litex/gen/migen/genlib/coding.py

   1 """
   2 Encoders and decoders between binary and one-hot representation
   3 """
   4
   5 from migen.fhdl.structure import *
   6 from migen.fhdl.module import Module
   7
   8
   9 class Encoder(Module):
  10     """Encode one-hot to binary
  11
  12     If `n` is low, the `o` th bit in `i` is asserted, else none or
  13     multiple bits are asserted.
  14
  15     Parameters
  16     ----------
  17     width : int
  18         Bit width of the input
  19
  20     Attributes
  21     ----------
  22     i : Signal(width), in
  23         One-hot input
  24     o : Signal(max=width), out
  25         Encoded binary
  26     n : Signal(1), out
  27         Invalid, either none or multiple input bits are asserted
  28     """
  29     def __init__(self, width):
  30         self.i = Signal(width)  # one-hot
  31         self.o = Signal(max=max(2, width))  # binary
  32         self.n = Signal()  # invalid: none or multiple
  33         act = dict((1<<j, self.o.eq(j)) for j in range(width))
  34         act["default"] = self.n.eq(1)
  35         self.comb += Case(self.i, act)
  36
  37
  38 class PriorityEncoder(Module):
  39     """Priority encode requests to binary
  40
  41     If `n` is low, the `o` th bit in `i` is asserted and the bits below
  42     `o` are unasserted, else `o == 0`. The LSB has priority.
  43
  44     Parameters
  45     ----------
  46     width : int
  47         Bit width of the input
  48
  49     Attributes
  50     ----------
  51     i : Signal(width), in
  52         Input requests
  53     o : Signal(max=width), out
  54         Encoded binary
  55     n : Signal(1), out
  56         Invalid, no input bits are asserted
  57     """
  58     def __init__(self, width):
  59         self.i = Signal(width)  # one-hot, lsb has priority
  60         self.o = Signal(max=max(2, width))  # binary
  61         self.n = Signal()  # none
  62         for j in range(width)[::-1]:  # last has priority
  63             self.comb += If(self.i[j], self.o.eq(j))
  64         self.comb += self.n.eq(self.i == 0)
  65
  66
  67 class Decoder(Module):
  68     """Decode binary to one-hot
  69
  70     If `n` is low, the `i` th bit in `o` is asserted, the others are
  71     not, else `o == 0`.
  72
  73     Parameters
  74     ----------
  75     width : int
  76         Bit width of the output
  77
  78     Attributes
  79     ----------
  80     i : Signal(max=width), in
  81         Input binary
  82     o : Signal(width), out
  83         Decoded one-hot
  84     n : Signal(1), in
  85         Invalid, no output bits are to be asserted
  86     """
  87
  88     def __init__(self, width):
  89         self.i = Signal(max=max(2, width))  # binary
  90         self.n = Signal()  # none/invalid
  91         self.o = Signal(width)  # one-hot
  92         act = dict((j, self.o.eq(1<<j)) for j in range(width))
  93         self.comb += Case(self.i, act)
  94         self.comb += If(self.n, self.o.eq(0))
  95
  96
  97 class PriorityDecoder(Decoder):
  98     pass  # same