Methods and apparatuses for transient analyses of a circuit using a
hierarchical approach. In one embodiment, the cells are grouped locally
on the power supply network according to average power dissipation. A
time varying current of each cell group is estimated using a
probabilistic approach to represent the cell group so that the
probability of a more severe waveform for the current of the cell group
is under a certain level. For example, the cells in a group are
partitioned as switching cells and non-switching cells using cell toggle
rates for the determination of the time varying current. The circuit
model of the power supply network includes the current sources according
to the estimated time varying currents for the cell groups, the power
supply wire resistance, the power supply to ground wire capacitance, well
capacitance and the de-coupling capacitance from non-switching cells.