Why should you prefer the -from/-to form of an exception over the -through-plus-clock form?
From PDVerse PrimeTime STA Interview Guide · pdVerse Mentor Guide
Ten-second Interview Answer
Because the special form - startpoint or endpoint given as -through with a clock as -from / -to - supports all valid start and end types but is more computationally intensive, especially on large designs. Plain -from / -to is cheaper.
Complete Technical Explanation
There is a special way to write a path specification in which the startpoint or endpoint is given as a -through while a clock object is supplied to -from or -to . It exists for good reason: it supports all valid start and end types, so it can express specifications that the plain form cannot. But it is more computationally intensive to evaluate, and that cost grows with design size - on a large design it can be a meaningful part of runtime, because the tool has less structure to work with when narrowing the candidate path set. Plain -from / -to is cheaper, so the guidance is to use it whenever it can express what you mean, and reserve the special form for the cases that genuinely require it. This is the same efficiency instinct that shows up all over exception writing: several forms are functionally correct, and the senior answer is the cheapest correct one. It matters most when the specification lands in an exception rather than a one-off report, since an exception written in the expensive form is re-evaluated on every path search in every scenario, multiplying a small per-run cost across the whole mode and corner matrix.
In Pd / Signoff
Runtime tuning on a large SoC. When a signoff run is slower than expected, the exception file is worth scanning for specifications written in the expensive -through -plus-clock form where a plain -from / -to would say the same thing - the cost is paid in every scenario of the matrix, not once.
Common Trap
Reaching for the -through -plus-clock form by habit because it always works, without checking whether plain -from / -to expresses the same intent. Both give correct results, so nothing flags it - you just pay the extra path-search cost on every run across the whole scenario matrix.
Expect Next
Which specifications genuinely require the -through -plus-clock form rather than plain -from / -to?
Topic And Primetime Commands
Timing Exceptions / Path Specification. No specific PrimeTime command is required for this conceptual answer.Keep building interview depth
Continue practising STA
Continue learning free
Get a practical low-power chapter
Receive the existing “Low Power and Multivoltage Fundamentals” PDF chapter and its download link by email.
Continue practising