o Max Transition (or Max Slew): This is a design rule constraint specified in the library (.lib) that defines the longest permissible time for a signal to transition from one logic level to another (e.g., 10% to 90% of Vdd).
o Purpose:
Ensures signal integrity and predictable cell behavior. Slow transitions can
cause:
§ Increased
sensitivity to noise (crosstalk).
§ Increased
short-circuit power consumption within the receiving cell.
§ Unreliable
timing (cell delays are characterized based on input slew; very slow slews
might fall outside characterization).
§ Potential
meta-stability if driving clock pins.
o Max
Capacitance (or Max Load): This constraint defines the maximum total
capacitance that the output pin of a cell is allowed to drive. This
capacitance includes subsequent gate input capacitances and interconnect (wire)
capacitance.
o Purpose:
Ensures the driving cell can charge/discharge the load within the expected
transition time limits and maintain signal integrity. Exceeding max cap leads
to:
§ Slow
transition times (violating max transition).
§ Increased
cell delay.
§ Potential
cell damage or reduced reliability over time (though less common now).
§ Fix
max cap even if trans is clean to ensure library characterization assumptions
hold
o Priority:
Max Transition is generally given higher priority during optimization
and fixing.
o Reason:
Fixing a max transition violation often implicitly fixes or helps the
corresponding max capacitance violation. If the transition is too slow, it's
usually because the load (capacitance) is too high or the driving cell is too
weak. Upsizing the driver or buffering the net addresses both the slow transition
and the high load. Conversely, simply meeting max capacitance doesn't guarantee
meeting max transition if the driving cell is very weak or the net has high
resistance. Furthermore, transition time directly impacts noise immunity and
power consumption, making it a critical signal integrity parameter.
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