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Old 02-22-2005, 12:26 PM
Ali Dixon
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Default virtex II register file

I'm trying to build a simple core in which decode and register file access occurs in the same cycle on a virtex II pro board.

Decode completes in the half clock cycle after the positive edge. Any ideas about how to make a dual ported RAM which operates in the half cycle after the negative edge?

Thanks in advance
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Old 02-22-2005, 04:13 PM
Peter Alfke
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Default Re: virtex II register file


Ali Dixon wrote:
> I'm trying to build a simple core in which decode and register file

access occurs in the same cycle on a virtex II pro board.
>
> Decode completes in the half clock cycle after the positive edge. Any

ideas about how to make a dual ported RAM which operates in the half
cycle after the negative edge?

Ali, I suppose you want to use the dual=ported BlockRAM in Virtex-II.
Those two ports are completely independent as far as addressing and
control is concerned. They only share the data. So you can use one port
to write on one clock edge, and use the other port to read out on the
other (or the same) clock edge (or even use a completely different
clock).
The issue will be speed. You obviously should not try to read something
that is not yet reliably stored in the data latches. So do a timing
analysis.
Our dual-ported BlockRAMs are really easy to understand, as long as you
realize that the incoming Address, Data, and WE control are all
registered. So nothing happens without a clock edge, even in read mode.
That's desirable in many cases, not desirable in others, but it is
"non-negotiable".
Also, we throw in a read data output even when you write, and you can
choose to read the old or the new data, or even to maintain the
previous data on that output port.
Now, if you want to use the LUTs as dual-ported RAM, then the structure
is different, and you can read without using a clock edge.
Peter Alfke, Xilinx Applications

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Old 02-22-2005, 05:41 PM
John_H
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Default Re: virtex II register file

For register file reads, why not access the BlockRAM with the lower address
bits anyway and only "use" the result if the BlockRAM was selected? This
assumes the register file is a contiguous chunk of memory and the read port
wouldn't be needed for something else that cycle.

"Ali Dixon" <[email protected]> wrote in message
news:[email protected]
> I'm trying to build a simple core in which decode and register file access

occurs in the same cycle on a virtex II pro board.
>
> Decode completes in the half clock cycle after the positive edge. Any

ideas about how to make a dual ported RAM which operates in the half cycle
after the negative edge?
>
> Thanks in advance



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  #4 (permalink)  
Old 02-22-2005, 06:17 PM
Vladislav Muravin
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Default Re: virtex II register file

Ali,

two ports of DPRAM block memory are completely independent, so the only
thing
you have to worry about is meeting the timing, in case the ports operate at
a different clock edges.

Regards,
Vladislav

"Ali Dixon" <[email protected]> wrote in message
news:[email protected]
> I'm trying to build a simple core in which decode and register file access

occurs in the same cycle on a virtex II pro board.
>
> Decode completes in the half clock cycle after the positive edge. Any

ideas about how to make a dual ported RAM which operates in the half cycle
after the negative edge?
>
> Thanks in advance



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