Prbs generation – Maxim Integrated HFAN-09.5.0: Pattern Creator/Converter Software User Manual
Page 3
Application Note HFAN-9.5.0 (Rev.1; 04/08)
Maxim Integrated
Page 3 of 8
2.1 PRBS Patterns
The Pseudo Random Bit Sequences (PRBS) are
generated using a software version of the
conventional hardware shift register with the taps of
minimum shift algorithms generated from Galois-
field arithmetic (Figure 2, Table 1, see References 1
& 2). The shift register length is determined by the
pattern title. For example, a PRBS 2
7
-1 pattern
would be generated using a shift register 7 bits long.
Where n is the register length, the PRBS pattern
contains every possible combination of n bits
(except one).
n
n-1
2
1
3
XOR
SHIFT DIRECTION
T
A
P
1
T
A
P
2
T
A
P
3
T
A
P
4
O
U
T
IN
n Bit Shift Register
PRBS Generation
PRBS Generation
Figure 2. PRBS Generation
The mark density (100%*(number of ones / total
bits)) and the transition density (100%*(number of
bit transitions / total number of bits)) of PRBS
patterns are approximately 50%. This approximation
improves as the pattern length (shift register length)
increases. Also, the maximum number of
consecutive identical digits (CIDs) is equal to the
register length (n) (e.g. a 2
23
-1 PRBS has a
maximum of 23 CIDs).
Pressing any of the PRBS buttons opens the option
list shown in Figure 3. Using this option list, the
PRBS sequence can be repeated an integer number
of times or the total number of bits to write of the
given PRBS can be selected. The PRBS can also be
written to the file inverted or non-inverted. It is
important to note that the mark density and the
transition density of the pattern sequence can deviate
from the 50% approximations if only part of the
PRBS is written to the file.
Table 1: PRBS Pattern Length / Tap Positions
Register Number (1 to n)
Register
Length
(n)
Pattern
Length
(2
n
-1)
Tap1
Tap2
Tap3
Tap4
3
7
3
2
4
15
4
3
5
31
5
3
6
63
6
5
7
127
7
6
8
255
8
7
3
2
9
511
9
5
10
1023
10
7
11
2047
11
9
12
4095 12
9 8 5
13
8191
13
12
10
9
14
16383
14
13
3
2
15
32767
15
14
16
65535
16
14
13
11
17
131071
17
14
18
262143
18
11
19
524287
19
18
14
13
20
1048575
20
17
21
2097151
21
19
22
4194303
22
21
23
8388607
23
18
Figure 3. PRBS Options
In this software program, each PRBS register is
initialized with all 1’s. Therefore, each PRBS
sequence will start with the maximum CID length
(e.g. the first seven bits generated from the non-
inverted PRBS 2^7-1 will be all ones).