Manual control of dedicated SPI CS?

Autonomous Machines Jetson & Embedded Systems Jetson TX2
1

Hi Guys,
I’m wondering if it is possible to manually control the hardware SPI Chip Select pin instead of them being controlled by the hardware?

I wish to use the SPI peripheral in Master mode and manually control the Chip Select level.

More specifically I’m interested in controlling SPI2_CS0# which is pin G16 (Tegra SPI1_CS0) - I see in the pin mux that this pin has no GPIO3 configuration possible and only SFIO2 being SPI1_CS0.

My end goal is to be able to do something like this pseudo code:

SPI2_CS_Pin = 0;

ioctl(file_spi, SPI_IOC_MESSAGE(1), &xfer_spi);
… do some processing and waiting required by the slave device…
ioctl(file_spi, SPI_IOC_MESSAGE(1), &xfer_spi);

SPI2_CS_Pin = 1;

Thanks for any advice you can offer.

Regards
Lasse

3

Why need manual control?
The chip select will be assert/deassert automatic when access the SPI device.

4

Hi @ShaneCCC

The device I’m talking to is made so that I need to keep CS asserted (low) between the transfers.

The “problem” is that I first have to send say 20 bytes and then depending on the response on MISO while sending these I have to send another 20 bytes which are depending on the result of the first - trouble is that I must keep CS asserted during this entire ordeal and only release it once I have all the bytes.

I saw that the SPI peripheral in the Tegra reference manual supports controlling the CS line by software using the bits:

CS_SW_HW and CS_SW_VAL

I’m just wondering if there’s support for it somehow?

Regards
Lasse

5

After consult with developer Jetson doesn’t support to control CS externally. We can use sw cs mode but only driver has control over it.

6

Hi @ShaneCCC

Thanks for checking!

Can you tell me (or point me to) how one can use sw cs mode using the before mentioned driver?

Regards
Lasse

7

You can check the source of …/kernel/nvidia/drivers/spi/spi-tegra114.c

9

Could it be cs_change that we need? The doc says:

(ii) When the transfer is the last one in the message, the chip may
* stay selected until the next transfer.  On multi-device SPI busses
* with nothing blocking messages going to other devices, this is just
* a performance hint; starting a message to another device deselects
* this one.  But in other cases, this can be used to ensure correctness.
* Some devices need protocol transactions to be built from a series of
* spi_message submissions, where the content of one message is determined
* by the results of previous messages and where the whole transaction
* ends when the chipselect goes inactive.

I see the driver seems to support it:

    		if (is_single_xfer && !(t->cs_change)) {
    			tspi->use_hw_based_cs = true;
    			command1 &= ~(SPI_CS_SW_HW | SPI_CS_SW_VAL);
    		} else {
    			tspi->use_hw_based_cs = false;
    			command1 |= SPI_CS_SW_HW;
    			if (spi->mode & SPI_CS_HIGH)
    				command1 |= SPI_CS_SW_VAL;
    			else
    				command1 &= ~SPI_CS_SW_VAL;
    		}

but I can’t find where is the DTS or spidev it should be configured, do you have any pointers? thanks!

1 Like
10

@ShaneCCC

Can you give any guidance?

Yt
Lasse

11

All of the dt setting can find here.

…/kernel/kernel-4.9/Documentation/devicetree/bindings/spi/nvidia,tegra114-spi.txt
…/kernel/kernel-4.9/Documentation/devicetree/bindings/spi/nvidia,tegra124-spi.txt

12

G16 can be made to work as a GPIO H.03 (#379). Try configuring it as a GPIO and manually setting it just as you have described. I would hope that the Tegra SPI driver will work, even though it is not in direct control of the SPI CS signal.

1 Like