GNU Radio Manual and C++ API Reference  3.7.4
The Free & Open Software Radio Ecosystem
 All Classes Namespaces Files Functions Variables Typedefs Enumerations Enumerator Properties Friends Macros Groups Pages
Public Member Functions | List of all members
gr::digital::lfsr Class Reference
Miscellaneous

Fibonacci Linear Feedback Shift Register using specified polynomial mask. More...

#include <gnuradio/digital/lfsr.h>

Public Member Functions

 lfsr (uint32_t mask, uint32_t seed, uint32_t reg_len)
 
unsigned char next_bit ()
 
unsigned char next_bit_scramble (unsigned char input)
 
unsigned char next_bit_descramble (unsigned char input)
 
void reset ()
 
void pre_shift (int num)
 
int mask () const
 

Detailed Description

Fibonacci Linear Feedback Shift Register using specified polynomial mask.

Generates a maximal length pseudo-random sequence of length 2^degree-1

Constructor: digital::lfsr(int mask, int seed, int reg_len);

Parameters
mask- polynomial coefficients representing the locations of feedback taps from a shift register which are xor'ed together to form the new high order bit.

Some common masks might be: x^4 + x^3 + x^0 = 0x19 x^5 + x^3 + x^0 = 0x29 x^6 + x^5 + x^0 = 0x61

Parameters
seed- the initialization vector placed into the register durring initialization. Low order bit corresponds to x^0 coefficient – the first to be shifted as output.
reg_len- specifies the length of the feedback shift register to be used. Durring each iteration, the register is rightshifted one and the new bit is placed in bit reg_len. reg_len should generally be at least order(mask) + 1

see http://en.wikipedia.org/wiki/Linear_feedback_shift_register for more explanation.

next_bit() - Standard LFSR operation 

Perform one cycle of the LFSR.  The output bit is taken from
the shift register LSB.  The shift register MSB is assigned from
the modulo 2 sum of the masked shift register.

next_bit_scramble(unsigned char input) - Scramble an input stream 

Perform one cycle of the LFSR.  The output bit is taken from
the shift register LSB.  The shift register MSB is assigned from
the modulo 2 sum of the masked shift register and the input LSB.

next_bit_descramble(unsigned char input) - Descramble an input stream 

Perform one cycle of the LFSR.  The output bit is taken from
the modulo 2 sum of the masked shift register and the input LSB.
The shift register MSB is assigned from the LSB of the input.

See http://en.wikipedia.org/wiki/Scrambler for operation of these last two functions (see multiplicative scrambler.)

Constructor & Destructor Documentation

gr::digital::lfsr::lfsr ( uint32_t  mask,
uint32_t  seed,
uint32_t  reg_len 
)
inline

Member Function Documentation

int gr::digital::lfsr::mask ( ) const
inline
unsigned char gr::digital::lfsr::next_bit ( )
inline

Referenced by pre_shift().

unsigned char gr::digital::lfsr::next_bit_descramble ( unsigned char  input)
inline
unsigned char gr::digital::lfsr::next_bit_scramble ( unsigned char  input)
inline
void gr::digital::lfsr::pre_shift ( int  num)
inline

Rotate the register through x number of bits where we are just throwing away the results to get queued up correctly

References next_bit().

void gr::digital::lfsr::reset ( )
inline

Reset shift register to initial seed value

The documentation for this class was generated from the following file: