gr::digital::lfsr Class Reference

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 during 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. During 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

lfsr()

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

inline

Member Function Documentation

mask()

int gr::digital::lfsr::mask ( ) const

inline

next_bit()

unsigned char gr::digital::lfsr::next_bit ( )

inline

Referenced by pre_shift().

next_bit_descramble()

unsigned char gr::digital::lfsr::next_bit_descramble ( unsigned char  input )

inline

next_bit_scramble()

unsigned char gr::digital::lfsr::next_bit_scramble ( unsigned char  input )

inline

pre_shift()

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().

reset()

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

inline

Reset shift register to initial seed value

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

• lfsr.h