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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.
12-1