Why is this an issue?

The preference for synchronous resets over asynchronous resets can be attributed to several key advantages:

1. Predictability and Timing Closure
   • Synchronous Resets: These are aligned with the clock edge, making the timing analysis simpler because the reset event behaves just like any other data event on the clock edge. This predictability helps in achieving timing closure in complex designs where timing is critical.
   • Asynchronous Resets: They can occur at any time, not just on the clock edge, which can complicate timing analysis. Recovery and removal times must be carefully managed to ensure the reset signal does not violate setup or hold times of the flip-flops, potentially leading to timing issues or metastability.
2. Simplicity in Design and Verification
   • With synchronous resets, the behavior is consistent with other state changes in the system, simplifying the design process and verification. Testbenches and simulation environments are easier to manage because the reset action is clearly defined at clock transitions.
   • Asynchronous resets can lead to race conditions or glitches, especially in synthesis or during simulation, where the exact timing of the reset might not match between simulation and real hardware.
3. Synthesis and Implementation
   • Modern synthesis tools often optimize synchronous resets better than asynchronous ones. They can be absorbed into other logic or timing paths, sometimes even becoming part of the flip-flop's data path, which can lead to smaller hardware footprint or better performance.
   • Asynchronous resets might require additional logic to ensure they are clean and do not cause timing violations, potentially increasing the complexity or area of the design.
4. Power Management
   • Synchronous resets generally consume less power because they only affect the flip-flops on clock edges, rather than potentially toggling at any moment with asynchronous resets which could lead to increased dynamic power consumption if not managed carefully.
5. Debugging and Maintenance
   • Debugging is often easier with synchronous resets because any issues related to reset are synchronized with the clock, making it straightforward to trace back problems in a sequential manner.

However, there are scenarios where asynchronous resets might be preferred or necessary:

• Immediate Reset Action: For safety-critical systems where an immediate reset is required regardless of clock phase.
• Power-Up Conditions: To ensure all registers are in a known state upon power-up before a clock starts.

Therefore, while synchronous resets are generally preferred for reasons of design predictability, timing, and power efficiency, the choice might also depend on specific system requirements or constraints. If you need to implement or analyze such a system, considering these points will help in making an informed decision.

How to fix it

Code examples

Noncompliant code example

process(clk, rst) is
begin
  if rst = '1' then -- Asynchronous reset
    s <= '0';
  elsif rising_edge(clk) then
    s <= '1';
  end if;
end process;

Compliant solution

process (clk) is
begin
 if rising_edge(clk) then
   if rst = '1' then  -- Synchronous reset
     s <= '0';
   else
     s <= '1';
   end if;
 end if;
end process;

Resources

Related rules

• Asynchronous resets should be preferred over synchronous resets