use only top bits of the data to display

tdc/rtl/debounce.v

@@ -0,0 +1,115 @@
+// Listing 5.6
+module debounce
+   (
+    input wire clk, reset,
+    input wire sw,
+    output reg db
+   );
+
+   // symbolic state declaration
+   localparam  [2:0]
+               zero    = 3'b000,
+               wait1_1 = 3'b001,
+               wait1_2 = 3'b010,
+               wait1_3 = 3'b011,
+               one     = 3'b100,
+               wait0_1 = 3'b101,
+               wait0_2 = 3'b110,
+               wait0_3 = 3'b111;
+
+   // number of counter bits (2^N * 10ns = 10ms tick)
+   localparam N =20;
+
+   // signal declaration
+   reg [N-1:0] q_reg;
+   wire [N-1:0] q_next;
+   wire m_tick;
+   reg [2:0] state_reg, state_next;
+
+   // body
+
+   //=============================================
+   // counter to generate 10 ms tick
+   //=============================================
+   always @(posedge clk)
+      q_reg <= q_next;
+   // next-state logic
+   assign q_next = q_reg + 1;
+   // output tick
+   assign m_tick = (q_reg==0) ? 1'b1 : 1'b0;
+
+   //=============================================
+   // debouncing FSM
+   //=============================================
+   // state register
+    always @(posedge clk, posedge reset)
+       if (reset)
+          state_reg <= zero;
+       else
+          state_reg <= state_next;
+
+   // next-state logic and output logic
+   always @*
+   begin
+      state_next = state_reg;  // default state: the same
+      db = 1'b0;               // default output: 0
+      case (state_reg)
+         zero:
+            if (sw)
+               state_next = wait1_1;
+         wait1_1:
+            if (~sw)
+               state_next = zero;
+            else
+               if (m_tick)
+                  state_next = wait1_2;
+         wait1_2:
+            if (~sw)
+               state_next = zero;
+            else
+               if (m_tick)
+                  state_next = wait1_3;
+         wait1_3:
+            if (~sw)
+               state_next = zero;
+            else
+               if (m_tick)
+                  state_next = one;
+         one:
+            begin
+              db = 1'b1;
+              if (~sw)
+                 state_next = wait0_1;
+            end
+         wait0_1:
+            begin
+               db = 1'b1;
+               if (sw)
+                  state_next = one;
+               else
+                 if (m_tick)
+                    state_next = wait0_2;
+            end
+         wait0_2:
+            begin
+               db = 1'b1;
+               if (sw)
+                  state_next = one;
+               else
+                 if (m_tick)
+                    state_next = wait0_3;
+            end
+         wait0_3:
+            begin
+               db = 1'b1;
+               if (sw)
+                  state_next = one;
+               else
+                 if (m_tick)
+                    state_next = zero;
+            end
+         default: state_next = zero;
+      endcase
+   end
+
+endmodule

tdc/rtl/tdc.v

@@ -12,6 +12,21 @@ module tdc #(parameter COUNTER_WIDTH=16)(
 reg [COUNTER_WIDTH-1:0] counter;
 assign o_data = counter;
 
+reg db_start, db_stop;
+debounce db1 (
+   // Outputs
+   .db                       (db_start),
+   // Inputs
+   .clk                      (i_clk),
+   .reset                    (i_reset),
+   .sw                       (i_start));
+debounce db2 (
+   // Outputs
+   .db                       (db_stop),
+   // Inputs
+   .clk                      (i_clk),
+   .reset                    (i_reset),
+   .sw                       (i_stop));
 // states
 localparam state_idle = 2'b00;
 localparam state_started = 2'b01;
@@ -20,7 +35,7 @@ localparam state_stopped = 2'b11;
 reg [1:0] current_state, next_state;
 
 // ensure that state changes each clock
-always @(posedge i_clk) begin
+always @(posedge i_clk, posedge i_reset) begin
    if (i_reset) begin
       current_state <= state_idle;
    end else begin
@@ -33,19 +48,19 @@ end
 always @(*) begin
    case (current_state)
       state_idle: begin
-         if (i_start && (~i_stop))
+         if (db_start && (~db_stop))
             next_state <= state_started;
          else
             next_state <= state_idle;
       end
       state_started: begin
-         if (~i_start && (~i_stop))
+         if (~db_start && (~db_stop))
             next_state <= state_running;
          else
             next_state <= state_started;
       end
       state_running: begin
-         if (~i_start && (i_stop))
+         if (~db_start && (db_stop))
             next_state <= state_stopped;
          else
             next_state <= state_running;

tdc/rtl/top.v

@@ -10,22 +10,25 @@ module top #(parameter WIDTH=24)(
    output wire [5:0] o_dataN,
    output wire o_led_row_0
 );
-   wire clk_3Hz;
+   wire clk_1Hz; // 1.4 Hz actually
    wire clk_100MHz;
    reg buf_led = 0;
    wire buf_ready;
-   wire [5:0] buf_data;
+   /* verilator lint_off UNUSED */
+   parameter TDC_COUNTER_WIDTH = 28;
+   wire [TDC_COUNTER_WIDTH-1:0] buf_data;
    assign o_readyN = ~buf_ready;
-   assign o_dataN = ~buf_data;
+   assign o_dataN = ~buf_data[TDC_COUNTER_WIDTH-1:TDC_COUNTER_WIDTH-6];
+   /* verilator lint_on UNUSED */
 
 /* verilator lint_off PINMISSING */
-   clk_gen #(.DIVISION(22)) clk_gen0 (
-                      .o_div_clk        (clk_3Hz),
+   clk_gen #(.DIVISION(26)) clk_gen0 (
+                      .o_div_clk        (clk_1Hz),
                       .o_clk_100MHz (clk_100MHz),
                       .i_clk            (i_clk));
 /* verilator lint_on PINMISSING */
 
-   tdc #(.COUNTER_WIDTH(6)) tdc0 (
+   tdc #(.COUNTER_WIDTH(TDC_COUNTER_WIDTH)) tdc0 (
             // Outputs
             .o_ready                    (buf_ready),
             .o_data                     (buf_data),
@@ -35,7 +38,7 @@ module top #(parameter WIDTH=24)(
             .i_stop                     (~i_stopN),
             .i_reset                   (~i_resetN));
 
-   always @(posedge clk_3Hz) begin
+   always @(posedge clk_1Hz) begin
       buf_led <= ~buf_led;
    end
 

tdc/sim/top.cc

@@ -46,9 +46,12 @@ int main(int argc, char **argv) {
    for (int k = 0; k < 3; k++) 
       tick(++tickcount, tb, tfp);
 
-   tb->i_startN = 0;
-   tick(++tickcount, tb, tfp);
-   tb->i_startN = 1;
+   for (int i = 0; i < 1000; i++) {
+      tb->i_startN = 0;
+      tick(++tickcount, tb, tfp);
+      tb->i_startN = 1;
+      tick(++tickcount, tb, tfp);
+   }
 
    for (int k = 0; k < 15; k++) 
       tick(++tickcount, tb, tfp);