What approach would you take to write HDL for a FSM with IDLE, READ, and WRITE states, transitioning on "op" input and resetting after 4 cycles?

Question Analysis

This question is asking about designing a Finite State Machine (FSM) using Hardware Description Language (HDL). The FSM has three states: IDLE, READ, and WRITE. The transitions between these states occur based on the "op" input signal. Additionally, the FSM should reset after 4 cycles. This implies that you need to manage both state transitions and a timing mechanism to count the cycles for resetting. The question requires understanding of FSM design principles, state encoding, and HDL syntax.

Answer

To write HDL for a FSM with IDLE, READ, and WRITE states, transitioning based on the "op" input and resetting after 4 cycles, follow these steps:

1. State Encoding:

   • Define the states using an enumerated type or constants. For example:

     type state_type is (IDLE, READ, WRITE);
     signal current_state, next_state: state_type;
     

2. State Transition Logic:

   • Create a process block that evaluates the "op" input to determine the next state:

     process(current_state, op)
     begin
       case current_state is
         when IDLE =>
           if op = '1' then
             next_state <= READ;
           else
             next_state <= IDLE;
           end if;
         when READ =>
           if op = '0' then
             next_state <= WRITE;
           else
             next_state <= READ;
           end if;
         when WRITE =>
           next_state <= IDLE;
       end case;
     end process;
     

3. Cycle Counting for Reset:

   • Implement a counter that increments every clock cycle and resets the state machine after 4 cycles.

     signal cycle_count: integer range 0 to 4;
     
     process(clk, reset)
     begin
       if rising_edge(clk) then
         if reset = '1' then
           cycle_count <= 0;
           current_state <= IDLE;
         else
           if cycle_count = 4 then
             cycle_count <= 0;
             current_state <= IDLE;
           else
             cycle_count <= cycle_count + 1;
             current_state <= next_state;
           end if;
         end if;
       end if;
     end process;
     

4. Output Logic:

   • If there are outputs associated with each state, define them based on the current state.

This structure ensures that the FSM correctly transitions between states based on the "op" input and automatically resets after 4 cycles. Adjust the logic as needed for specific requirements or additional conditions.