Die size of BRAM/DSP48 in CLBs

I would have thought it is more a question of what you have available - go for the fastest solution if both are going spare!

I would not evaluate them based on area, since you do not have the freedom to exchange one for the other in silicon.

For an arbitrary design, I would use the ratio of multipliers to CLBs on th e FPGA device. If the FPGA device has a hundred multipliers and and 10,000 CLBs, then a multiplier is worth 100 CLBs on that device.

The information is available on Xilinx's website to evaluate the ratios.

For your specific design, when contemplating an optimization or additional functionality, I would use the ratio of available (remaining) multipliers & CLBs. If the virgin ratio is 1:100, but the remaining ratio is 1:1000, it would indicate that multipliers are a little more precious than CLBs for yo ur design on that device than perhaps for other designs on that device.

Andy

Virtex 5/6/7 part.) I was wondering, in terms of die size, whether it's better to use a multiplier or, say, four 36-bit fabric adders?

What else are you going to use the multipliers for?

The BRAMs and DSP blocks are on the die whether you use them or not. The 'trick' is to not run out of them or CLBs when fitting your design.

--------------------------------------- Posted through

Yes, I see the point that since the number of CLBs and DSP48s isn't variabl e, like it would be on an ASIC, I should just see how much I have left of e ach. I guess I was thinking more in terms of a rule of thumb for reusable IP that I might want to use in different designs on different parts, like " If you're multiplying by more than a 4-bit number, a DSP48 is better." But then it still depends on that particular design.

A similar question would relate to power consumption. How many CLBs equals 1 DSP48?

I believe some of the earlier families had versions available of about the same size, with and without DSP48 or other processors. Maybe also with/without BRAM and multipliers. Otherwise, you might be able to tell from a photograph of the chip or mask. DSP48, BRAM, and multipliers have to fit in rectangles in the array, unless they are all on the edge.

-- glen