Why zero hold time is prefered?

[yuhiub90]

I've read an article said that today's libraries tend toward rising-edge active clocks, zero hold times, and positive set-up times. So why zero hold time is prefered to positive hold time?
Thank you.

 

[rca]

Well it is always a trade-off between the logic you add to increase the hold time margin. So the minimum is zero hold time, but if with adding "few" std cells, you could increased the hold time margin, then, you are in safe side.

The hold time distribution will show you how much path are close to "zero" and then how many std cell is needed to increase your margin.