DekTec DTC-300 StreamXpress - Manual (Page 73)

DTC-300-SP – StreamXpress
USER MANUAL
73
Appendix B. PRBS generation
This appendix describes in detail how StreamXpress generates the PRBS signal as defined in ITU
O.151.
A 23-bit register (prbs_reg) is initialized to all 1’s. Then, for each bit of output:
- Output-bit =~(prbs_reg_bit17 ^ prbs_reg_bit22
That is, bits 17 and 22 (zero-indexed) are exclusive or-ed and the result of that is negated.
The result of this computation is written to the output buffer.
- Prbs_reg = ((prbs_reg<<1) | (((prbs_reg>>22) ^ (prbs_reg>>17)) & 1)) & 0x7FFFFF
Bits 17 and 22 are exclusive or-ed. Then all bits in prbs_reg are shifted to the left and the
result of the exclusive or of the previous bits 17 and 22 is stored as new bit 0.