By: David W. Hess (dwhess.delete@this.banishedsouls.org), September 22, 2007 9:10 pm
Room: Moderated Discussions
David Kanter (dkanter@realworldtech.com) on 9/22/07 wrote:
---------------------------
>>>>Small question - how come they didn't leverage the PCI >Express and needed a new
>>>>bus (excuse me, p2p interconnect) altogether?
>>>
>>>PCI Express isn't coherent, it's also fairly high latency since it uses 8B/10B clock encoding.
>>
>>How much latency does 8B/10B encoding contribute? If I am >reading this correctly,
>>Lattice has a programmable logic implementation optimized >for throughput with only
>>2 clocks of latency on the encoder and 3 clocks on the >decoder when working exclusively with serial bit streams:
>>
>>http://www.latticesemi.com/dynamic/view_document.cfm?document_id=5653
>
>So I think the issue is that you'd need at least 10 data transfers to occur before
>you can get usable data extracted from the symbols.
That is what I thought at first before reading about the Lattice design which is pipelined and operates at the serial level. As near as I can tell, the added latency including the encoding and decoding is 5 clocks and the symbols are decoded one bit at a time which is what caught my attention. Just a cursorily study of the 5B/6B and 3B/4B code tables convinces me that I must have misunderstood something. Now I am going to be up all night thinking about it.
---------------------------
>>>>Small question - how come they didn't leverage the PCI >Express and needed a new
>>>>bus (excuse me, p2p interconnect) altogether?
>>>
>>>PCI Express isn't coherent, it's also fairly high latency since it uses 8B/10B clock encoding.
>>
>>How much latency does 8B/10B encoding contribute? If I am >reading this correctly,
>>Lattice has a programmable logic implementation optimized >for throughput with only
>>2 clocks of latency on the encoder and 3 clocks on the >decoder when working exclusively with serial bit streams:
>>
>>http://www.latticesemi.com/dynamic/view_document.cfm?document_id=5653
>
>So I think the issue is that you'd need at least 10 data transfers to occur before
>you can get usable data extracted from the symbols.
That is what I thought at first before reading about the Lattice design which is pipelined and operates at the serial level. As near as I can tell, the added latency including the encoding and decoding is 5 clocks and the symbols are decoded one bit at a time which is what caught my attention. Just a cursorily study of the 5B/6B and 3B/4B code tables convinces me that I must have misunderstood something. Now I am going to be up all night thinking about it.