Electronics Terminology: MISR
, a MISR
is a M
egister, (also called Multi-Input Shift
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
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.