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ZAhma1's avatar
ZAhma1
Icon for New Contributor rankNew Contributor
6 years ago
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ALTERA_FP_FUNCTIONS Single Precision Multiplier doesn't seem to support subnormal numbers

I am trying to use a Single Precision Floating Point multiplier as generated by QSys in Quartus Prime 16.1.2. We are using an Arria 10 device so the option I have is to use the ALTERA_FP_FUNCTIONS mo...
  • AnandRaj_S_Intel's avatar
    AnandRaj_S_Intel
    6 years ago

    Yes, I got the problem.

    However Altera floating-point IP cores do not support denormal/subnormal number inputs. If the input is a denormal value, the IP core forces the value to zero and treats the value as a zero before going through any operation.

    Hence we are seeing output zero.

    Regards

    Anand

AnandRaj_S_Intel's avatar
AnandRaj_S_Intel
Icon for Regular Contributor rankRegular Contributor
6 years ago

Hi Ahmad,

Check your testbench rest signal. Its is not properly handled and also reset is active high.

without testbench IP works as expected.

Regards

Anand

  • ZAhma1's avatar
    ZAhma1
    Icon for New Contributor rankNew Contributor
    6 years ago

    Hi Anand,

    I don't understand what you mean by "reset is not properly handled". The testbench applies an active high reset at the start after which the test begins and reset is pulled low. I see you've done the same thing in your test.

    Also if you see the testbench there are three tests. Test 1 is *supposed* to pass. This is the same output in your run and my testbench.

        report( lf &
            "------------------ TEST 1 ------------------------"  & lf &
            "    This should pass"
          );
    wait until falling_edge(s_clk);
    s_a <= X"3fc00000";           -- 1.5
    s_b <= X"3fc00000";           -- 1.5
    expected_q := X"40100000";    -- 2.25

    Test 2 and 3 fail with the IP. These are the tests that show the IP doesn't support subnormal numbers. But you don't seem to have run these tests.

        report( lf &
            "------------------ TEST 2 ------------------------"              & lf &
            "    Multiply subnormal number to a regular number"               & lf &
            "    The output should be a regular number, but will likely be 0" & lf &
            "    This will probably fail"
          );
    wait until falling_edge(s_clk);
    s_a <= X"00781136";           -- 1.10264e-38    (subnormal)
    s_b <= X"3fc00000";           -- 1.5            (not subnormal)
    expected_q := X"01ba0cf5";    -- 6.83442e-38    (not subnormal)
        report( lf &
            "------------------ TEST 3 ------------------------"                      & lf &
            "    Multiply two regular numbers that should produce a subnormal number" & lf &
            "    The output should be a subnormal number, but will likely be 0"       & lf &
            "    This will probably fail"
          );
    wait until falling_edge(s_clk);
    s_a <= X"1ff7f0d6";           -- 1.05007e-19    (not subnormal)
    s_b <= X"1ff7f0d6";           -- 1.05007e-19    (not subnormal)
    expected_q := X"00781150";    -- 1.10265e-38    (subnormal)