[sdiy] Long LFSRs (Was Psych Tone)
David Manley
dlmanley at sonic.net
Sun Jan 6 19:57:51 CET 2019
A maximal length LFSR by definition has 2^n - 1 states.
This means these LFSRs will - by definition - eventually create every
pattern on N-bits, except for the '-1' state which is either all-0s or
all-1's depending on whether XOR or XNOR feedback is used.
So for any maximal length LFSR it doesn't really matter where the taps
are, eventually the states with many Consecutive Identical Digits
('CID') will be emitted. Different tap selections will just change the
order in which the states are generated.
To put it in simpler terms: even a maximal length LFSR will generate a
'1' followed by N-1 zeroes if you let it run long enough. So a 127-bit
LFSR will generate at some point these states (again not in this order,
but at some point):
10000000000...0
11000000000...0
11100000000...0
...
11111111111...0
...
0...00000000111
0...00000000011
0...00000000001
They'll also generate:
10101010
01010101
11001100
00110011
etc...
LFSRs are commonly used for scramblers in bit-serial transmission
protocols. The CID problem is one factor that determines how good the
PLLs used for clock and data recovery need to be. For PLLs to operate
they need edges to determine the phase of the received signal, and if
you have too many consecutive identical digits, you have no edges, and
the PLL can drift off phase/frequency if designed incorrectly. Since
the data being transmitted gets EXORed with the LFSR output you can also
have CID if the data transmitted just happens to match the LFSR output.
Typicaly this is avoided by using a data coding such as 8B10B or 64B66B
where transitions are guaranteed to occur.
-Dave (seems like I've been dealing with bit-serial transmission
protocols my whole career...)
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