Chapter 12. Test
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Chapter 12
Test
This chapter provides an overview of test features of CF4e. Some of the features, such as
MBist hardware, are included in the CF4e design. The scan and wrapper methodology,
described later in the chapter, are part of the CF4e design but are described here as a
reference for properly designing CF4e for test.
Because it is less accessible, an embedded core device is more difficult to test. The solution
to applying the test to an embedded core should not be less efficient, should not lessen the
quality, and should not significantly increase integration costs by the need for extra signals,
routes, and logic. High quality levels and efficient test vectors can result from using the
proper mix of test techniques to address the embedded market.
Testing the structure of general combinational and sequential logic in the core is typically
done by application of vectors measured against the stuck-at fault model. The test vectors
applied may be functional or scan based; however, full-scan testing is the most efficient test
architecture for generating and applying stuck-at vectors. A further optimization to a
full-scan test architecture that allows the reduction of the shift cost (number of clock cycles
required to load in a state) associated with the scan architecture is the support of a
parallel-pin architecture with multiple, simultaneously operational scan chains.
Testing memory arrays in an embedded core is most efficiently done with a memory
built-in-self test (MBIST) architecture. One major goal is to reduce the overall test time by
testing each embedded memory simultaneously with at-speed data transfers, reads, and
writes. Another goal is to reduce the signal interface involved in testing multiple embedded
memories by requiring only the invoke, done, and fail indicators (as a minimum).
All logic internal to the boundary of an embedded core can be tested efficiently with support
from test selection, scan, and MBIST architectures. These architectures, in conjunction
with tester pauses and current measurement techniques, allow all test and DFT goals to be
met. In addition, the optional test wrapper scan architecture at the embedded core hierarchy
boundary permits testing beyond the embedded core.
The embedded CF4e is designed to be tested independently of the rest of the chip in all test
modes. It also allows noncore logic to be tested up to the core interface. No reliance on
internal core logic or specific core test modes should be required to test non-core logic.
Considerations are taken to ensure that the CF4e can be put in the functional (nontest) mode
with no residual effect or interference from the test logic.
F
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