Embedded multiplier delay

Altera_Forum · ‎01-02-2012

Do embedded multipliers (by '*' VHDL operator) in Cyclone II device have delay only through "combinational logic" or do they require some 250MHz clock cycles (in Cyclone II Device Handbook I've read there is 250MHz clock inside)?

Does it take longer than 20ns to calculate for 18x18bit multiplier?

I've found similar topic, but without clear answer.

Second question is which multiplication algorithm is used by embedded multiplier?

Altera_Forum · ‎01-02-2012

You will need a clock. There is no internal clock, you have to provide one. The 250Mhz you've seen is the max clock frequency.

Altera_Forum · ‎01-02-2012

What clock you are talking about? How to provide it? I'am only using '*' VHDL operator.

Altera_Forum · ‎01-02-2012

The embedded multipliers will achieve highest performs when used with it's associated registers. But they can be also used in an unregistered "combinational" mode, e.g when combining multiply and addition operation. The embedded multiplier block diagram in Cyclone hardware manual will clarify about the avaliable options.

When inferring multipliers from HDL code, which is probably the most popular method to use it, you can control registering by performing signal assignments under a clock edge sensitive condition.

When using multipliers at moderate clock speeds, e.g. 30 to 50 MHz, it's not generally necessary to enable registering for each multiplier output or even additionally for inputs. Timing analysis will tell you what's possible.

Altera_Forum · ‎01-02-2012

If I understand correctly, such VHDL code

c <= a * b;

assigns a and b directly to the multiplier input, not through input register (Embedded Multiplier consists of multiplier module, input register and output register).

So delay only depends on propagation time.

Altera_Forum · ‎01-02-2012

Depends on. For an output register, write:

if rising_egge(clk) then
  c <= a * b;
end if;

Or input- and output register

if rising_egge(clk) then
  as <= a;
  bs <= b;
  c <= as * bs;
end if;