HP HP-Bench-Briefs-1996-01-03 free download

File name HP-Bench-Briefs-1996-01-03.pdf

HEWLETT PACKARD SERVICE INFORMATION FROM HEWLETT-PACKARD 1st Quarter 1996 Failure Modes o Qigital ICs b Jim BechtoldlHewlett-Packard Open Output Bond cause all inputs driven by that output to float to a bad level since 1.5 volts is In the case of an open output bond (Fig- less than the high threshold level of 2.0 In order to troubleshoot efficiently, it ure l),the inputs driven by that output volts and greater than the low thresh- is important to understand the type of are left to float. In TTL and DTL circuits old level of .4 volt. In TTL and DTL, a failures found in digitalcircuits.IC fail- a floating input rises to approximately floating input is interpreted as a high ures can be categorized into two main 1.4 to 1.5 volts and usually has the ef- level. Thus the effect will be that these classes; internal IC failure and a failure fect on circuit operation as a high logic inputs will respond to this bad level as in the circuit external to the IC. level. Thus an open output bond will though it were a static high signal. Internal Failures There are four types of failures that can occur internally to an IC. These are: An open bond on either an input or output A short between an input or output and Vcc or ground A short between two pins (neither of which are Vcc or ground) A failure in the internal circuitry (of- ten called the steering circuitry) of the IC External Failures In addition to these four failures inter- nal to an IC, there are four failures that can occur in the circuit external to the IC. These are: A short between a node and Vcc or ground A short between two nodes (neither of which are Vcc or ground) An open signal path A failure of an analog component 1 Effects on Circuit Operation DTL INPUTS AS A HIGH STATE The first failure internal to an IC men- tioned was an open bond on either an input or output. This failure has a dif- Figure 1. The Effect of an Open Output Bond Upon Circuit Operation. An open output ferent effect depending upon whether bond allows all inputs driven by that output to float to a "bad level." This level is usually it is an open output bond or an