation/iio/ad4695.rst > +++ b/Documentation/iio/ad4695.rst > @@ -179,12 +179,38 @@ Gain/offset calibration > System calibration is supported using the channel gain and offset registers via > the ``calibscale`` and ``calibbias`` attributes respectively. > > +Oversampling > +------------ > + > +The chip supports per-channel oversampling when SPI offload is being used, with > +available oversampling ratios (OSR) of 1 (default), 4, 16, and 64. Enabling > +oversampling on a channel raises the effective number of bits of sampled data to > +17 (OSR == 4), 18 (16), or 19 (64), respectively. This can be set via the > +``oversampling_ratio`` attribute. > + > +Setting the oversampling ratio for a channel also changes the sample rate for > +that channel, since it requires multiple conversions per 1 sample. Specifically, > +the new sampling frequency is the PWM sampling frequency divided by the > +particular OSR. This is set automatically by the driver when setting the > +``oversampling_ratio`` attribute. For example, if the device's current > +``sampling_frequency`` is 10000 and an OSR of 4 is set on channel ``voltage0``, > +the new reported sampling rate for that channel will be 2500 (ignoring PWM API > +rounding), while all others will remain at 10000. Subsequently setting the > +sampling frequency to a higher value on that channel will adjust the CNV trigger > +period for all channels, e.g. if ``voltage0``'s sampling frequency is adjusted > +from 2500 (with an OSR of 4) to 10000, the value reported by > +``in_voltage0_sampling_frequency`` will be 10000, but all other channels will > +now report 40000. Ah. I forgot there is another series in flight for this and was going to say that we needed a statement on the frequencies being a common control. That is there in the spi offload series so all good! > + > +For simplicity, the sampling frequency of the device should be set (considering > +the highest desired OSR value to be used) first, before configuring oversampling > +for specific channels. > + > Unimplemented features > ---------------------- > > - Additional wiring modes > - Threshold events > -- Oversampling > - GPIO support > - CRC support > > @@ -233,3 +259,11 @@ words, it is the value of the ``in_voltageY_sampling_frequency`` attribute > divided by the number of enabled channels. So if 4 channels are enabled, with > the ``in_voltageY_sampling_frequency`` attributes set to 1 MHz, the effective > sample rate is 250 kHz. > + > +With oversampling enabled, the effective sample rate also depends on the OSR > +assigned to each channel. For example, if one of the 4 channels mentioned in the > +previous case is configured with an OSR of 4, the effective sample rate for that > +channel becomes (1 MHz / 4 ) = 250 kHz. The effective sample rate for all > +four channels is then 1 / ( (3 / 1 MHz) + ( 1 / 250 kHz) ) ~= 142.9 kHz. Note > +that in this case "sample" refers to one read of all enabled channels (i.e. one > +full cycle through the auto-sequencer). >
