Timing margin

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Timing Margin is an expression of the difference between the actual change in a signal and the latest time at which the signal can change in order for the circuit to function correctly. It is used in the design of digital electronics.

In this image, the lower signal is the clock, the upper signal is the data. Data is recognized by the circuit at the positive edge of the clock. There are two times illustrated in this image. One is the Setup time, the other is the timing margin. The setup time is illustrated in red in this image; the timing margin is illustrated in green.

The edges of the signals can shift around in a real-world electronic system due to various reasons. If the clock and the data signal are shifted relative to each other, this may increase or reduce the timing margin; as long as the data signal changes before the Setup time is entered, the data will be interpreted correctly. If you know from experience that the signals can shift relative to each other by as much as 2 microseconds, for instance, designing your system with at least 2 microseconds of timing margin will prevent incorrect interpretation of the data signal by the receiver.

If you change the physical design of your circuit, for example by lengthening the wire that the clock signal is transmitted on, the edge of the data signal will move closer to the positive edge of the clock signal, reducing the timing margin. If you have designed your signals with enough timing margin, this will not cause incorrect data to be received.