Document SPI offload support for the ad4695 driver. Signed-off-by: David Lechner <dlechner@xxxxxxxxxxxx> --- v5 changes: new patch in v5 --- Documentation/iio/ad4695.rst | 68 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 68 insertions(+) diff --git a/Documentation/iio/ad4695.rst b/Documentation/iio/ad4695.rst index 9ec8bf466c15..8009a0c272bc 100644 --- a/Documentation/iio/ad4695.rst +++ b/Documentation/iio/ad4695.rst @@ -47,6 +47,36 @@ In this mode, CNV and CS are tied together and there is a single SDO line. To use this mode, in the device tree, omit the ``cnv-gpios`` and ``spi-rx-bus-width`` properties. +SPI offload wiring +^^^^^^^^^^^^^^^^^^ + +When used with a SPI offload, the supported wiring configuration is: + +.. code-block:: + + +-------------+ +-------------+ + | GP0/BUSY |-------->| TRIGGER | + | CS |<--------| CS | + | | | | + | ADC | | SPI | + | | | | + | SDI |<--------| SDO | + | SDO |-------->| SDI | + | SCLK |<--------| SCLK | + | | | | + | | +-------------+ + | CNV |<-----+--| PWM | + | | +--| GPIO | + +-------------+ +-------------+ + +In this case, both the ``cnv-gpios`` and ``pwms`` properties are required. +The ``#trigger-source-cells = <2>`` property is also required to connect back +to the SPI offload. The SPI offload will have ``trigger-sources`` property +with cells to indicate the busy signal and which GPx pin is used, e.g +``<&ad4695 AD4695_TRIGGER_EVENT_BUSY AD4695_TRIGGER_PIN_GP0>``. + +.. seealso:: `SPI offload support`_ + Channel configuration --------------------- @@ -158,6 +188,27 @@ Unimplemented features - GPIO support - CRC support +SPI offload support +=================== + +To be able to achieve the maximum sample rate, the driver can be used with the +`AXI SPI Engine`_ to provide SPI offload support. + +.. _AXI SPI Engine: http://analogdevicesinc.github.io/hdl/projects/ad469x_fmc/index.html + +.. seealso:: `SPI offload wiring`_ + +When SPI offload is being used, some attributes will be different. + +* ``trigger`` directory is removed. +* ``in_voltage0_sampling_frequency`` attributes are added for setting the sample + rate. +* ``in_voltage0_sampling_frequency_available`` attributes are added for querying + the max sample rate. +* ``timestamp`` channel is removed. +* Buffer data format may be different compared to when offload is not used, + e.g. the ``in_voltage0_type`` attribute. + Device buffers ============== @@ -165,3 +216,20 @@ This driver supports hardware triggered buffers. This uses the "advanced sequencer" feature of the chip to trigger a burst of conversions. Also see :doc:`iio_devbuf` for more general information. + +Effective sample rate for buffered reads +---------------------------------------- + +When SPI offload is not used, the sample rate is determined by the trigger that +is manually configured in userspace. All enabled channels will be read in a +burst when the trigger is received. + +When SPI offload is used, the sample rate is configured per channel. All +all channels will have the same rate, so only one ``sampling_frequency`` +attribute needs to be set. Since this rate determines the delay between each +individual conversion, the effective sample rate for each sample is actually +the sum of the periods of each enabled channel in a buffered read. In other +words, it is the value of the ``sampling_frequency`` attribute divided by the +number of enabled channels. So if 4 channels are enabled, with the +``sampling_frequency`` attributes set to 1 MHz, the effective sample rate is +250 kHz. -- 2.43.0
