Fixedpoint Multiply/Accumulate in DSP48

Kolja, You can easily make an 18x85 multiplier with two DSP48s (see fig 1-20 of

-Kevin

Hi, thanks for that. It got crowded in the chip so I kept banging my head against the user guide schematics and came up with the following. To compute A18*B35 + C35 I chain two DSP48 to form a 18x35 multiplier. Then I set C48

Kolja, I'm not sure exactly what you are describing. You could take the 30 bits from the upper (msb) slice's P output and leftshift 17 bits and route them to the C input so they can be summed with AxB from the lower slice, but then you still can't accumulate. When you multiply, you have to use both the X and Y muxes for the multiplier's partial sums, so then you are left with the Z mux, which you can use to add in either C, PCIN, or P. So you can't multiply, accumulate, and add in a third thing at the same time.

I'm not sure what you mean by your suggestion about the mux to split the C between the two slices. The C input already goes to both slices. (By the way, the DSP48E in the V5 has independent C inputs for each slice. And it does a 25x18 multiply.) -Kevin

I don't want to do P = a*b + c + p I just want P = a*b* + c

I can't add C in the MSB tile, because the Z mux is used by PCIN. I can add C in the lower tile but I can't reach the full dynamic of the multipliers result because only 30 bits will end up in the upper tile.

Does the Virtex-5 DSP slice also resolve the MUX bottleneck?

Kolja