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8B/10B vs. Start/Stop for SERDES

I'm doing a new design with a SERDES at 1.6Gbps. For cost reasons, I'm using an FPGA without serial IO, so I'm looking at using one of the following TI parts:

- TLK2501

- TLK2521

- TLK2701

I am having trouble choosing between them. They all look basically the same except that the 2521 is 18-bits using Start/Stop bits, while the other 2 use 8B/10B coding. The 2701 gives you direct access to the K-codes, while the 2501 just gives you sync and error signals.

My question is this: Is there a reason I should prefer 8B/10B over

18-bit? I don't really need the additional 2 bits, but my concern is with maintaing sync. I need to burst packets through, but there is no way to retransmit anything if there is any kind of loss. It needs to be obvious when packets start and end.

I'd also like to be able to eventually talk to one of these with a Rocket-IO, so that sort of compatibility is necessary.

Any advice would be appreciated.

Thanks, Matt

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M E
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RocketIO uses 8B/10B encoding, so if you want to talk to them in the future probably better to use that scheme. Plus, in my opinion, 8B/10B is better than a high-speed RS-232 like protocol. Because of the encoding, it gives you extra control characters and maintains a balance of 0s and 1s on the line.

---Matthew Hicks

Reply to
Matthew Hicks

The technical comparison depends on your application. If you aregoing to drive a cable between two different pieces of equipment, then 8B/10B encoding is superior for a number of reasons. If you are going point to point (between two boards in a chassis) then start/stop may be sufficient.

Start/stop has a lower overhead than 8B/10B encoding, so your power will be a little lower (effective line rate at 1.6Gb.s throughput ->

1.8Gb/s) than 8B/10B encoding (line rate 1.92Gb/s), but start/stop has run lengths of up to 17 bits where 8B/10B encoding has a maximum run length of 5. This simplifies coupling (for AC coupled signals, such as across cables).

The shorter run length also simplifies the interconnect ac model (by limiting the low frequency end of the effective bandwidth).

8B/10B encoding also has low DC wander (zero across long times) whereas start/stop does not.

Much depends on 'what you want to connect to' and how.

Cheers

PeteS

Reply to
PeteS

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