The objective of this chapter consists of introducing the students to the subject of mixed-signal testing, using the IEEE 1149.4 standard.
From 1149.1 to 1149.4
It was already mentioned in previous chapters that the IEEE 1149.1 standard (BST) was developed with the specific purpose of addressing the structural test of printed circuit boards. The narrow scope of BST simplified its acceptance and helped to accelerate the introduction of BST products, since there were few compromise solutions requiring an agreement among the sectors involved (the design and test community, semiconductor and ATE manufacturers, CAD companies, etc.). The effectiveness of this test technology was a strong contributor to its success, but it takes more than a good solution to gain market acceptance. On the other hand, its narrow scope meant that a reasonable number of problems would remain in the queue waiting for a broader solution. One of the areas which was left behind was the testing of mixed-signal circuits, which became a visible R&D trend in recent years due to the increased market share of this type of products. One of the most promising approaches towards an effective mixed-signal test technology is the 1149.4 standard, which will be presented in this chapter.
Scope of the IEEE 1149.4 standard
The 1149.4 IEEE standard was approved in June of 1999 and consists of an extension to 1149.1, defining the structures that are to be added to an 1149.1-compliant chip, in order to simplify the testing of mixed-signal circuits.
It is important to stress the fact that the 1149.4 infrastructure widens the application domain of BST, but keeps exactly the same characteristics of a serial access scan infrastructure. This means that areas such as functional testing will still be insufficiently served by this new test technology, because of the serial nature of scan and also due to the difficulty of synchronising functional events and test operations. However, the structural test operations enabled by the BST infrastructure will now be able to address mixed-signal circuits as well. Moreover, some parametric test operations will now become possible, such as measuring the impedance of discrete components placed between 1149.4 pins, or checking the DC threshold levels in digital circuit inputs and outputs.
The introduction of 1149.4 may therefore be seen as an important extension of the original 1149.1 concept, even if still very much restricted to structural test. The addition of optional instructions to 1149.4-compliant circuits will however enable wider application areas, eventually including hardware debugging, functional test operations and real-time fault detection (such as is already possible with enhanced 1149.1 infrastructures).