On Thu, 22 Nov 2001 09:47:17 +1100, you wrote:

>On 03:50 PM 21/11/2001 -0500, [EMAIL PROTECTED] said:
>>Mr. Lomax
>>I would hesitate to let the pcb routing dictate the FPGA configuration. It
>>has been my experience in the past that this can result in disastrous
>>synthesis. I would suggest that unless you have full control over the FPGA
>>simulation, synthesis and layout, that it is best to let the FPGA
>>programming (Xilinx Foundation or fascimile) dictate what pins get connected
>>to what signal.
>>Regards,
>>Lloyd Good
>
>I agree, Lloyd.
>
>It is rare to find a programmable logic architecture that is completely 
>non-blocking or has no limits on product terms or expanders etc.  So I 
>always let the fitter do the initial pin allocation.  Problem is I am not 
>convinced that all the fitters will try to optimize resource usage once 
>they have achieved a fit.  So, the pin allocation given by the initial fit 
>may not be the most optimum in terms of maximizing future expandability anyway.

Yep, hard to know what you are letting yourself in for. In the past I have
artificially expanded the design adding redundant inputs and outputs and
hooked them up with existing signals to give the fitter a bit more work to
do and hopefully come up with a pin allocation which accomodate future real
design changes. 

If there is an option to go up or down on device size with the same
footprint it is also wise to make sure the fitter is happy with the same
pin allocation for all device sizes. 


Cheers, Terry.

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