The Clock and Timing Diagram

The Clock:

In digital system all waveforms are synchronized with a basic timing waveform called clock.
The clock is a periodic waveform in which each interval between pulses ( the period ) equals the time for one bit.
Each bit in a sequence occupies defined time interval called a bit time. When waveform is HIGH, a binary '1' is present and when waveform is LOW a binary '0' is present.

In clock waveform, each change in level of waveform A occurs at the leading edge of clock waveform. In other cases, level change occur at the trailing edge of clock waveform. The clock waveform doesnot carry any information.

Timing Diagram:

A timing diagram is a graph of digital waveforms showing the actual time relationship of two or more waveforms and how each waveform changes in relation to the others.
By looking at timing diagram, we can determine states ( HIGH or LOW ) of all the waveforms at any specified point in time and the exact time that a waveform changes state to the other waveforms.

In above example of timing diagram, the three waves A, B, C are 'HIGH' only during bit time '7' and they all change back 'LOW' at the end of bit time '7'.