Verifying the safety of digital in-circuit testing

The author describes a method of verifying the safety of in-circuit tests by using a software package to examine the test. During a device test inputs are driven high and low, and outputs are being monitored. Most input legs of devices on a board are electrically connected to output pins, whether on the same device or not, so corresponding outputs are driven as well. This is referred to as backdriving. To obtain the best performance of speed vs. accuracy a two level design is the best compromise, because typically 80% of tests will pass the first stage. The ability to follow flow control in both the generation and execution of tests enables the effect of faults causing branches to possibly hazardous code can be evaluated