Signals in VHDL

Here’s an idiomatic VHDL example demonstrating signal handling, inspired by the concept in the provided HTML content:

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.NUMERIC_STD.ALL;

entity SignalHandler is
    Port ( clk : in STD_LOGIC;
           reset : in STD_LOGIC;
           signal_in : in STD_LOGIC;
           signal_received : out STD_LOGIC);
end SignalHandler;

architecture Behavioral of SignalHandler is
    type state_type is (IDLE, SIGNAL_DETECTED, SHUTDOWN);
    signal current_state, next_state : state_type;
    
    -- Signal for graceful shutdown
    signal shutdown_request : STD_LOGIC := '0';
begin
    -- State register process
    process(clk, reset)
    begin
        if reset = '1' then
            current_state <= IDLE;
        elsif rising_edge(clk) then
            current_state <= next_state;
        end if;
    end process;

    -- Next state logic and output
    process(current_state, signal_in, shutdown_request)
    begin
        next_state <= current_state;
        signal_received <= '0';
        
        case current_state is
            when IDLE =>
                if signal_in = '1' then
                    next_state <= SIGNAL_DETECTED;
                end if;
            
            when SIGNAL_DETECTED =>
                signal_received <= '1';
                if shutdown_request = '1' then
                    next_state <= SHUTDOWN;
                elsif signal_in = '0' then
                    next_state <= IDLE;
                end if;
            
            when SHUTDOWN =>
                -- Perform shutdown operations here
                null;
        end case;
    end process;

    -- Simulating an asynchronous shutdown request
    process
    begin
        wait for 100 ns;
        shutdown_request <= '1';
        wait;
    end process;

end Behavioral;

This VHDL example demonstrates a simple signal handling mechanism, which is conceptually similar to the Go example provided. Here’s an explanation of the code:

1. We define an entity SignalHandler with input ports for clock, reset, and an incoming signal, and an output port to indicate when a signal is received.

2. The architecture uses a state machine with three states: IDLE, SIGNAL_DETECTED, and SHUTDOWN.

3. We use a process sensitive to the clock and reset to update the current state.

4. Another process handles the next state logic and output based on the current state and inputs.

5. When a signal is detected (signal_in = ‘1’), the state machine moves to the SIGNAL_DETECTED state and asserts the signal_received output.

6. We simulate an asynchronous shutdown request using a separate process that sets the shutdown_request signal after 100 ns.

7. When the shutdown request is received while in the SIGNAL_DETECTED state, the state machine transitions to the SHUTDOWN state.

This example showcases how VHDL can handle signals and implement a simple state machine for signal processing. While it doesn’t directly correspond to Unix signals like in the Go example, it demonstrates the concept of signal handling in the context of hardware description languages.

To simulate this design:

1. Save the code in a file named SignalHandler.vhd.
2. Use a VHDL simulator like ModelSim or GHDL.
3. Create a testbench to provide clock and input signals.
4. Compile the VHDL file and run the simulation.

This example follows VHDL best practices by using synchronous design principles, clearly defined states, and separate processes for different functionalities.