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Copyright (C) 2003-2008 Fons Adriaensen <fons@kokkinizita.net>
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, write to the Free Software
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Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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//---------------------------------------------------------------------------
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// Pseudo random binary sequence generator using polynomial
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// division in GF (2).
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// There are two ways to built such a generator. Both use some
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// form of shift register.
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// 1. The first type feeds back the parity (XOR) of the taps corresponding to
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// the non-zero elements of the polynomial into the input of the register.
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// This is the most efficient way to do it in hardware.
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// 2. In the seond form, when the bit shifted out is 1, the contents of the
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// register are XORed with a bit pattern representing the polynomial.
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// This is the best way to do it in software.
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// Mutatis mutandis the two forms are equivalent. Any sequence that can be
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// generated by one of the realisations can also be produced by the other.
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// This software obviously uses the second form. It can use any polynomial
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// up to (and including) a degree of 32.
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// Defines the polynomial to be used. The value of p is found from the
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// sequence of coefficients (0 or 1) of the polynomial starting with the
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// constant term, and dropping the highest one
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// Example: P = x^7 + x^6 + 1 --> 1 0 0 0 0 0 1 1 --> 1000001 --> 0x41
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// To emulate the first form described above, start with the highest
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// exponent and drop the constant term.
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// Example: P = x^7 + x^6 + 1 --> 1 1 0 0 0 0 0 1 --> 1100000 --> 0x60
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// Also sets the state to all ones.
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// Sets the initial state to x.
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// Returns the next pseudo random bit.
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// This sets the generator in a state as if the last N (= degree) bits
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// were those defined by x (the LSB of x represents the oldest bit).
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// This can be used to synchronise a BER counter to a received bit stream,
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// or to set the initial state when emulating a generator of the first form
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// when the output is taken from the feedback.
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// This sets the generator in a state so that the first N (= degree) output
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// bits will be those defined by x (the LSB of x will be the first output bit).
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// This can be used to set the initial state when emulating a generator of
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// the first form when the output is taken from the shifted out bit.
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//---------------------------------------------------------------------------==
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// Some polynomials for maximum length seqeunces.
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void set_poly (uint32_t poly);
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void set_stat (uint32_t stat);
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void sync_forw (uint32_t bits);
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void sync_back (uint32_t bits);
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uint32_t stat (void) const;
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uint32_t poly (void) const;
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uint32_t mask (void) const;
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uint32_t hbit (void) const;
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int degr (void) const;
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inline Prbsgen::Prbsgen (void)
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: _stat (0), _poly (0), _mask (0), _degr (0)
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inline Prbsgen::~Prbsgen (void)
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inline void Prbsgen::set_poly (uint32_t poly)
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while (_mask < _poly)
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_mask = (_mask << 1) | 1;
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_hbit = (_mask >> 1) + 1;
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inline void Prbsgen::set_stat (uint32_t stat)
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_stat = stat & _mask;
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inline int Prbsgen::step (void)
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if (bit) _stat ^= _poly;
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inline void Prbsgen::sync_forw (uint32_t bits)
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for (int i = 0; i < _degr; i++)
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if (bits & 1) _stat ^= _poly;
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inline void Prbsgen::sync_back (uint32_t bits)
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for (int h = _hbit; h; h >>= 1)
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if (bits & h) _stat ^= _poly;
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inline void Prbsgen::crc_in (int b)
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bit = (_stat & 1) ^ b;
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if (bit) _stat ^= _poly;
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inline int Prbsgen::crc_out (void)
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inline uint32_t Prbsgen::stat (void) const { return _stat; }
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inline uint32_t Prbsgen::poly (void) const { return _poly; }
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inline uint32_t Prbsgen::mask (void) const { return _mask; }
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inline uint32_t Prbsgen::hbit (void) const { return _hbit; }
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inline int Prbsgen::degr (void) const { return _degr; }
added
removed
source/prbsgen.h
