What official function should I call to genertate a sum of products in VHDL

[Weng Tianxiang]

Hi,
What official functions should I call to genertate a sum of products
in VHDL?

S(...) <= A0*B0(...) + A1*B1(...) + A2*B2(...) + ... + An*Bn(...);

Ax is a type of std_logic or bool; Bx() is a type of std_logic_vector
or unsigned.

I use the following two libraries:
USE ieee.std_logic_1164.all;
use ieee.numeric_std.all;

Thank you.

Weng

 

[Ralf Hildebrandt]

What official functions should I call to genertate a sum of products
in VHDL?

S(...) <= A0*B0(...) + A1*B1(...) + A2*B2(...) + ... + An*Bn(...);

Ax is a type of std_logic or bool; Bx() is a type of std_logic_vector
or unsigned.

Looks similar to standard multiplication. What about:

process(A,B)
variable result : unsigned(result_bits-1 downto 0);
begin
result:=to_unsigned(0,result'length);
for N in A'range loop
if (A(N)='1') then
result:=result+resize(unsigned(B(N)),result'length);
end if;
end loop;
s<=result;
end process;

(code not checked for any errors - just typed)

This may result in synthesis in a slow carry-ribble array. As an
alternative you could use this for-loop to first generate a 2D-vector
table containing eigther zero vectors or B-vectors (depending on A).
This 2D-Array could be added as multipliers do: Carry-Save-Array,
Wallace-Tree...


Another option would be to make each A-Bits a vector:
A_vec(N)<=('0' & A(N));
Such a vector can be easily converted to unsigned and you can just type
you sum of products, as you have done it in your question.


Ralf

 

[Paul]

Weng,

If Ax is of type std_logic, you only has 2 very trivial cases to
cover. Using multipliers would be a gigantic waste of FPGA
resources... since you have no "non-trivial" cases to concern
yourself with, you have a much simpler option than multipliers.
Personally, I don't think students should be given answers from a
message board, so that's as much of a hint as I am personally willing
to give you (sorry, the problem is trivial enough to be a quite
obvious homework assignment) But, think about what I said and why
your 2 cases are very trivial....

-Paul

P.S. - It's actually trivial enough that a decent synthesizer would
not synthesize multipliers if you did it that way anyway.... at least
it shouldn't.