Re: TransWarp IIgs GAL's - Upgrade now possible

[Eric Smith <eric@brouhaha.com>]

Henry wrote:
> I'm also not too sure about the FPGA, what 
> it does or doesn't do.  I need the data sheet on that as well.  Now the fun 
> part - start looking for it.  Anyone?  I'm sure an email to Xilinx will 
> solve that, but maybe it's already on-line somewhere?

http://www.datasheet4u.com/html/X/C/2/XC2000_Xilinx.pdf.html

> Using an FPGA, I don't see why breaking 
> the 14 MHz barrier would be that big of a deal.

In the XC2000 family, the speed grades were specified by the "toggle
rate" in MHz of the flip-flops in the FPGA.  Standard speed grades for
the XC2064 were -70, -100, and -130.  That sounds really fast, but in
actuality the real performance you get out of the FPGA depends very much
on the number of levels of combinatorial logic and the routing delays.
So the "70 MHz FPGA" may be marginal for running a TWGS at 14 MHz or
more.

Note also that switching to a -130 part does not in actuality mean that
the FPGA will operate 85% faster than the -50 part.  The ratings are
what Xilinx guaranteed, but the -50 part probably has much more
headroom (overclocking capability) than the -130 part.

However, since in practice many people have reported success running
their TWGS at 14 MHz or more using the original FPGA, it is reasonably
plausible that anyone having FPGA-related trouble might be able to solve
it by switching to the -100 or -130 FPGA.

The XC2064 has been out of product for quite a while, but parts brokers
still have them.  I have no idea what kind of prices they'd charge.

Eric