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bsps/imxrt: Document GPIO CS pins for LPSPI

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Christian Mauderer 2023-11-21 15:54:36 +01:00
parent 500ac0df65
commit 6744793931
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user/bsps/arm/imxrt.rst
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@ -198,10 +198,38 @@ Note that the SPI-pins on the evaluation board are shared with the SD card.
Populate R278, R279, R280, R281 on the IMXRT1050-EVKB (Rev A) to use the SPI
pins on the Arduino connector.
By default, the native chip selects are used. If you want to use GPIOs as chip
select instead, you can use the `cs-gpios` and `num-cs` attributes just like on
a Linux SPI controller. A maximum of `IMXRT_LPSPI_MAX_CS` pins can be used.
The hardware doesn't support selecting no native chip select during a transfer.
Therefore one native chip select has to be reserved as a dummy if you want to be
able to use GPIOs. The pin function for this chip select must not be configured
on any pin. Dummy will be the first of the first four chip selects that is not a
native one. Example configuration::
&lpspi4 {
status = "okay";
pinctrl-0 = <&my_pinctrl_lpspi4>;
cs-gpios = <0>, <0>, <&gpio1 1 0>, <0>, <&gpio11 5 1>;
num-cs = <5>;
}
In this case, CS2 will be the dummy chip select and no pin must be configured
with that function. CS0, CS1 and CS3 are just native chip selects and should be
used via pin functions. GPIO1.1 is used as a high active CS and GPIO11.5 a low
active one.
Limitations:
* Only a basic SPI driver is implemented. This is mostly a driver limitation and
not a hardware one.
* GPIO CS pins on i.MXRT10xx are not tested. The chip has a lot of errate so
they might not work.
* Switching from one mode (CPOL/CPHA) to another one can lead to single wrong
edges on the CLK line if GPIO CS pins are involved. Make sure to stuff a dummy
transfer with `SPI_NO_CS` set if you use multiple modes together with a GPIO
CS.
Network Interface Driver
------------------------