Duty cycle: Duty cycle of a clock is defined as the fraction of a period of clock during which the clock is in active state. Duty cycle of a clock is normally expressed as a percentage. For instance, figure below shows a clock having an active state of '1' stays low for 2 ns during its period of 10 ns. It is, therefore, said to have a duty cycle of 20%.
How duty cycle impacts timing: Duty cycle of clock plays a big role in timing closure of designs. We need to consider following factors related to duty cycle variation while timing:
- Half cycle timing paths: If there are both positive and negative edge-triggered flip-flops in the design, duty cycle of the clock matters a lot. For instance, if we have a clock of 100 MHz with 20% duty cycle; For a timing path from positive edge-triggered flip-flop to negative edge-triggered flip-flop, we get only 2 ns for setup timing for positive-to-negative path and 8 ns for negative-to-positive path as compared to 10 ns for a full cycle path. However, if the same clock had duty cycle of 50%, we would have got 5 ns for the same half cycle timng path.
- Minimum pulse width requirements: At high frequencies, duty cycle matters a lot. For instance, every sequential element has requirement of minimum pulse width that should reach it (read this). If the duty cycle of the clock is not close to 50%, we are limited in providing high frequency even if we are capable of meeting timing at even higher frequencies. Let us take an example. If the minimum pulse width requirement of a flip-flop is 500 ps, then with 50% duty cycle clock, we can use a clock of 1 GHz (1 ns clock period). But if we use a clock of duty cycle of 20%, we cannot use a clock greater than 400 MHz.
With the above things in mind, it makes sense to use a clock with duty cycle as close to 50%. However, in many scenarios, it may not be feasible to do so. So, one needs to decide the priorities; i.e., architecture complexities vs timing complexities. Generating a divided clock of 50% duty cycle is not always possible and there are a few complexities involved in architecture. For instance, clock waveform synchronization between the clocks if there are multiple dividers. Also, for odd division factors like divide_by_3 etc., we need more complex divider circuitry than what may be required for divide_by_2 or divide_by_4 etc.