supersonic_528
Newbie level 5
Hi,
I am a bit confused about how scan works in ASIC designs, especially when we mix clocks in the same chain. On searching this forum, I found the following answer from a post on similar topic: https://www.edaboard.com/threads/199460/
You can mix the clocks on the same chain.
For shifting, having lockup latches can give you a solution to avoid metastability issue.
For capturing, you just can't toggle all the clocks at once. You need to toggle one clock at a time and all works fine.
However, it is still not very clear to me. I have the following questions.
(1) Firstly, here are we talking about the same clock being used in scan shift and scan capture modes?
(2) If we do mix clocks in a scan chain, can the clocks be of any frequency or is there any restriction on that? I am not very clear on how scan shift will work with clocks of two different frequencies even if we insert lockup latches. If the clock used at the beginning of the chain has greater frequency and data is scanned in using that clock, then the flop-flops on the other clock will miss data during shift, correct?
(3) Why can we not toggle all the clocks in the chain during scan capture? What is the problem with that?
Also, if anyone can point me to some source explaining clock mixing in scan chain in greater detail (preferably with some diagrams and examples), that will be great.
Thank you.
I am a bit confused about how scan works in ASIC designs, especially when we mix clocks in the same chain. On searching this forum, I found the following answer from a post on similar topic: https://www.edaboard.com/threads/199460/
You can mix the clocks on the same chain.
For shifting, having lockup latches can give you a solution to avoid metastability issue.
For capturing, you just can't toggle all the clocks at once. You need to toggle one clock at a time and all works fine.
However, it is still not very clear to me. I have the following questions.
(1) Firstly, here are we talking about the same clock being used in scan shift and scan capture modes?
(2) If we do mix clocks in a scan chain, can the clocks be of any frequency or is there any restriction on that? I am not very clear on how scan shift will work with clocks of two different frequencies even if we insert lockup latches. If the clock used at the beginning of the chain has greater frequency and data is scanned in using that clock, then the flop-flops on the other clock will miss data during shift, correct?
(3) Why can we not toggle all the clocks in the chain during scan capture? What is the problem with that?
Also, if anyone can point me to some source explaining clock mixing in scan chain in greater detail (preferably with some diagrams and examples), that will be great.
Thank you.