什么是useful skew, global skew, local skew？

参考closing the gap between ASIC and full custom design

book by david chinnery, kurt keutzer, from UCB

clock skew: the difference between the time clock signals arrive at the clock sinks excluding ignore pins(clock sinks include the CK pin of sequential cells, such as transparent latches, flip-flops, ICG cells, and memory blocks, and ignore pins include explicit ones and implicit ones, such as output, pins without timing info, etc )
global skew: the clock skew between all the valid end points of the clock tree, no matter those points are logically/physically adjacent or not
local skew: only count the skwe when the two points are adjacent
useful skew: when setup/hold violation happens, we can adjust the arrival time of clock signals to meet the timing constraint. this technique is not so useful when on-chip-variation become more and more significant.