Enums in VHDL

Enumerated types (enums) are a special case of sum types. An enum is a type that has a fixed number of possible values, each with a distinct name. In VHDL, enums can be directly defined using the enumerated type feature.

Our enum type ServerState is defined using the type keyword in VHDL.

-- Define the ServerState enumerated type
type ServerState is (StateIdle, StateConnected, StateError, StateRetrying);

The possible values for ServerState are defined within the type declaration.

By implementing a function to convert ServerState values to strings, we can print out or convert these values when needed.

-- Function to convert ServerState to string
function ServerStateToString (ss : ServerState) return string is
begin
    case ss is
        when StateIdle =>
            return "idle";
        when StateConnected =>
            return "connected";
        when StateError =>
            return "error";
        when StateRetrying =>
            return "retrying";
    end case;
end function;

Now, we’ll write the transition function, which emulates a state transition for a server. It takes the existing state and returns a new state.

-- Function to handle state transitions
function transition(s : ServerState) return ServerState is
begin
    case s is
        when StateIdle =>
            return StateConnected;
        when StateConnected | StateRetrying =>
            return StateIdle;
        when StateError =>
            return StateError;
        when others =>
            assert false report "unknown state: " & ServerStateToString(s);
            return StateError;  -- default case for safety
    end case;
end function;

To test our transition function, we can write a process in VHDL that simulates the state transitions and prints the results.

-- Test process for state transitions
process
    variable currentState : ServerState := StateIdle;
begin
    currentState := transition(currentState);
    report ServerStateToString(currentState);  -- should output "connected"

    currentState := transition(currentState);
    report ServerStateToString(currentState);  -- should output "idle"

    -- Add more tests as needed
    wait;
end process;

To run the VHDL code, you can use a VHDL simulator like GHDL. Here is an example of how to run the simulation.

$ ghdl -a yourfile.vhdl
$ ghdl -e your_entity
$ ghdl -r your_entity

This outputs the results of the state transitions:

connected
idle

Now that we have demonstrated basic enumerated types and state transitions in VHDL, let’s explore more about the language.

Interfaces in VHDL

Struct Embedding in VHDL

Learn X By Example

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Enums in VHDL