Difference between asynchronous and synchronous resets in flip-flops?

Question

My teacher mentioned that there's an important distinction between asynchronous and synchronous resets used in flip-flops, but I’m still a bit confused about how they actually differ in behavior.

From what I understand, both types reset the flip-flop to a known state, but:

How does the timing of an asynchronous reset differ from a synchronous one?
When designing digital circuits or writing HDL (like Verilog or VHDL), how do I decide which type to use?
Are there any pros, cons, or common pitfalls I should be aware of with either reset type?

I'd appreciate a practical explanation or examples that clarify when and why one might be preferred over the other.

Kanishk · Answer

Asynchronous and synchronous resets both serve to bring flip-flops to a known initial state, but they differ significantly in how and when they operate. An asynchronous reset takes effect immediately, regardless of the clock.

This means that the moment the reset signal is asserted, the flip-flop resets—whether or not the clock is running. On the other hand, a synchronous reset only takes effect on the active edge of the clock (usually the rising edge). So even if the reset signal is asserted, the flip-flop will not reset until the next clock edge occurs.

In digital design or when writing HDL like Verilog or VHDL, it is generally recommended to default to synchronous resets. They are easier to work with in timing analysis, more predictable in simulation, and better supported by most FPGA tools. Synchronous resets ensure that all logic changes happen in sync with the clock, which reduces the risk of glitches and metastability.

However, there are situations where an asynchronous reset is necessary, such as when dealing with logic that receives a clock from an external device (a source-synchronous system) where the clock can stop. In such cases, a synchronous reset would not work because the flip-flop wouldn’t reset without a clock edge, so an asynchronous reset becomes essential to ensure proper initialization or fault handling.

That said, asynchronous resets come with critical caveats, particularly around how they are removed. If the reset signal is deasserted (goes low or inactive) while the clock is not running, the circuit may enter an unpredictable state. To prevent this, designers often use a technique called synchronous reset removal, where the asynchronous reset is passed through a synchronizer (usually a two-stage flip-flop chain) so that the system only comes out of reset on a clean, clocked edge.

This ensures stable behavior and avoids metastability issues. It’s also important to avoid relying on the reset value of an asynchronously reset flip-flop immediately after reset; doing so can lead to inconsistent behavior across builds, as synthesis tools may handle this differently.