When power is first applied, each flip flop settles into 0 or 1 more or less at random. A power-on reset puts the circuit into a known starting state before it begins to work.
How it is done: a reset signal is held active for a short time after power-up, and it drives the asynchronous clear (and sometimes asynchronous preset) inputs of the flip-flops. Asynchronous inputs are the right tool because they work even before the clock oscillator has started and stabilized.
Why it matters:
- A CPU's program counter must start at the first instruction.
- A counter should start from 0, not from a random value.
- A ring counter must start with exactly one 1. If it powers up as
0000, it stays there forever, and if it powers up with two 1s it circulates the wrong pattern. - A state machine must start in its initial state.
The reset is usually active-low and released cleanly, often through a synchronizer, so that every flip-flop leaves reset on the same clock edge.
Pressing the reset button on a microcontroller board triggers the same process without cycling the power.