Channel Directions in VHDL

When using signals as procedure parameters, you can specify if a signal is meant to only send or receive values. This specificity increases the type-safety of the program.

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_TEXTIO.ALL;
use STD.TEXTIO.ALL;

entity ChannelDirections is
end ChannelDirections;

architecture Behavioral of ChannelDirections is
    signal pings : std_logic_vector(7 downto 0);
    signal pongs : std_logic_vector(7 downto 0);
    
    procedure ping (signal pings : out std_logic_vector(7 downto 0); 
                    constant msg : in string) is
    begin
        for i in 1 to msg'length loop
            pings <= std_logic_vector(to_unsigned(character'pos(msg(i)), 8));
            wait for 10 ns;
        end loop;
    end procedure;
    
    procedure pong (signal pings : in std_logic_vector(7 downto 0);
                    signal pongs : out std_logic_vector(7 downto 0)) is
    begin
        pongs <= pings;
        wait for 10 ns;
    end procedure;
    
begin
    process
        variable l : line;
    begin
        ping(pings, "passed message");
        pong(pings, pongs);
        
        write(l, character'val(to_integer(unsigned(pongs))));
        writeline(output, l);
        
        wait;
    end process;
end Behavioral;

This ping procedure only accepts a signal for sending values. It would be a compile-time error to try to read from this signal within the procedure.

The pong procedure accepts one signal for receives (pings) and a second for sends (pongs).

In the main process, we create two signals pings and pongs. We then call the ping procedure with the pings signal and a message. After that, we call the pong procedure with both signals. Finally, we print the received message.

To run this VHDL code, you would typically use a VHDL simulator such as ModelSim or GHDL. The exact commands may vary depending on your simulation environment, but it might look something like this:

$ ghdl -a channel_directions.vhd
$ ghdl -e channel_directions
$ ghdl -r channel_directions
passed message

This example demonstrates how VHDL can use signal directions to enforce type safety in signal communication between procedures, similar to channel directions in other languages.

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Channel Directions in VHDL