Spartan3 multiplier

Hello, Can anyone give me an approximate time for an 18 by 18 multiply using the spartan3. I cannot seem to find a specification for this in the data sheet. I realize that the time is a function of the number of bits. Also, I assume that this multiplier is not clocked. In otherwords, does the value eventually ripple through to the output, or is this system in some fashion clocked. An input latch is quite acceptable, I just would rather not deal with a clocked delay through the multiplier. Thanks, Theron

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Theron Hicks
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Theron Hicks wrote: : Hello, : Can anyone give me an approximate time for an 18 by 18 multiply using : the spartan3. I cannot seem to find a specification for this in the data : sheet. I realize that the time is a function of the number of bits. Also, : I assume that this multiplier is not clocked. In otherwords, does the value : eventually ripple through to the output, or is this system in some fashion : clocked. An input latch is quite acceptable, I just would rather not deal : with a clocked delay through the multiplier.

Look at the Virtex datasheet to get a feeling of the multiplier behaviour.

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Uwe Bonnes                bon@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik  Schlossgartenstrasse 9  64289 Darmstadt
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Uwe Bonnes

The worst-case delay in a 18-bit x18-bit =36-bit multiply is from bit 0 of the input to bit 35 of the product. Using the -4 speed grade, which is the slowest, this delay is reported as 7.98 ns.

If your application can tolerate a few clock cycles of extra latency, then you can fully pipeline the multiplier (registered input, pipelined multiplier, registered output) and operate at ~150 MHz using the techniques described in application note XAPP636.

Here are two application note references that may be of interest.

XAPP467: Using Embedded Multipliers in Spartan-3 FPGAs

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XAPP636: Optimal Pipelining of the I/O Ports of Virtex-II Multipliers

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--------------------------------- Steven K. Knapp Applications Manager, Xilinx Inc. Spartan-3/II/IIE FPGAs

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--------------------------------- Spartan-3: Make it Your ASIC

Also,

value

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Steven K. Knapp

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