Properly Managing Common Tri-State Control Cells Boosts Fault Coverage

Posted on December 16, 2009

As the boundary scan community continues looking for new ways to improve test procedures and achieve higher and higher fault coverage, we expect the test tools to compensate for shortcomings in silicon devices or board design. We are constantly exposed to new situations in boundary scan test and new approaches to using boundary scan to achieve better test fault coverage. One item standing in the way of higher, more accurate fault coverage is common tri-state control cells.

Common tri-state control cells are groups of pins on a common net.  Just as the name suggests, they are tri-state pins, grouped together by a common function, sharing a boundary scan cell. Though this is efficient for circuitry, it poses some issues during JTAG test. For example, when a single pin on the common cell drives or senses a value, all the pins associated with that cell are forced to perform the same function, simultaneously. This is represented in the following drawing.

Tri-State Control Cell diagram for boundary scan JTAG test
Un-handled common tri-state cells can have a negative impact on boundary scan test, dramatically reducing accuracy and fault coverage of opens and shorts tests because multiple pins sharing a common net drive in the same test vector, as displayed in the screen capture below.

onTAP JTAG Test showing Common Tri-State Control Cell un-tested
In this image, you can see pins U23.AA14 and U36.AE30 are on net U23_AA14. This test is not accounting for the tri-state pins on the common control cell, ultimately compromising fault coverage.

The yellow 0 and 1 characters show drive, or boundary register update, values at each test vector, and the green L and H values show expected boundary register capture values on the vectors following an update.

Mv64360 boundary scan device multiple pins sharing common tr-state cells

This image shows the netlist view of the Mv6430 boundary scan device, while the following image displays the pins in an expanded view.

onTAP Netlist Browser displays common tristate control cells for JTAG device

 

ProScan test view showing additional vectors added to JTAG test for tristate condition

 

This image shows how the test was revised to account for the tri-state pins on the common control cell.  The result is a boost in fault coverage, with the added benefit of making the test more accurate.
As explained in the text box in this screen shot, onTAP accounts for the shared cells and adds additional test vectors to ensure pins do not drive simultaneously, allowing for more comprehensive tests that deliver higher, and most importantly, more accurate, test fault coverage.

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