PRBS

Coding principles

The principle of PRBS coding is that the current codeword is obtained by performing the PRBS coding on the codeword, so the verification uses this principle to verify.

PRBS is the abbreviation of Pseudo Random Binary Sequence, which means "pseudo random binary sequence". The PRBS code has a "random" characteristic, because in the PRBS code stream, the binary numbers "0" and "1" appear randomly, but it is different from the real random code. This "random" characteristic is only partial , That is, within the cycle, "0" and "1" appear randomly (after the code stream generation function and the initial code are determined, the sequence of the code stream is also fixed), but the code stream in each cycle is exactly the same , So we call it "" pseudo-random code.

The period length of the PRBS code is related to its order. Commonly used orders are 7, 9, 11, 15, 20, 23, 31, which is what we often say PRBS7, PRBS9, PRBS11, PRBS15, PRBS20, PRBS23, PRBS31.

For n-th order PRBS codes, the sequence length of each cycle is 2^n-1. In each cycle, "0" and "1" are randomly distributed, and the number of "1" is more than that of "0" One more number, the maximum number of consecutive "1" is n, and the maximum number of consecutive "0" is n-1 (after reversal, it is n-1 consecutive "1" and n consecutive "0").

When performing bit error tests on high-speed signal links, PRBS bit streams are basically used to simulate the real line network bit stream environment, because in the online network, all the data appears randomly, there is no rule at all, The PRBS code stream has this kind of "random data" characteristic to a certain extent, binary "0" and "1" appear randomly, and its spectral characteristics are very close to white noise. The higher the order of the PRBS code stream, the richer the code patterns it contains, the closer it is to the real line network environment, and the more accurate the test results.

A PRBS sequence can be serial/parallel converted into multiple channels (2, 4, 8, 16, .), the output rate of each channel is reduced, but still retains all the characteristics of the original sequence; otherwise, the same clock source low rate multiple channel (2 , 4, 8, 16, .) The same n number of PRBS can be converted into high-speed n-order PRBS via parallel/serial conversion.

The PRBS pattern generator consists of a shift register and XOR gate (XOR).

In the standard PRBS code stream, the ratio of binary numbers "0" and "1" is equal, but in some specific test occasions, it is required to increase the proportion of "0" or "1" in the code stream, which has Variable Mark Ratio Quasi-PRBS, this code is also based on PRBS code, common Mark Ratio are 1/8, 1/4, 3/4 and 7/8 (standard PRBS code Mark Ratio is 1/2)

Reverse the 1/4, 3/4 Mark RatioPRBS code stream to get 3/4, 7/8 Mark Ratio PRBS code stream.

1). In the 1/4 Mark Ratio PRBS code stream, the proportion of binary numbers "1" accounts for 1/4, and the proportion of "0" accounts for 3/4;

2). In the 3/4 Mark Ratio PRBS code stream, the proportion of binary numbers "1" accounts for 3/4, and the proportion of "0" accounts for 1/4;

3). In the 1/8 Mark Ratio PRBS code stream, the proportion of binary numbers "1" accounts for 1/8, and the proportion of "0" accounts for 7/8;

4). In the 7/8 Mark Ratio PRBS code stream, the proportion of binary numbers "1" accounts for 7/8, and the proportion of "0" accounts for 1/8