CHAPTER 12
TEST CAPABILITIES
The Inte1386 DX microprocessor contains built-in features that enhance its testability. These features are derived from signature analysis and proprietary test techniques. All the regular logic blocks of the Intel386 DX microprocessor, or about half of all its internal devices, can be tested using these built-in features.
The Intel386 DX microprocessor testability features include aids for both internal and board-level testing. This chapter describes these features.
12.1 INTERNAL TESTS
Allowances have been made for two types of internal tests: automatic self-test and Translation Lookaside Buffer (TLB) tests. The automatic self-test is controlled com- pletely by the Intel386 DX microprocessor. The designer needs only to initiate the test and check the results. The TLB tests must be externally developed and applied. The Intel386 DX microprocessor provides an interface that makes this test development simple.
12.1.1 Automatic Self-Test
The Inte1386 DX microprocessor can automatically verify the functionality of its three major Programmable Logic Arrays (PLAs) (the Entry Point, Control, and Test PLAs) and the contents of its Control ROM (CROM). The automatic self-test is initiated by setting the BUSY# input active during initialization (as described in Chapter 3). The test result is stored in the EAX register of the Inte1386 DX microprocessor.
The self-test progresses as follows (see Figure 12-1):
1.Normal PLA or CROM inputs are disabled. '
2.A pseudo-random count sequence, generated by an internal Linear Feedback Shift register (LFSR), provides all possible combinations of PLA and CROM inputs.
3.PLA and CROM outputs for each input combinations are directed to a parallel-load LFSR.
4.Through the action of this LFSR, a signature of all output results is accumulated.
5.After all input combinations have been sequenced, the final contents of the LFSR are XORed with a signature constant stored in the Intel386 DX microprocessor. If the LFSR contents match the signature constant, the result will be all zeroes, indi- cating functional PLA and CROM.
6.The result is loaded into the EAX register.
