07-22-2019 08:06 AM
I have nine 8-bit values that I want to add using the dsp slices.
As an example I tried this code from the Xilinx answer records(https://www.xilinx.com/support/answers/66429.html).
I want to implement the second approach: "Two of the inputs are free to come from any source and one input comes from an internal DSP48 feedback signal as in a MAC."
But after synthesis even the original verilog code is inferring 2 DSP slices.
Am I interpreting things wrong or is there any bug in the code?
I created this VHDL equivalent of the Verilog code from Xilinx.
library IEEE; use IEEE.STD_LOGIC_1164.ALL; use IEEE.NUMERIC_STD.ALL; entity tri_adr is GENERIC( CONSTANT SIZEIN : NATURAL := 48; -- Input size CONSTANT SIZEOUT : NATURAL := 48 -- Output size ); PORT( clk : in STD_LOGIC; -- Clock resetn_glb : in STD_LOGIC; -- Global Reset from the Zynq PS resetn_lc : in STD_LOGIC; -- Local Reset if necessary operand_i1 : in STD_LOGIC_VECTOR (SIZEIN-1 downto 0); -- 1st input to dsp operand_i2 : in STD_LOGIC_VECTOR (SIZEIN-1 downto 0); -- 2nd input to dsp operand_i3 : in STD_LOGIC_VECTOR (SIZEIN-1 downto 0); -- 3nd input to dsp avg_out : out STD_LOGIC_VECTOR (SIZEOUT-1 downto 0) -- Averged output ); attribute USE_DSP : string; attribute USE_DSP of tri_adr: entity is "YES"; end ENTITY; architecture behav of tri_adr is signal a : std_logic_vector (26 downto 0); signal b : std_logic_vector (17 downto 0); signal pcout : std_logic_vector (SIZEOUT-1 downto 0); signal avg_t : std_logic_vector (SIZEOUT-1 downto 0); begin avg:PROCESS (clk) --check every clk begin if rising_edge(clk) then if resetn_glb = '0' then --negative assert avg_t <= (OTHERS => '0'); pcout <= (OTHERS => '0'); else pcout <= std_logic_vector((signed(a) * signed(b)) + signed(operand_i1)); avg_t <= std_logic_vector(signed(pcout) + signed(operand_i2) + signed(operand_i3)); -- avg_t <= std_logic_vector(signed(operand_i1) + signed(operand_i2) + signed(operand_i3)); end if; end if; end PROCESS; -- Output result avg_out <= avg_t; end behav;
I will be adding three 8-bit numbers using one DSP slice and create a cascaded adder logic to do addittion of all nine values.
Thanks in advance.
07-24-2019 01:58 PM
you can't do that. Ternary addition in the DSP48E2 can only be done if you DON'T use the multiplier. The output stage of the multiplier needs 2 of the 3 adder inputs. But I think you will be frustrated in actually getting DSP48E2 inferral to work exactly the way you want. Your best bet is to instantiate the DSP48E2 yourself and then configure it the way you want. But I don't see any easy or simple way to use the DSP48E2 to produce the sum of 9 values, since you can really only pump in fresh data through the A:B and C ports.
But in adding 9 values of 8 bits, why not use slice logic? Ternary adders work very well in the fabric, and an 8 bit ternary adder will run at 500 MHz. This should only take 5 slices and 2 clock cycles in an Ultrascale device.