In our post "Setup and hold - origin", we discussed that every device captures data within a certain window known as "setup + hold window". During this time, data must be held stable so that it can be captured properly. Outside this window, data is allowed to toggle.
Figure above shows "setup+hold window". This window is characterized by the setup and hold times of the device. The width of this window is essentially the sum of setup time and hold time. Thus, if the sum of setup and hold time is positive, it means there is a finite window wherein the device is allowed to capture the data. On the other hand, a negative sum of setup time and hold time indicates that the width of this window is negative. In other words, the window does not exist. So, a negative setup and hold time implies that the device cannot capture the data at all!!
Thus, for a functional device, we always need the sum of setup and hold times to be positive. :-)
But is itn't contradicting to the possibility of having negative setup and hold times in data-to-data checks ?
ReplyDeleteVery good question. It is very difficult to explain in comment. Can you ping me on vlsiuniverse@gmail.com. We can have a call and discuss. :-)
DeleteHi,
ReplyDeleteWe can have negative setup and hold times as well right? For ex. in case of data-to-data checks.
Hi
DeleteYes, we can have negative setup and hold times, but the sum of setup and hold times should always be positive. It holds a special significance in that the setup + hold window needs to exist which is only possible when the sum is positive