144_pdfsam_VLSI TEST PRINCIPLES &amp; ARCHITECTURES

# 144_pdfsam_VLSI TEST PRINCIPLES & ARCHITECTURES -...

This preview shows page 1. Sign up to view the full content.

Logic and Fault Simulation 113 To resolve the problem of information loss, one would have to assign to each flip-flop a unique unknown symbol u i and associate with u i the following rules: NOT ±u i ² = u ± i NOT ±u ± i ² = u i u i · u ± i = 0 u i + u ± i = 1 Let us revisit the example in Figure 3.5. Based on the above rules, the output of G 3 will be u ± instead of u , and finally one has K = 0, the correct answer. The problem with this approach is that signals that are affected by multiple unknown symbols have to be expressed as Boolean expressions of u i ’s. As the number of unknown symbols grows, the required symbolic simulation becomes cumbersome. 3.2.2.2 High-Impedance State Z Until now, the logic signal states that we have discussed are 1 and 0, indicating that the signal is connected to either V dd or ground. (The unknown symbol indicates
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: uncertainty; however, the signal of interest is still 1 or 0.) In addition to 1 or 0, tristate gates have a third, high-impedance state, denoted by logic symbol Z . Tristate gates permit several gates to time-share a common wire, called a bus . A signal is in the Z state if it is connected to neither V dd nor ground. Figure 3.6 depicts a typical bus application. In this example, three bus drivers ( G 1 , G 2 , and G 3 ) drive the bus wire y . Each driver G i is controlled by an enable signal e i , and its output o i is determined as follows: o i = ± x i if e i = 1 Z if e i = pull-up or down DFF e 1 G 1 G 2 G 3 e 2 x 1 o 1 o 2 o 3 x 2 x 3 e 3 y Resolution Function ± FIGURE 3.6 Tristate circuits....
View Full Document

## This note was uploaded on 05/16/2011 for the course ENGINEERIN mp108 taught by Professor Elbarki during the Spring '08 term at Alexandria University.

Ask a homework question - tutors are online