Edge Detection

Edge detection is the process of sending a signal at the moment a signal transitions.

A rising edge refers to the instant when a signal changes from off to on.

A falling edge refers to the instant when a signal changes from on to off.

Construction

Raising edge detector

You can detect a rising edge by checking whether the input signal was off some number of ticks ago but is now on.

This can be done using an AND Gate and an Inverter. Connect one input of the AND gate directly to your signal, and connect the other input through the inverter.

When the input signal turns on, the direct input to the AND gate is immediately on. The inverter output, however, is delayed by 1 tick, so it remains on for one more tick. Since both inputs of the AND gate are on, it becomes marked for activation in the next tick.

On the next tick, the AND gate turns on while the inverter output turns off. At this point, not all AND gate inputs are on, so it will be marked to turn off on the following tick.

The pulse length is determined by the delay between the direct and inverted inputs of the AND gate. You can extend the pulse to 2 ticks using a Buffer, or make it 2 ticks or longer with a Delayer.

Example Signal Diagram

Below is an example timing diagram showing how a rising edge detector behaves:

• Input shows the original signal toggling between off and on.
• Output is the short pulse generated on each rising edge of the input signal.

		0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15
Input
Output

NOTE: This diagram is simplified for educational purposes. While this diagram suggests that the output turns on immediately when the input turns on, in reality, logic gates create a delay, and the output signal will lag behind the input signal by at least one tick.

How it looks in the Logic World:

Raising Edge on oscilloscope