pre,post CTS useful skew

what is the effect of preCTS useful skew,post cts useful skew,


what is lacal& glabal skew,

please tell me some thing about scheduling,latency files

thanku

Hi
As far as I know, there will be no pre CTS useful skew.. AS the clock is considered as ideal in pre CTS stage. Post CTS useful skew helps in meeting the timing for critical paths..

If you have a critical path violating timing by 1ns in a 10ns clock and the next flop is meeting the timing by 8ns itself (when period is 10ns), then you will have huge violation in the path.. In that case you delay the clock to the first flop by around 1.2ns. This will reduce the timeperiod for the data to arrive at the next flop by 1.2ns. But the timing violation at the first flop is met.... comming to the second flop, it has only 8.8ns for the data to reach the next stage. But as it only takes 8ns to reach the next stage, here also timing is met. so, In this way the timing for the both flops are met.. hence the timing violation of that critical path is resolved..

This is how the useful skeew helps us in meting timing.. If this is not there you need to use multicycle path(insrert onemore flop in the first stage). This will increase onemore clock cycle for that stage and hence slows down the design speed..

This is what drizzle is trying to explain saying that using useful skews at the local stages to increase the fastness of the design..

In the case if you are going for useful skew.. take care of holdtime.It is very important.

Please correct me if there are any errors..

Added after 2 minutes:

Local skew and global skew: If you give some skew to the individual clocks in a specified area.. then it is called local skew.. and if you give the same skew to a specified and complete block itself in the top level .. then it is called as a global skew...

Hi,
One comment regarding local and global skew.
In CTS stage the tool divides into several clock domain and does CTS. Local skew is the skew seen within that clock domain. Global skew is the skew seen between any two clock domain. Generally P&R tools try to reduce global skew as near to zero, but what u specify in clock specification file as max skew will be local skew.