Multiplication by constant

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Hi all. Just found out that Quartus does not instantiate hard block multiplier if multiplication is done by a constant. Even if i define multstyle = "dsp", why?

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what value is your constant?

With a constant there are some sizable reductions that can be done during synthesis to make it more palatable in logic. That being said, if you have a DSP block available, why not use it. Perhaps try keeping the constant, e.g.

(* keep *) wire mult_const;

assign mult_const = 16'ha9a9;

I'm guessing something like that will get it to synthesize a DSP.

If the constant value is 2^n, then the result is simply a bitshift.

Similarly, if it has just a couple of bits set, it may be easier just to create an adder.

Thanx for fast reply, however it was a simple typo, so now synthesis multstyle works fine :). So not to create one more thread, ill ask here, for example i need to partially connect some modules output (let it be 8 most significant bits of 16). The straightforward solution is:

mult mult_inst(
  .a(a_8bit),
  .b(b_8bit),
  .res(c_16bit)
);
assign c_8bit = c_16bit

May there be more elegant approach (without this intermediate wire) ?