3 files changed, 27 insertions, 6 deletions
diff --git a/nandgame/computer.sv b/nandgame/computer.sv
index 3286bfe..3794270 100644
--- a/nandgame/computer.sv
+++ b/nandgame/computer.sv
@@ -80,7 +80,8 @@ counter PC (
   .clk_in(clk_in),
   .counter_out(PC_addr_int),
   .X_in(reg_A_int),
-  .st_store_X_in(cpu_do_jump_int)
+  .st_store_X_in(cpu_do_jump_int),
+  .count_enable_in('b1)
 );
 
 `ifdef VERILATOR
diff --git a/nandgame/counter.sv b/nandgame/counter.sv
index 2089194..87ecdd7 100644
--- a/nandgame/counter.sv
+++ b/nandgame/counter.sv
@@ -1,11 +1,12 @@
 // nandgame counter
+// counts on *falling* edge
 
 `timescale 1us/1us
 
 `ifndef NANDGAME_COUNTER
 `define NANDGAME_COUNTER
 
-module  counter #(
+module counter #(
   parameter DATA_WIDTH = 16
 ) (
   // input / value to store in counter
@@ -14,6 +15,9 @@ module  counter #(
   input wire st_store_X_in,
   // clock
   input wire clk_in,
+  // active high, whether to count
+  // st_store_X_in overrides this in any case.
+  input wire count_enable_in,
 
   // counter value
   output wire [(DATA_WIDTH-1):0] counter_out
@@ -45,7 +49,7 @@ end
 always @(negedge clk_in) begin
   if (st_store_X_in)
     counter_int <= X_in;
-  else
+  else if (count_enable_in)
     counter_int <= counter_int + 1;
 end
 
diff --git a/nandgame/counter_tb.sv b/nandgame/counter_tb.sv
index 1603a31..b725475 100644
--- a/nandgame/counter_tb.sv
+++ b/nandgame/counter_tb.sv
@@ -2,14 +2,15 @@
 
 module counter_tb;
 
-logic tst_clk, tst_store_X;
+logic tst_clk, tst_store_X, tst_count;
 logic [15:0] tst_in_X, tst_result;
 
 counter uut (
   .clk_in(tst_clk),
   .X_in(tst_in_X),
   .counter_out(tst_result),
-  .st_store_X_in(tst_store_X)
+  .st_store_X_in(tst_store_X),
+  .count_enable_in(tst_count)
 );
 
 string filename;
@@ -23,12 +24,23 @@ initial begin
   tst_clk = 0;
   tst_store_X = 0;
   tst_in_X = 0;
+  tst_count = 0;
 end
 
 always #10 tst_clk = ~tst_clk;
 
 initial begin
-  #200
+  // start of with count disabled
+  repeat(5) @(posedge tst_clk);
+  assert (tst_result == 0)
+  else   $error("Expected 0, got %d", tst_result);
+  tst_count = 1;
+
+  // now that counting is enabled
+  repeat(5) @(posedge tst_clk);
+  assert (tst_result == 5)
+  else   $error("Expected 5, got %d", tst_result);
+
   @(posedge tst_clk);
   tst_store_X = 1;
   tst_in_X = 16'd834;
@@ -36,8 +48,12 @@ initial begin
     @(posedge tst_clk);
     assert (tst_result == 834)
     else   $error("Expected 834, got %d", tst_result);
+    // this shouldn't matter
+    tst_count = !tst_count;
   end
   tst_store_X = 0;
+  tst_count = 1;
+
   @(posedge tst_clk);
   assert (tst_result == 835)
   else   $error("Expected 835, got %d", tst_result);