Search code examples
vhdlxilinxvivado

Directly Instansiating a DSP Slice Without IP Core

The Problem

I want:

p <= (d-a) * b

Trying to directly instantiate a DSP block by using a DSP48E1 instead of simply writing p <= (d-a) * b plus it helps me understand how this block works for the future. So far I've had little luck with it though.

Referencing this article:

http://www.xilinx.com/support/documentation/user_guides/ug479_7Series_DSP48E1.pdf

Attempt

These are my current settings:

a <= std_logic_vector(to_unsigned(5, 30));
b <= std_logic_vector(to_unsigned(1, 18));
d <= std_logic_vector(to_unsigned(20, 25));

    dsp : DSP48E1
    generic map (
        USE_DPORT => True,
        ALUMODEREG => 0, 
        CARRYINREG => 0,
        CARRYINSELREG => 0,
        CREG => 0
        )
    port map(
        clk => clk,
        acin => std_logic_vector(to_unsigned(1, 30)), -- cascaded data input
        alumode => "0000", -- control bits to select logic unit inputs
        bcin => std_logic_vector(to_unsigned(1, 18)), -- cascaded data input 
        carrycascin => '0', -- cascaded data input
        carryin => '0',  -- carry input
        carryinsel => "000", -- selects carry source
        cea1 => '1', -- clock enable if AREG = 2 or INMODE0 = 1
        cea2 => '1', -- clock enable if AREG = 1 or 2
        cead => '1', -- clock enable for AD pipeline register
        cealumode => '0', -- clock enable for ALUMODE --0
        ceb1 => '1', -- clock enable if BREG = 1 or INMODE4 = 1
        ceb2 => '1', -- clock enable if BREG = 1 or 2
        cec => '0', -- clock enable for C
        cecarryin => '0', -- clock enable
        cectrl => '0', -- clock enable for OPMODE and CARRYINSEL ctrl registers
        ced => '1', -- clock enable for D
        ceinmode => '0',-- **** clock enable input registers
        cem => '0', -- clock enable for the post multiply M register and the internal multiply round CARRYIN register
        cep => '1', -- clock enable 
        inmode => "01101", -- *selects functionality of preadder [3] = sign, [4] = B multiplier sel
        multsignin => '0', -- MACC extension sign
        opmode =>  "0000101", -- *** Control input to A, Y and Z multiplexers
        pcin => std_logic_vector(to_unsigned(1, 48)), -- cascade input
        rsta => rst,
        rstallcarryin => '0',
        rstalumode => '0',
        rstb => rst,
        rstc => '0',
        rstctrl => rst,
        rstd => rst,
        rstinmode => rst,
        rstm => rst,
        rstp => rst,
        a => a,--_dsp, -- bits 29:25 used in second stage preadder 
        b => b,--_dsp,
        c => c_dsp,
        d => d,--_dsp,
        p => p_dsp
    );

I always get p = 0 even if I force d = 20, a = 5, b = 1.

I figured I should keep ALUMODE and OPMODE at 0 since I'm skipping the final stage and just want a straight subtraction and multiplication.

Photos

DSP

Solution

  • Look at table 2-7 from the DSP48E1 user guide page 34.

    Your current configuration performs, at the post-adder, P = Z + X + Y + CIN, with Z = 0, X = 0 and Y = 0. You see the problem here?

    The OPMODE signals control the value of the multiplexers. You want OPMODE(6 downto 4) = "000", so that Z keeps its null value. However, you want OPMODE(3 downto 0) = "0101" to set X/Y to the multiplier output M. ALUMODE should keep it's current value, it's fine.