[CC112x] RX FIFO access using standard FIFO access

Copied answer from another direction since respondent could not post this with some reason.

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Hi Enok

I am not sure I understand your question so I will try to explain how the FIFO works.

I an application where you use variable packet length and you want to receive the whole packet before reading the RXFIFO, the max length byte you can send is 127.

In the RX FIFO you will than have: 127, Data₁, Data₂, …, Data₁₂₇.

The RX FIFO is now full and you cannot receive any more packets before reading out the packet that is already there. In this case, append status must be disabled as there are no room for the status bytes

Assume you start to read the FIFO and have read the following: 127, Data₁, Data₂, …, Data₅₀.

You have now read 51 bytes of the 128 bytes that you received, meaning that there will now be room for 51 new bytes in the FIFO. If you strobe SRX again you can for example receive a packet with length byte 50, without having the FIFO to overflow.

If you have an empty FIFO and you enter RX mode you for example receive the following 4 packet before reading the FIFO:

31, Data₁, Data₂, .., Data₃₁, 31, Data₁, Data₂, .., Data₃₁, 5, Data₁, Data₂, .., Data₅, 57, Data₁, Data₂, …, Data₅₇

To summaries: In variable packet length mode you must always have a length byte as the first byte after sync. This byte should tell how many bytes there are in the following payload (excluding CRC/status bytes.) Max length byte is 127 when you want to receive the whole packet before reading the FIFO, and 255, if you want to read the FIFO during reception. In an application where you do not always empty the FIFO before a new packet is received it is the application that must make sure that the FIFO is emptied fast enough to avoid overflow.

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Hi Siri,

Thanks for your clarification. I understood the variable length always requires length byte. 

My original question is come from Direct FIFO Access. I have seen following description on Users Guide, and it looks the length byte are omitted. Therefore I understood the length information is stored to another memory. I know the question is unrealistic but that is why I'm wondering how does it work when the maximum number of packet.

Which scenarios this execution will be issued? Or is there any misunderstanding of mine?

Best Regards,

Enok