+++ /dev/null
-# SPDX-License-Identifier: LGPL-2.1-or-later
-# See Notices.txt for copyright information
-from nmigen import Signal, Module, Const, Cat, Elaboratable
-from nmigen.cli import verilog, rtlil
-
-
-class LFSRPolynomial(set):
- """ implements a polynomial for use in LFSR
- """
- def __init__(self, exponents=()):
- for e in exponents:
- assert isinstance(e, int), TypeError("%s must be an int" % repr(e))
- assert (e >= 0), ValueError("%d must not be negative" % e)
- set.__init__(self, set(exponents).union({0})) # must contain zero
-
- @property
- def max_exponent(self):
- return max(self) # derived from set, so this returns the max exponent
-
- @property
- def exponents(self):
- exponents = list(self) # get elements of set as a list
- exponents.sort(reverse=True)
- return exponents
-
- def __str__(self):
- expd = {0: "1", 1: 'x', 2: "x^{}"} # case 2 isn't 2, it's min(i,2)
- retval = map(lambda i: expd[min(i,2)].format(i), self.exponents)
- return " + ".join(retval)
-
- def __repr__(self):
- return "LFSRPolynomial(%s)" % self.exponents
-
-
-# list of selected polynomials from https://web.archive.org/web/20190418121923/https://en.wikipedia.org/wiki/Linear-feedback_shift_register#Some_polynomials_for_maximal_LFSRs # noqa
-LFSR_POLY_2 = LFSRPolynomial([2, 1, 0])
-LFSR_POLY_3 = LFSRPolynomial([3, 2, 0])
-LFSR_POLY_4 = LFSRPolynomial([4, 3, 0])
-LFSR_POLY_5 = LFSRPolynomial([5, 3, 0])
-LFSR_POLY_6 = LFSRPolynomial([6, 5, 0])
-LFSR_POLY_7 = LFSRPolynomial([7, 6, 0])
-LFSR_POLY_8 = LFSRPolynomial([8, 6, 5, 4, 0])
-LFSR_POLY_9 = LFSRPolynomial([9, 5, 0])
-LFSR_POLY_10 = LFSRPolynomial([10, 7, 0])
-LFSR_POLY_11 = LFSRPolynomial([11, 9, 0])
-LFSR_POLY_12 = LFSRPolynomial([12, 11, 10, 4, 0])
-LFSR_POLY_13 = LFSRPolynomial([13, 12, 11, 8, 0])
-LFSR_POLY_14 = LFSRPolynomial([14, 13, 12, 2, 0])
-LFSR_POLY_15 = LFSRPolynomial([15, 14, 0])
-LFSR_POLY_16 = LFSRPolynomial([16, 15, 13, 4, 0])
-LFSR_POLY_17 = LFSRPolynomial([17, 14, 0])
-LFSR_POLY_18 = LFSRPolynomial([18, 11, 0])
-LFSR_POLY_19 = LFSRPolynomial([19, 18, 17, 14, 0])
-LFSR_POLY_20 = LFSRPolynomial([20, 17, 0])
-LFSR_POLY_21 = LFSRPolynomial([21, 19, 0])
-LFSR_POLY_22 = LFSRPolynomial([22, 21, 0])
-LFSR_POLY_23 = LFSRPolynomial([23, 18, 0])
-LFSR_POLY_24 = LFSRPolynomial([24, 23, 22, 17, 0])
-
-
-class LFSR(LFSRPolynomial, Elaboratable):
- """ implements a Linear Feedback Shift Register
- """
- def __init__(self, polynomial):
- """ Inputs:
- ------
- :polynomial: the polynomial to feedback on. may be a LFSRPolynomial
- instance or an iterable of ints (list/tuple/generator)
- :enable: enable (set LO to disable. NOTE: defaults to HI)
-
- Outputs:
- -------
- :state: the LFSR state. bitwidth is taken from the polynomial
- maximum exponent.
-
- Note: if an LFSRPolynomial is passed in as the input, because
- LFSRPolynomial is derived from set() it's ok:
- LFSRPolynomial(LFSRPolynomial(p)) == LFSRPolynomial(p)
- """
- LFSRPolynomial.__init__(self, polynomial)
- self.state = Signal(self.max_exponent, reset=1)
- self.enable = Signal(reset=1)
-
- def elaborate(self, platform):
- m = Module()
- # do absolutely nothing if the polynomial is empty (always has a zero)
- if self.max_exponent <= 1:
- return m
-
- # create XOR-bunch, select bits from state based on exponent
- feedback = Const(0) # doesn't do any harm starting from 0b0 (xor chain)
- for exponent in self:
- if exponent > 0: # don't have to skip, saves CPU cycles though
- feedback ^= self.state[exponent - 1]
-
- # if enabled, shift-and-feedback
- with m.If(self.enable):
- # shift up lower bits by Cat'ing in a new bit zero (feedback)
- newstate = Cat(feedback, self.state[:-1])
- m.d.sync += self.state.eq(newstate)
-
- return m
-
-
-# example: Poly24
-if __name__ == '__main__':
- p24 = rtlil.convert(LFSR(LFSR_POLY_24))
- with open("lfsr2_p24.il", "w") as f:
- f.write(p24)
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