compiling Verilog code in Quartus

Question

I'm new to verilog HDL and my first project is to implement a simple stopwatch counter using a set of registers. I'm using Altera Quartus.

When I tried compiling the code below, I keep getting an error for each and everyone of the registers. one of the error messages looks like this:

Error (10028): Can't resolve multiple constant drivers for net "sec0[3]" at test_interface.v(127)

Anyone can help? The code simulates fine in Modelsim.

Here's the fragment of code that's causing problems:

always @ (posedge clk)

  if (qsoutput == 1)
      sec0 = sec0 + 1;
  else if (sec0 == 4'b1010) begin
      sec1 = sec1 + 1;
      sec0 = 4'b0000;
  end else if (sec1 == 4'b0110) begin
      min0 = min0 + 1;
      sec1 = 4'b0000;
  end else if (min0 == 4'b1010) begin
      min1 = min1 + 1;
      min0 = 4'b0000;     
  end else if (min1 == 4'b0110) begin
      sec0 = 4'b0000;
      sec1 = 4'b0000;
      min0 = 4'b0000;
      min1 = 4'b0000;
  end

Answer 1

Based on your code in Dropbox, you are assigning registers in multiple always blocks. This is illegal for synthesis and cosponsors to the Altera Quartus error message is referring to. A reg type should only be assigned with in one always block.

As an example, sec0 is defined in always @(posedge reset_reg) and the code provided in your question. The code in Dropbox is even worse because you split the counter logic into 4 separate always blocks that assign sec0.

I suggest you put all sec* and min* resisters one clock synchronous always block with an asynchronous:

always(@posedge clk or posedge reset_reg)
begin
  if(reset_reg)
  begin
    // ... asynchronous reset code ...
  end
  else
  begin
    // ... synchronous counter code ...
  end
end

This paper goes into detail about good verilog coding practices for synthesis: http://www.sunburst-design.com/papers/CummingsSNUG2000SJ_NBA.pdf

---

Other issues you will have:

• Use non-blocking (<=) when assigning registers. This is discussed in Cliff's paper mentioned earlier.
• Get rid of the initial block. I understand some FPGA synthesizers allow it and some ignore it. It is a better practice to have an asynchronous to put everything into a known and predictable value.
• The block starting with always @ (clk or start_reg or lap_reg or reset_reg) has a bizarre sensitivity list and will likely give you problems. you wither want @(*) if you want combination logic or @(posedge clk or posedge reset_reg) for synchronous flops.
• Very rarely dual edge flops are used. The line always @ (posedge clk or negedge clk) should be always @ (posedge clk) for synchronous or always @(*) for combination logic.