In our previous posts, we discussed about the duty cycle, duty cycle variation and duty cycle degradation. Bad duty cycle impacts half cycle timing paths and has impact in meeting timing for minimum pulse width checks of flip-flops. However, there are certain techniques available that can help you in improving the duty cycle of the clock. We will discuss these techniques in this post as below:
1. Dual inversion in clock branch: A certain category of logic cells are more probable of having one of the rise or fall delays greater than the other. A chain of such cells will make either high pulse of clock shorter or low pulse of clock shorter. One can use an inverter in the middle of the chain as shown in figure below to tackle this. Doing this, what we are essentially doing is converting rise edges to fall and vice-versa. So, the shortening of pulse of first few elements is balanced with the rest of the elements. In the below figure, there are 20 buffers, each shortening the pulse by 10 ps. The output of 10th buffer will have a shorter pulse as compared to clock source. The inverter at the output of 10th buffer will feed an inverted clock to 11th buffer. This will have high pulse which is greater than low pulse. Rest of the chain will try to reduce this pulse. In the end, we get a pulse which is equal to what was available at the source.
One can also try an all-inverter clock tree. In an all-inverter clock tree, every element will change the sense of clock pulse; thereby minimizing the clock pulse distortion.
However, this kind of delay balancing will only work where there is inherent variation of delays in rise vs fall. It will not work in case of OCV variations. So, if the chain length is arbitrarily large, our second method will come to rescue.
2. Even division to tackle duty cycle degradation: Suppose there is source clock with very poor duty cycle (say 10%) and you divide down the clock by 2 with a flip-flop divider. What we observe is amazing. The resulting clock is having almost 50% duty cycle. So, whenever we need an output clock with perfect duty cycle, we can use a divider to divide down the clock. The only drawback of this method is that we need a source clock of frequency twice than what is required to be timed!!
There are a few things to be kept in mind for this method:
- This method will improve the duty cycle of clock at the output of the flop. Degradation in duty cycle happening after the divider, if any, will be there.
- Duty cycle of the input clock at flip-flop must be within the limits of what is required to be minimum pulse width at the flip-flop.