VHDL Coding Example With Unexpected Results - 2021.1 English

Vivado Design Suite User Guide: Logic Simulation (UG900)

Document ID
UG900
Release Date
2021-06-16
Version
2021.1 English
clk_b <= clk;
clk_prcs : process (clk)
begin
  if (clk'event and clk='1') then
    result <= data;
  end if;
end process;
clk_b_prcs : process (clk_b)
begin
  if (clk_b'event and clk_b='1') then
    result1 <= result;
  end if;
end process;

In this example, there are two synchronous processes:

  • clk_prcs
  • clk_b_prcs

The simulator performs the clk_b <= clk assignment before advancing the simulation time. As a result, events that should occur in two clock edges occur in one clock edge instead, causing a race condition.

Recommended ways to introduce causality in simulators for such cases include:

  • Do not change clock and data at the same time. Insert a delay at every output.
  • Use the same clock.
  • Force a delta delay by using a temporary signal, as shown in the following example:
    clk_b <= clk;
    clk_prcs : process (clk)
    begin 
      if (clk'event and clk='1') then
        result <= data;
      end if;
    end process;
    result_temp <= result;
    clk_b_prcs : process (clk_b)
    begin
    if (clk_b'event and clk_b='1') then
        result1 <= result_temp;
      end if;
    end process;

Most event-based simulators can display delta cycles. Use this to your advantage when debugging simulation issues.