[oc] Maximum speed of TCK in boundary scan chain in SoC

[¹èÇüÈ£ <recreate@nownuri.net>]

Hello 
I'm studying the infra structure for SoC. As you know, due to the lack of access from outside, there is infrastructure need for looking inside signals of the SoC. One of the solutions are boundary scan. For I know, the limitation of boundary scan clock(TCK) is limited 20MHz in general commercial products. But it is not sufficient for real-time signal acquisition.
What is the maximum speed of TCK in nowaday ASIC or SoC? And what is the limiting factor of TCK? 

thanks
Hyeoungho
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