TRANSITION DENSITY - A NEW MEASURE OF ACTIVITY IN DIGITAL CIRCUITS

Reliability assessment is an important part of the design process of digital integrated circuits. We observe that a common thread that runs through most causes of runtime failure is the extent of circuit activity, i.e., the rate at which its nodes are switching. We propose a new measure of activity, called the transition density, which may be defined as the ''average switching rate'' at a circuit node. Based on a stochastic model of logic signals, we also present an algorithm to propagate density values from the primary inputs to internal and output nodes. To illustrate the practical significance of this work, we demonstrate how the density values at internal nodes can be used to study circuit reliability by estimating 1) the average power and ground currents; 2) the average power dissipation; 3) the susceptibility to electromigration failures; and 4) the extent of hot-electron degradation. The density propagation algorithm has been implemented in a prototype density simulator. Using this, we present experimental results to assess the validity and feasibility of the approach. In order to obtain the same circuit activity information by traditional means, the circuit would need to be simulated for thousands of input transitions. Thus this approach is very efficient, and makes possible the analysis of VLSI circuits, which are traditionally too big to simulate for long input sequences.