Is it possible to start SPIS after CSN is already low?

RE: Is it possible to start SPIS after CSN is already low?

Jared — Mon, 18 Sep 2023 14:24:51 GMT

[quote user="anthony.asterisk"]

This means that a dummy/empty SPI slave buffer must always initially be read out, and only then the SPI slave can reload with the newly generated payload.

In short, my motivation to wait for the CSN to trigger spi_transceive is so that I can determine at the latest possible moment if the slave has a payload to send back to the master.

[/quote]

I see, unfortunately, I don't think there is any "solution" to this as this is a result of our design choice for SPIS peripheral. At least in the case that you use our drivers ( which we recommend 🙂 )

However, the semaphore handling doesn't restrict you to update the RX and TX pointers to only when you have it, you should be able to do it regardless. It will only tell you when it's safe, naturally our drivers use this semaphore to update the RX and TX pointers, but you should be able to make your own driver that doesn't follow this. Of course, you risk corrupted data being sent if you do this.

Documentation:

regards

Jared

RE: Is it possible to start SPIS after CSN is already low?

Anthony Ambuehl — Fri, 15 Sep 2023 16:19:01 GMT

Since the only indication that the SPI master will soon start a SPI transaction is the CSN, and the SPI master can generate a payload to send at any time, therefore I must immediately start the spi_transceive operation. After I start the spi_transceive operation, if an asynchronous SPI slave payload is generated, I cannot insert the new payload into the SPIS since the spi_transceive has already started. This means that a dummy/empty SPI slave buffer must always initially be read out, and only then the SPI slave can reload with the newly generated payload.

In short, my motivation to wait for the CSN to trigger spi_transceive is so that I can determine at the latest possible moment if the slave has a payload to send back to the master.

RE: Is it possible to start SPIS after CSN is already low?

Jared — Fri, 15 Sep 2023 11:40:12 GMT

Hi,

[quote user=""]. However during testing I find that the spi_transceive operation if started after CSN is already low does not start until after CSN goes high and low again.[/quote]

Is any data clocked out in this case? What is the state of the MISO?

I think the issue is that the CPU can't set the TX or RX buffer while CSN is low because it doesn't have the semaphore. If can first update the RX and TX pointers when it acquires a semaphore after the CSN signal goes high.

[quote user=""]Since the SPIS spi_transceive operation is blocking until completion, I was hoping to delay calling spi_transceive until an ISR detects that CSN has gone low. [/quote]

But first, can you elaborate a bit more on your motivation for doing this?

nrfx SPIS driver use a callback handler so that driver API shouldn't be blocking, I'm not that familiar with the Zephyr SPIS API.

regards

Jared