src/soc/TLB/AddressEncoder.py

   1 from nmigen import Module, Signal, Elaboratable
   2 from nmigen.lib.coding import Encoder, PriorityEncoder
   3
   4 class AddressEncoder(Elaboratable):
   5     """Address Encoder
   6
   7        The purpose of this module is to take in a vector and
   8        encode the bits that are one hot into an address. This module
   9        combines both nmigen's Encoder and PriorityEncoder and will state
  10        whether the input line has a single bit hot, multiple bits hot,
  11        or no bits hot. The output line will always have the lowest value
  12        address output.
  13
  14        Usage:
  15        The output is valid when either single or multiple match is high.
  16        Otherwise output is 0.
  17     """
  18     def __init__(self, width):
  19         """ Arguments:
  20             * width: The desired length of the input vector
  21         """
  22         # Internal
  23         self.encoder = Encoder(width)
  24         self.p_encoder = PriorityEncoder(width)
  25
  26         # Input
  27         self.i = Signal(width)
  28
  29         # Output
  30         self.single_match = Signal(1)
  31         self.multiple_match = Signal(1)
  32         self.o = Signal(max=width)
  33
  34     def elaborate(self, platform=None):
  35         m = Module()
  36
  37         # Add internal submodules
  38         m.submodules.encoder = self.encoder
  39         m.submodules.p_encoder = self.p_encoder
  40
  41         m.d.comb += [
  42             self.encoder.i.eq(self.i),
  43             self.p_encoder.i.eq(self.i)
  44         ]
  45
  46         # Steps:
  47         # 1. check if the input vector is non-zero
  48         # 2. if non-zero, check if single match or multiple match
  49         # 3. set output line to be lowest value address output
  50
  51         # If the priority encoder recieves an input of 0
  52         # If n is 1 then the output is not valid
  53         with m.If(self.p_encoder.n):
  54             m.d.comb += [
  55                 self.single_match.eq(0),
  56                 self.multiple_match.eq(0),
  57                 self.o.eq(0)
  58             ]
  59         # If the priority encoder recieves an input > 0
  60         with m.Else():
  61             # Multiple Match if encoder n is invalid
  62             with m.If(self.encoder.n):
  63                 m.d.comb += [
  64                     self.single_match.eq(0),
  65                     self.multiple_match.eq(1)
  66                 ]
  67             # Single Match if encoder n is valid
  68             with m.Else():
  69                 m.d.comb += [
  70                     self.single_match.eq(1),
  71                     self.multiple_match.eq(0)
  72                 ]
  73             # Always set output based on priority encoder output
  74             m.d.comb += self.o.eq(self.p_encoder.o)
  75         return m