A driver sees the wire plus every sink pin as one capacitance load. Larger loads mean slower edges and longer delays.
Three limits guard the result. Each is stated as a constraint, and each repair in this topic targets one of them.
Rule | What it limits | EXAMPLE limit |
|---|---|---|
Max fanout | Sink count on one net | 32 sinks |
Max transition | Slowest allowed edge (slew) | 0.5 EXAMPLE ns |
Max capacitance | Total load on one driver | 10 EXAMPLE fF |
Load is the wire capacitance plus one pin capacitance per sink. The calc block checks an EXAMPLE net against an EXAMPLE limit.
EXAMPLE limits (illustrative, not any tool's defaults):
max_capacitance 10 EXAMPLE fF, max_fanout 32 sinks
EXAMPLE net: wire 4 fF, each sink 1 fF
6 sinks: 4 + 6 x 1 = 10 fF -> exactly at the limit
7 sinks: 4 + 7 x 1 = 11 fF -> violates by 1 fF
40 sinks against max_fanout 32 -> violates by 8 sinksThe three SDC-style commands below are a tool-neutral sketch. The names are standard SDC; the limits are EXAMPLE values.
# illustrative SDC sketch (not run; EXAMPLE limits, not any tool's defaults)
set_max_fanout 32 [current_design]
set_max_transition 0.5 [current_design]
set_max_capacitance 10 [current_design]6 sinks: 4 + 6 = 10 fF, exactly at the limit. The 7th sink reaches 11 fF and violates.
It violates by 8 sinks. No: the repaired net must still be checked against max transition and max capacitance.