English
Menu

Synchronous vs Asynchronous Reset: Key Differences Explained

Both are mechanisms for putting flip-flops into a defined initial value, but they differ in when the reset takes effect. A synchronous reset captures the reset signal at the clock edge, just like other data. An asynchronous reset uses the flip-flop's reset pin directly and takes effect immediately, regardless of the clock. In practice, a common approach is to assert the reset asynchronously so it acts at once, and synchronize only its release to the clock.

AspectSynchronous resetAsynchronous reset
When it takes effectWhen a clock edge arrives while reset is activeThe instant reset becomes active. It does not wait for the clock
Clock requiredYes. If the clock is stopped, the circuit cannot be resetNo. It works even before the clock starts or while it is stopped
Verilog codingalways @(posedge clk) if (!rst_n) q <= 0; else q <= d;always @(posedge clk or negedge rst_n) if (!rst_n) q <= 0; else q <= d;
Circuit implementationReset logic is placed before the data inputUses flip-flops with a reset pin
Area and data pathGates are added to the data path, which can increase delay and areaThe cell is larger by the reset pin, but no logic is added to the data path
Glitch immunityStrong. Glitches other than at the edge instant are not capturedWeak. A glitch on the reset line can cause a false reset
Reset pulse widthMust stay active for at least one clock periodEven a short pulse works
Timing analysisCan be analyzed with setup and hold as an ordinary data inputRelease timing (recovery and removal times) is checked separately
Care at reset releaseSynchronized to the clock, so problems are unlikely within the same clock domainIf release is close to a clock edge, flip-flops may leave reset in different cycles or go metastable. A reset synchronizer aligns the release to the clock
Suited toCircuits whose clock always runs, circuits where the reset signal is prone to noiseCircuits that must be initialized right after power-up, circuits whose clock may stop

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

Synchronous reset
When the clock always runs and the reset signal may pick up glitches, or when you want to time the reset like an ordinary data path.
Asynchronous reset
When you need to initialize immediately even before the clock runs or while it is stopped, such as circuits whose outputs must quickly go to a safe state.
Combined
Asynchronous assertion with synchronous release (a reset synchronizer) is widely used. Assertion acts at once, and release is aligned to the clock edge by two flip-flops.

FAQ

Why synchronize the release of an asynchronous reset to the clock?

If release happens near a clock edge, recovery or removal time is violated, and one flip-flop may start in the current cycle while another starts in the next, or metastability may occur. This can make a state machine start in an illegal state, so the release is aligned to the clock with two flip-flops before being distributed.

What happens with a synchronous reset if the clock is stopped?

Even if the reset signal is asserted, the flip-flops are not initialized. For circuits that need a safe state before the clock stabilizes, such as right after power-up, an asynchronous reset is used.

Does every flip-flop need a reset?

No. Registers that are always written before use, such as in the data path, may omit reset to save area and wiring. Things whose initial value affects operation, such as state machines and control signals, are reset.

Check your understanding

What is the difference between synchronous and asynchronous resets?

  1. AA synchronous reset acts regardless of the clock; an asynchronous one only at clock edges
  2. BA synchronous reset takes effect only at clock edges; an asynchronous reset takes effect immediately regardless of the clock
  3. CThey behave identically; only the names differ
Show answer and explanation

Answer: B. A synchronous reset takes effect only at clock edges; an asynchronous reset takes effect immediately regardless of the clock

A synchronous reset samples the reset signal at clock edges like any other input, which eases timing analysis, but it cannot reset without a running clock. An asynchronous reset uses the flip-flop's reset pin directly, so it acts at once even without a clock, but if release happens near a clock edge, flip-flops may come out of reset at different cycles.

In circuits using asynchronous reset, why is reset release (the 0-to-1 transition) synchronized to the clock through two flip-flops before distribution?

  1. ATo delay asserting reset by two clocks for safety
  2. BTo make the reset signal twice as fast
  3. CIf release lands near a clock edge, flip-flops may leave reset on different cycles or become unstable, so release is aligned to the clock
Show answer and explanation

Answer: C. If release lands near a clock edge, flip-flops may leave reset on different cycles or become unstable, so release is aligned to the clock

If an asynchronous reset is released just before or after a clock edge, the timing requirements for release are violated, and one flip-flop may start in this cycle while another starts in the next. This can start a state machine in an illegal state. A reset synchronizer applies reset immediately but aligns only its release to the clock through two flip-flops before distributing it.

In simulation, a register without reset starts as X. What happens in the real chip?

  1. AAt power-on it becomes either 0 or 1, but which one is not determined
  2. BIt stays stuck at a voltage midway between 0 and 1
  3. CIt is always 0 in the real chip
Show answer and explanation

Answer: A. At power-on it becomes either 0 or 1, but which one is not determined

A real flip-flop settles to a stable 0 or 1 at power-on, but which one varies with the chip, location, temperature and so on and is not determined. The X in simulation represents "unknown which". If that value affects circuit behavior, it must be set by reset. Registers in data paths that are always written before use sometimes omit reset.

  1. What type of reset does the code above have, and how does it behave?
  2. What is the state called when a flip-flop receiving an asynchronous signal violates setup or hold time and its output is temporarily neither 0 nor 1?

Practice HDL & Verification questions

Key terms explained

Synchronous reset
A reset sampled at clock edges to initialize flip-flops. Timing is easier to analyze, but it does not act while the clock is stopped.
Asynchronous reset
A reset that initializes flip-flops as soon as the reset signal is asserted, regardless of the clock. Release should be synchronized to the clock.
Reset synchronizer
A circuit that applies an asynchronous reset immediately but aligns its release to the clock through two flip-flops before distributing it.
Metastability
A state in which a flip-flop's output stays between 0 and 1 for a while when setup or hold time is violated. Asynchronous signals are passed through two flip-flops (a synchronizer) to guard against it.
Glitch
An unwanted momentary pulse on an output, caused for example by differing delays along signal paths. In combinational logic it is also called a hazard; in DACs it appears when the code changes.

Related comparisons