**2. Standard library: Liberty format (LIB)**

Liberty format (LIB) (Synosys, 2003) is the most popular library format at gate level to store the timing information and the average energy consumption of each cell in the standard library. Those data are stored using some look-up tables. The definitions of some commonly used variables are listed as follows. They will be used later to derive the proposed current waveform model.

**Transition Time**: This is defined as the duration time of a signal from 10% to 90% VDD in the rising case and from 90% to 10% VDD in the falling case. TR(X) is defined as the transition time of the node X in the rising case. TF(X) is defined as the transition time of the node X in the falling case.

**Propagation Time**: This is defined as the duration time from the input signal crossing 50% VDD to the output signal crossing 50% VDD. TDR(XÆY) is defined as the propagation delay from the related pin X to the output Y when the output Y is rising. D represents the propagation delay and R represents the rising case. TDF(XÆY) is defined as the propagation delay from the related pin X to the output Y when the output Y is falling. F represents the falling case.

**Setup Time**: This is a timing constraint of the sequential cell, which is defined as the minimum time that the data input D must remain stable before the active edge of the clock CK to ensure correct functioning of the cell. In other words, it is the duration from D crossing 50% VDD to CK crossing 50% VDD if the output value can be evaluated successfully. TSR(D) is defined as the setup time when the data input D is rising. S represents the setup time and R represents the rising case. TSF(D) is the setup time when the data input D is falling. F represents the falling case.

**Load**: This is the total capacitance at a node. Load(Y) is defined as the capacitance at the node Y.

**Internal Powe**r: This is the internal energy consumption of a cell without the energy consumed on its output loading. EINT is defined as the internal energy consumption of the cell.

**Changing Time**: T(X) is defined as the time that the signal X is crossing 50% VDD, which is the signal transition point in logic simulators recorded in VCD (Value Changed Dump) files.

**Voltage Definitions**: VDD is defined as the supply voltage. VT is defined as the threshold voltage of the transistor.
