Mid-state or resistive shorts are a significant problem that often goes undetected by a typical boundary scan test. As a global industry leader, the professionals at Flynn Systems recognized this “hole” in boundary scan testing and quickly designed a solution to improve test fault coverage. As the first company to test for mid-state shorts, we realize that knowledge must also be extended beyond innovative solutions.

In this article, we are taking a look at mid-state and resistive shorts and why further testing is a crucial step.

onTAP detects and diagnoses mid-state shorts

Mid-state shorts are bridging faults that result in a mid-state voltage level rather than a hard-low or high level. In our experience working with customers, our team has found the condition occurs where short circuits exist:

1. Between boundary-scan pins on FPGA devices, when only one boundary scan pin is present on a PCB net.
2. Across pins on resistor networks where the resistors lie between the shorts fault and scannable pins, and again on nets with only one boundary-scan pin.

If left undetected and uncleared, mid-state shorts, like any bridging fault, result in system level application failures. The difficulty is that these shorts remain undetected by a traditional boundary scan test. Shorts detect algorithms with zero or one logic level. The failure concerns the captured cell on single-pin nets when the capture value is equal to the value being actively driven from the same pin.

In the resistor network, it is easy to see how the resistors isolate the capture value from the value measured at the physical short. In the case of the FPGA pins, the reasons are not so straightforward. Sufficient impedance exists between the bidirectional cells and physical pins, which allows the input cell to recognize the value driven from the drive cell, not the value measured at the physical short.

Again, the capture value is equal to the expected value, producing a PASS.

How mid-state shorts are detected

As expected, MID-STATE shorts can be detected by a boundary scan test pattern using tri-state, high impedance values, Z, in combination with zero and one values. However, one difficulty is that Z is a passive condition. Thus, this approach is likely to result in unpredictable numbers of false failures. Another problem lies within the potential number of test scans required.

onTAP addresses these difficulties in several ways. First, to achieve a manageable number of test patterns, standard Wagner test, popular in boundary scan testing and known for their compactness and effectiveness, are utilized. However, instead of using only 0-1 test patterns, 0-Z and 1-Z patterns are also used. This modified pattern is not only effective, but it also detects the mid-shorts.
In response to potential false failures, onTAP actively interrogates all indicted net combinations at run time with a more exhaustive 0-1-Z pattern. This pattern eliminates false failures and indicts any MID-STATE shorts conditions.

For more information on the innovative solutions of Flynn Systems, contact their experts at (603) 598-4444.