Electronics Terminology: MISR
In
electronics, a
MISR is a
Multi-
Input
Signature
Register, (also called Multi-Input
Shift Register).
The MISR is a sequential electronic circuit whose structure is essentially a Linear Feedback Shift Register, (LFSR), which has been modified so that an exclusive or gate drives the input of one or more of the LFSR's flip-flops.
Where are they used?
MISR's are typically used in testing random
logic portions of
electronic circuits as part of a
BIST-based
DFT methodology. They are used to compress
parallel streams of
data from test points within a
circuit into a "
signature". They are favoured in
BIST-based test beacuse of their low probability of "
aliasing".
How do they work?
One input of each
EXOR gate in the MISR is connected to the
LFSR feedback point. The second
EXOR input, (the "test data" input), is connected to an output of the
logic being tested. The "
circuit under test" will usually be stimulated by a
LFSR pattern generator clocked at the same rate as the MISR. The "
signature" of the test, as read on the MISR's
flip-flop outputs', is a function of its regular, LFSR sequence and the inputs from the "
logic under test". Unexpected input values from the latter, (caused by a "
stuck-at fault", for example), will cause the MISR
signature to deviate from the expected value - the circuit
fault has been detected at this point. From this point in time forward, the values stored within the MISR will be different to the "fault-free" version, even if no more faults are detected.
At the end of the test sequence, the final signature can be compared to the "expected" final value to determine if a fault has been detected.