A false-path exception marks paths that cannot switch in the real design, such as crossings between unrelated test and mission logic. The tool stops optimising them against the clock.
A false path must be truly impossible, not merely slow. Marking a real path false hides its failure instead of fixing it.
A multicycle exception gives a path more than one clock period, for logic that is validly sampled late, such as an enable-gated datapath. The setup check moves out to the stated number of cycles (a multicycle value of 2 allows two cycles from launch to capture).
How the tool pairs the setup and hold checks for multicycle paths varies, so state both parts explicitly. The listing below sketches the setup part in tool-neutral form.
The listing below is illustrative tool-neutral SDC and is not run by any tool.
illustrative SDC sketch (not run by any tool):
set_false_path -from [get_ports test_en] -to [get_clocks CLK]
set_multicycle_path -setup 2 -from [get_cells slow_a] -to [get_cells slow_b]
# hold pairing for multicycle paths varies between tools; state it explicitlyCase analysis pins a pin or port to a constant value, so the tool analyses only the paths that are live under that value. It models a mode tie without changing the RTL.
A maximum-delay exception bounds named paths directly in EXAMPLE nanoseconds, independent of the clock period. It suits paths whose budget is not one period, such as a short handshake.
The listing below is illustrative tool-neutral SDC and is not run by any tool.
illustrative SDC sketch (not run by any tool):
set_case_analysis 0 [get_ports test_en]
set_max_delay 4.0 -from [get_cells slow_a] -to [get_cells slow_b]Exception | Effect | Risk if wrong |
|---|---|---|
False path | Path is not timed against the clock | A real path escapes timing entirely. |
Multicycle path | Setup check moves out to the stated number of cycles | A one-cycle path is given time it does not have. |
Case analysis | Only the tied value is analysed | Paths live under the other value are ignored. |
Maximum delay | Path is bounded in EXAMPLE ns | A budget detached from the clock drifts when the period changes. |
The tool stops timing it, so its failure is hidden. A false path must be truly impossible.
The hold pairing, which varies between tools. State both parts explicitly.