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Registered: ‎07-23-2019

Can comma symbols be added freely?

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If I have a serial transceiver where there is the possibility of not having data for a few periods, can I send comma symbols until data is available without disrupting communications?

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Teacher
Teacher
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Registered: ‎07-09-2009

Re: Can comma symbols be added freely?

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Assuming you have a packet type interface,
which Aurora is ,

Then when you are not sending data, the link auto inserts the required codes to pad out and keep clock synchronisation.
<== If this was helpful, please feel free to give Kudos, and close if it answers your question ==>

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Moderator
Moderator
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Registered: ‎07-30-2007

Re: Can comma symbols be added freely?

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It would be best for the CDR and equalization to send more variety and back to back comma's are not prefered.  It has to process one comma before it gets the next.  I think the ideal would be periodic commas and an 8B10B encoded PRBS data in between.




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Xilinx Employee
Xilinx Employee
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Registered: ‎08-13-2007

Re: Can comma symbols be added freely?

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It depends on the protocol in question, but in general, yes they get added - but not necessarily freely as in as soon as you run out of data, e.g. transmit FIFO empty.

FibreChannel has the "IDLE ordered set", http://www.fc-av.info/putting_it_all_together.html

So not only do IDLEs got added between frames, they may get added/removed between nodes based on rate adaption logic since the TX & RX are at slightly different rates.

But you need to be careful, you probably don't want these in the middle of your frame, you may need to terminate the frame with other data (e.g. CRC, end of frame, etc.) before the K character, etc.

And the K characters likely only occur at a particular place in the alignment (e.g. most significant 8b of a 32b value, before the 8b/10b encoding).

It is usually addressed during the protocol design and not necessarily trivial with you consider alignment, running disparity, etc. So it is a bit more complicated often in practical than just adding a comma/K if that makes sense. I'd recommend looking at some of protocols like ANSI/VITA 17.1/Serial FPDP, JESD204b, etc.

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Registered: ‎07-23-2019

Re: Can comma symbols be added freely?

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Thanks, @barriet , @roym 

Maybe some more details are needed. The protocol is not yet decided. This is a P2P connection, so no network layer and associated complications. Actually, what I'm asking for shouldn't be needed by design, it is just a precaution. I will have a transceiver sending a large (GB) file where data will be DMA'ed from RAM, but (probably, not decided yet) not in a single chunk, but in pieces of 64, 128 KB. I thought about sending commas in case the DMA cannot, for whatever reason, produce data. Probably it is better to have a large enough FIFO.

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Xilinx Employee
Xilinx Employee
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Registered: ‎08-13-2007

Re: Can comma symbols be added freely?

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In my experience with various protocols (including custom), things can quickly get more complex than initially expected when you consider synchronization (e.g. K/comma), alignment, error detection, error handling (even with a 1E-15 error rate than can be more often than you might expect with 10+Gbps SERDES), rate adaption, etc.

Aurora is another protocol designed to be "simple" and handles much of this for you... You can find the protocol spec here:

https://www.xilinx.com/support/documentation/ip_documentation/aurora_8b10b_protocol_spec_sp002.pdf

It has been around a long time - since V2P/VIrtex-II Pro for 15+ years.

And you can find that IP used to build that might be interesting to you, e.g.

https://www.xilinx.com/support/documentation/application_notes/xapp1216-axi-chip2chip-aurora.pdf

 

Cheers,

bt

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Teacher
Teacher
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Registered: ‎07-09-2009

Re: Can comma symbols be added freely?

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Assuming you have a packet type interface,
which Aurora is ,

Then when you are not sending data, the link auto inserts the required codes to pad out and keep clock synchronisation.
<== If this was helpful, please feel free to give Kudos, and close if it answers your question ==>

View solution in original post