[sdiy] FracRak digital module?

[Scott Gravenhorst]

Neil Johnson <neil.johnson97 at ntlworld.com> wrote:
>Hi,
>
>On 22/04/2011 18:22, Eric Brombaugh wrote:
>> I think Scott has the answer - market analysis shows that most 'sockets'
>> for FPGAs are in designs which need a lot of I/O.
>>
>> Another consideration - I've made some DIY boards using FPGAs with the
>> minimum pincount package which is a 100pin VQFP in Xilinx. The actual
>> I/O pins on this are actually limited to around 60% of the total pins
>> due to the overhead/support required: multiple power supplies for I/O,
>> core & auxiliary, lots of ground pins to improve signal integrity,
>> configuration and debug pins. Given the number of support pins needed
>> for even a small FPGA, if you tried to squeeze it down to a 28-pin
>> package you'd be left with only a dozen or so pins available for general
>> I/O.
>
>I think there's yet another factor as well.  For a given size (gate 
>count, CLB count, whatever metric you choose) there will be a required 
>amount of silicon.  Ideally the chip is square, and as already mentioned 
>market analysis will indicate where the feature sweet spots are.  So you 
>end up with a piece of silicon a certain size.  Then add in all the IOs 
>that you can to give the market boys a bigger number than the competition.
>
>But, this then means that you have a slab of silicon than won't fit into 
>your ordinary DIP package once you add in the pin frame, bonding, 
>packaging, etc.  So in simple terms it just doesn't fit.  Unless you 
>fancy handling a 64-pin DIP?  Eeeshhh....

Great points.

>Why not do smaller FPGAs that will fit into a DIP?  They already exist: 
>PLDs.  Fond memories of the 22V10 :)

With the selection of fast microprocessors including dsPIC, as project scale goes
smaller, I can see in today's world where it might be easier and less expensive to use
a fast microprocessor rather than a small FPGA.  And such microprocessors can still be
had in DIP packages as well as easily soldered SMD.

But your comment prompts a question from me - do PLDs have hardware dedicated
multipliers?  Or do they have to be built from logic fabric?  (I do not know, but I'd
guess not)  One of the reasons that FPGAs are attractive for DSP is the set of several
dedicated hardware multipliers provided (as well as RAM).  If those multipliers aren't
there and have to be built from logic fabric, I think a significant amount of real
estate might be spent on them leaving less for the rest of the project.  The same is
true of the RAM if it must be built from flipflops.  As for an FPGA that can fit into a
DIY solderable package, I think that the competition against the fast microprocessors
of today would be hard to beat.  The 16 bit dsPIC can operate at around 40 MIPS.

-- ScottG
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-- Scott Gravenhorst
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