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SPI Slave: inspect data before transfer complete?

Andrew Kohlsmith

I'm trying to emulate an nRF24L01+ SPI slave interface with an nRF51822. The nRF24L01+ interprets the first byte in the SPI transfer as a "command", and the data that it will shift out in the same SPI transaction depends on the first byte received from the master.

Looking at the SPIS peripheral in the nRF51 reference manual, it does not appear that I emulate this behaviour. If I set MAXRX to 1, the rest of the SPI transaction will be ignored, and it doesn't look like I can implement a polling method to see when the first byte comes in.

Is there a way that I can emulate an nRF24L01+ SPI implementation with the nRF51822?

edit: Just to be clear: I'm not trying to emulate the whole nRF24L01+; I want to fake out a device that talks to an nRF24L01+ over a 400kHz SPI link just enough to be able to say "I have a new packet, here is the data" and that's about it. The data for the packet would of course come from BLE instead of a proprietary 2.4GHz link.

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    The master controls the flow of the communication. You can first send out one byte from master and end the communication by pulling the CSN high. That will generate END event on the SPI slave. Then you can check the byte and fill the SPI buffers on the SPI slave with appropriate data. After you give SPI slave enough time to do its work, you can have the SPI master set the CSN low again to get the data form the SPI slave. Try using timers or delay functions.

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    The master controls the flow of the communication. You can first send out one byte from master and end the communication by pulling the CSN high. That will generate END event on the SPI slave. Then you can check the byte and fill the SPI buffers on the SPI slave with appropriate data. After you give SPI slave enough time to do its work, you can have the SPI master set the CSN low again to get the data form the SPI slave. Try using timers or delay functions.

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  • 0 in reply to yd

    That's not what the original poster is asking. He's not looking to change the protocol or re-implement how the master works, he's looking to implement a slave for an existing master, he even makes it clear in the original question (by putting it in italics) that the data must go out in the same transaction.

  • 0 in reply to yd

    I read that. He also said in another post (devzone.nordicsemi.com/.../ that it is not possible to do any data processing between first and second bytes. If the data gets clocked out on the third byte, he will have to change the master anyways. So, I gave him an alternative. Changing the master to set the pin high for a few microseconds is pretty easy and it ensures data integrity instead of possibly running into a race condition trying to set bytes while the SPI communication is active. This is a better way to communicate with SPI slaves. Send a command, wait for the processing and get data.

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