verilog counter implementation unexpected behaviour

Question

I made a counter in verilog and realized it in hardware. But I am unable to explain the behaviour The code is:

module clock_test(clk_in,led,rst);
input wire clk_in;
input wire rst;
output wire [7:0] led;

reg [23:0] counter = 24'b0;

assign led = counter[23:16];

always @(posedge clk_int)  begin
    if(rst) begin
     counter <= 0;
  end
else begin
     counter <= counter +1;
   end
 end
 endmodule // clock_test

In the hardware, when I hit rst, the LEDs freeze at the point it was counting. Its not becoming exactly zero. Assert rst, and you can see some random pattern other than zero, which dont change unless I release rst.

My question is : when if(rst) begin block executes, counter is set to 0. Since leds are assigned as combo logic from counter, should'nt it reflect immediately ?

Answer 1

Since you have a synchronous reset the reset value would not take effect until the next clock edge after reset asserts.

When you assert reset does that also stop the clock? That would seem to be the most likely cause since otherwise your code looks correct (excepting the syntax error noted by Greg).

Answer 2

It looks like the clk is typod and it was a sync reset, rather than an async.

Try this :

module clock_test(
  input        clk_in,
  input        rst,
  output [7:0] led
);

reg [23:0] counter;

assign led = counter[23:16];

always @(posedge clk_in or posedge rst)  begin
  if(rst) begin
    counter <= 0;
  end
  else begin
    counter <= counter +1;
  end
 end
 endmodule // clock_test

NB: you are using an active high reset (1 implies reset). If you are actually using active low (0 applies reset) the you need to change the following:

always @(posedge clk_in or negedge rst)  begin
  if(~rst) begin
    counter <= 0;