FSM POR/Reset State on Cyclone 10 LP

Altera_Forum · ‎05-22-2018

This should be trivial, but I've never experienced this issue and I can't seem to solve it! I have a very simple FSM that I use to emulate a switch (given the press of a button). I debounce the button and I detect it's edge -- this is it what drives my state transitions. Now to the issue: even though I have specified the Reset state of my FSM to be the OFF state (1'b0), it is powering up into the ON state (1'b1) and I have to manually hit my reset_n push button to force it into the OFF state. Why is this? I have never had this issue with any other Altera (or Xilinx) FPGAs! (Is it a hardware issue?). I have checked all my signals in SignalTap and the only incorrect signal is "state"... Here is the simple FSM:

 module pb_sensor_emulator(
    input wire clk,
    input wire rst_n,
    
    input wire button_in,
    //output wire sensor_out
    output reg sensor_out
);
    reg sensor_reg;
    
    
    /* debounce & detect edge of button */
    wire button_edge;
    reg button_in_d1;
    reg button_db = 1'b0;
    
    // debounce
    debouncer deb(
        .clk(clk),
        .PB(button_in),
        .PB_state(button_db)
    );    
        
    // detect edge
    always @(posedge clk)
    begin
            button_in_d1<=button_db;
    end
    assign button_edge = button_db & ~button_in_d1;
    /********************************/
    
    /* define states for sensor emulator FSM */
    enum logic {OFF = 1'b0, ON = 1'b1} state, nextstate;
     
    /* state transitions */
    always_ff @(posedge clk or negedge rst_n) 
    begin
        if (!rst_n)
            state <= OFF;
        else
            state <= nextstate;
    end
    
    /* state outputs */
    always_comb 
    begin
        /* set defaults */
        sensor_out = 1'b0;
        /* begin output combinational logic */  
        case(state)
            OFF:
            begin
            end
            
            ON:
            begin
                sensor_out = 1'b1;
            end
        endcase
    end
    
    /* next state logic */   
    always_comb
    begin
        /* default next-state */        
        nextstate = OFF;
        /* begin next-state combinational logic */ 
        case(state)
            
            OFF:
            begin
                if(button_edge)
                    nextstate = ON;
                else
                    nextstate = OFF;
            end
            
            ON:
            begin
                if(button_edge)
                    nextstate = OFF;
                else
                    nextstate = ON;
            end
        endcase
    end
    endmodule

I appreciate your help!

Altera_Forum · ‎05-22-2018

Hi,

Have you checked this code work as you expected by using test bench? Have you got the correct result after simulation?

For this code simulation always showing OFF state for rst_n (Reset) either High or Low.

Check screenshot.

Best Regards

Vikas Jathar

(This message was posted on behalf of Intel Corporation)