Module 2 : Behavioral modeling TOPIC : Modeling a Digital pulse UNIT 1: Modeling and Simulation

Introduction Variations between input-output signals ◦ Output value varies according to the function ◦ Arrival of output varies w.r.t input Complete behavior of the circuit is described by amplitude and time variations of the output w.r.t to time. The model which enables the complete description of the block is called as behavioral model.

Representation of any Gate Any gate can be represented by ideal gate and adding delay model to it. Ideal gate represents the function model of the gate. Functional modeling is a part of behavioral modeling.

Why do we need behavioral modeling Any pulse in a digital circuit has a certain ◦ Amplitude ◦ Width ◦ Rise time ◦ Fall time To get the exact behavior of a circuit we need transient analysis of the circuit for a digital circuit.

Modeling a Digital pulse – type 1 We can model the pulse into a square signal. The pulse is assumed to be start at 50% of the of the signal. Amplitude. Pulse Width = t 2 - t 1 t1t1 t2t2

Rise time and Fall time Rise time: time taken by the signal to change its value from 10% of its amplitude to 90%. Fall time: time taken by the signal to change its value from 90% of its amplitude to 10%. t r and t f are also called Rise time and fall time delay. Rise timeFall time

Propagation delay Propagation delay of a gate defines how quickly it responds to a change at its input(s). It is measured between the 50% transition points of the input and output waveforms : Input and output pulse for an inverter

Modeling a Digital pulse – type 2 Due to Rise and fall delay another way of modeling a pulse as square pulse seems to be insufficient. Trapezium is used to model a pulse. t1t1 t2t2

Delay model as a Table Delay typeDelay Value T r (Rise time)1 ns T f (Fall time)1 ns T d (Propagation delay) 1.4 ns Pulse width4.2 ns Green lines : 10% and 90% values Red lines : 50% values