Path Bit Research Articles

A design-for-testability technique for register-transfer level circuits using control/data flow extraction

In this paper, we present a technique for extracting functional (control/data flow) information from register-transfer level controller/data path circuits, and illustrate its use in design for hierarchical testability of these circuits. This scheme does not require any additional behavioral information. It identifies a suitable control and data flow from the register-transfer level circuit, and uses it to test each embedded element in the circuit by symbolically justifying its precomputed test set from the system primary inputs to the element inputs and symbolically propagating the output response to the system primary outputs. When symbolic justification and propagation become difficult, it inserts test multiplexers at suitable points to increase the symbolic controllability and observability of the circuit. These test multiplexers are mostly restricted to off-critical paths. Testability analysis and insertion are completely based on the register-transfer level circuit and the functional information automatically extracted from it, and are independent of the data path bit width owing to their symbolic nature. Furthermore, the data path test set is obtained as a byproduct of this analysis without any further search. Unlike many other design-for-testability techniques, this scheme makes the combined controller-data path very highly testable. It is general enough to handle control-flow-intensive register-transfer level circuits like protocol handlers as well as data-flow intensive circuits like digital filters. It results in low area/delay/power overheads, high fault coverage, and very low test generation times (because it is symbolic and independent of bit width). Also, a large part of our system-level test sets can be applied at speed. Experimental results on many benchmarks show the average area, delay, and power overheads for testability to be 3.1, 1.0, and 4.2%, respectively. Over 99% fault coverage is obtained in most cases with two-four orders of magnitude test generation time advantage over an efficient gate-level sequential test pattern generator and one-three orders of magnitude advantage over an efficient gate-level combinational test pattern generator (that assumes full scan). In addition, the test application times obtained for our method are comparable with those of gate-level sequential test pattern generators, and up to two orders of magnitude smaller than designs using full scan.

Design for hierarchical testability of RTL circuits obtained by behavioral synthesis

In recent years, there has been growing interest in behavioral (high-level) synthesis for testability. This is due to the fact that testability features, such as scan or the built-in self-test, may incur large overheads if introduced during logic synthesis in the later phase of the design cycle. Related previous work attempted to generate system-level test sets using hierarchical testability during behavioral synthesis. There, the test generation scheme is independent of bit width and is, therefore, capable of handling complex controller/data path circuits with large data path bit widths (e.g., 32), which has posed a serious challenge to logic-level sequential test generators. However, this previous work is not applicable when another high-level synthesis system is used. In this paper, we present techniques that add minimal test hardware to a given register-transfer level (RTL) circuit obtained by behavioral synthesis in order to ensure that the embedded elements in the circuit are hierarchically testable. An important byproduct of our design for testability (DFT) procedure is a system-level test set that delivers precomputed test sets to each element in the RTL circuit. This eliminates the need to apply gate-level sequential test generation to the combined controller/data path. We performed extensive experiments with several complex controller/data path circuits synthesized by three different high-level synthesis systems which do not target testability. The key advantages of our method, illustrated by these experiments, include: 1) the area, delay, and power overheads incurred for testability are very low (the average area, delay, and power overheads for a large number of benchmarks are 3.5, 0.5, and 3.4%, respectively), 2) both the DFT hardware addition and test generation algorithms are independent of the data path bit width.