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

The question is asking you to outline an approach for designing a Finite State Machine (FSM) using a Hardware Description Language (HDL). The FSM has three states: IDLE, READ, and WRITE. The transitions between these states are determined by an input called "op". Additionally, the FSM needs to reset after 4 cycles, which means there should be a mechanism to count the cycles and reset the FSM appropriately.

To approach this problem:

1. Understand FSM Basics: You need to be familiar with the concept of Finite State Machines, which are used to design sequential logic circuits. FSMs have a finite number of states and transition between these states based on input signals.

2. Define States: Identify and define the states needed for the FSM (IDLE, READ, WRITE).

3. Transitions: Determine how the FSM transitions between these states based on the "op" input.

4. Cycle Counter: Implement a counter that tracks the number of cycles and resets the FSM after 4 cycles.

5. HDL Syntax: Be familiar with writing the state logic using an HDL like Verilog or VHDL.

Answer

To design an FSM in HDL for the given requirements, follow the steps below:

1. Define the States:

• Use an enumerated type or parameter definition to represent the states IDLE, READ, and WRITE.

typedef enum logic [1:0] {IDLE = 2'b00, READ = 2'b01, WRITE = 2'b10} state_t;
state_t current_state, next_state;

2. State Transition Logic:

• Implement a sequential block to update the current state based on the next state.
• Use a combinational block to define the next state logic based on the "op" input and current state.

always_ff @(posedge clk or posedge reset) begin
    if (reset) begin
        current_state <= IDLE;
        cycle_count <= 0;
    end else begin
        current_state <= next_state;
        cycle_count <= cycle_count + 1;
        if (cycle_count == 4) begin
            cycle_count <= 0;
            current_state <= IDLE;
        end
    end
end

always_comb begin
    case (current_state)
        IDLE: begin
            if (op == READ_OP)
                next_state = READ;
            else if (op == WRITE_OP)
                next_state = WRITE;
            else
                next_state = IDLE;
        end
        READ: begin
            // Define transitions from READ
            next_state = IDLE;  // Example logic
        end
        WRITE: begin
            // Define transitions from WRITE
            next_state = IDLE;  // Example logic
        end
    endcase
end

3. Cycle Counter:

• Incorporate a counter that resets the state machine after 4 cycles.

int cycle_count;

always_ff @(posedge clk or posedge reset) begin
    if (reset) begin
        cycle_count <= 0;
    end else if (cycle_count == 4) begin
        cycle_count <= 0;
    end else begin
        cycle_count <= cycle_count + 1;
    end
end

4. Reset Logic:

• Ensure the FSM resets to the IDLE state either on an external reset signal or after the specified number of cycles.

This approach ensures that you have a robust design that meets the requirements of transitioning between states based on the "op" input and resetting after 4 cycles.