dummy/linux-stable-mirror
1
0
Fork 1
mirror of https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ synced 2025-04-19 20:58:31 +09:00
Code Issues Packages Projects Releases Wiki Activity

doc: iio: ad4695: describe oversampling support

Browse Source 
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@baylibre.com>
Tested-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250109-ad4695-oversampling-v2-2-a46ac487082c@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
This commit is contained in:
Trevor Gamblin 2025-01-09 13:47:24 -05:00 committed by Jonathan Cameron
parent 67d63185db
commit c26b0854eb
1 changed files with 35 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

36
Documentation/iio/ad4695.rst
View File

@ -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.
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).