VHDL Code for Crossbar Switch: Syntax and Example

A crossbar switch in VHDL connects multiple inputs to multiple outputs with selectable paths. You can implement it using a with-select statement or multiplexers to route inputs to outputs based on control signals. The code defines input and output ports and uses a control signal to select which input connects to each output.

Syntax

A crossbar switch in VHDL typically uses a with-select or multiplexers to route inputs to outputs. The main parts are:

• Entity: Defines input and output ports and control signals.
• Architecture: Contains the logic to select inputs for each output.
• Control signals: Select which input connects to each output.

vhdl

entity CrossbarSwitch is
    generic (
        N : integer := 4  -- Number of inputs and outputs
    );
    port (
        inputs  : in  std_logic_vector(N-1 downto 0);
        select_lines : in std_logic_vector(N*2-1 downto 0); -- 2 bits per output to select input
        outputs : out std_logic_vector(N-1 downto 0)
    );
end CrossbarSwitch;

architecture Behavioral of CrossbarSwitch is
begin
    process(inputs, select_lines)
    begin
        for i in 0 to N-1 loop
            case select_lines((i*2)+1 downto i*2) is
                when "00" => outputs(i) <= inputs(0);
                when "01" => outputs(i) <= inputs(1);
                when "10" => outputs(i) <= inputs(2);
                when "11" => outputs(i) <= inputs(3);
                when others => outputs(i) <= '0';
            end case;
        end loop;
    end process;
end Behavioral;

Example

This example shows a 4x4 crossbar switch where each output selects one of four inputs using 2-bit control signals. The select_lines input has 8 bits (2 bits per output) to choose the input for each output.

vhdl

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

entity CrossbarSwitch is
    port (
        inputs  : in  std_logic_vector(3 downto 0);
        select_lines : in std_logic_vector(7 downto 0); -- 2 bits per output
        outputs : out std_logic_vector(3 downto 0)
    );
end CrossbarSwitch;

architecture Behavioral of CrossbarSwitch is
begin
    process(inputs, select_lines)
    begin
        for i in 0 to 3 loop
            case select_lines((i*2)+1 downto i*2) is
                when "00" => outputs(i) <= inputs(0);
                when "01" => outputs(i) <= inputs(1);
                when "10" => outputs(i) <= inputs(2);
                when "11" => outputs(i) <= inputs(3);
                when others => outputs(i) <= '0';
            end case;
        end loop;
    end process;
end Behavioral;

Common Pitfalls

Common mistakes when coding a crossbar switch in VHDL include:

• Not matching the width of select_lines to the number of outputs (2 bits per output).
• Forgetting to cover all cases in the case statement, which can cause latches or unintended behavior.
• Using blocking assignments (:=) inside processes instead of signal assignments (<=).
• Not including a default when others clause in the case statement.

Example of a wrong and right case usage:

vhdl

process(inputs, select_lines)
begin
    for i in 0 to 3 loop
        case select_lines((i*2)+1 downto i*2) is
            when "00" => outputs(i) <= inputs(0);
            when "01" => outputs(i) <= inputs(1);
            -- Missing other cases causes latch
        end case;
    end loop;
end process;

-- Correct version includes all cases and when others
process(inputs, select_lines)
begin
    for i in 0 to 3 loop
        case select_lines((i*2)+1 downto i*2) is
            when "00" => outputs(i) <= inputs(0);
            when "01" => outputs(i) <= inputs(1);
            when "10" => outputs(i) <= inputs(2);
            when "11" => outputs(i) <= inputs(3);
            when others => outputs(i) <= '0';
        end case;
    end loop;
end process;

Quick Reference

Crossbar Switch VHDL Tips:

• Use 2 bits of control per output to select inputs (for 4 inputs).
• Use case statements inside a process for clear routing logic.
• Always include when others in case to avoid latches.
• Match input/output vector sizes carefully.
• Test with different select_lines values to verify routing.

Key Takeaways

A crossbar switch routes multiple inputs to outputs using control signals in VHDL.

Use 2-bit select signals per output to choose among 4 inputs with a case statement.

Always include a default case to avoid unintended latches or errors.

Match vector sizes of inputs, outputs, and select signals carefully.

Test your design with various select inputs to ensure correct routing.