src/nmutil/picker.py

   1 """ Priority Picker: optimised back-to-back PriorityEncoder and Decoder
   2
   3     The input is N bits, the output is N bits wide and only one is
   4     enabled.
   5 """
   6
   7 from nmigen import Module, Signal, Cat, Elaboratable
   8
   9 class PriorityPicker(Elaboratable):
  10     """ implements a priority-picker.  input: N bits, output: N bits
  11     """
  12     def __init__(self, wid):
  13         self.wid = wid
  14         # inputs
  15         self.i = Signal(wid, reset_less=True)
  16         self.o = Signal(wid, reset_less=True)
  17
  18     def elaborate(self, platform):
  19         m = Module()
  20
  21         res = []
  22         ni = Signal(self.wid, reset_less = True)
  23         m.d.comb += ni.eq(~self.i)
  24         for i in range(0, self.wid):
  25             t = Signal(reset_less = True)
  26             res.append(t)
  27             if i == 0:
  28                 m.d.comb += t.eq(self.i[i])
  29             else:
  30                 m.d.comb += t.eq(~Cat(ni[i], *self.i[:i]).bool())
  31
  32         # we like Cat(*xxx).  turn lists into concatenated bits
  33         m.d.comb += self.o.eq(Cat(*res))
  34
  35         return m
  36
  37     def __iter__(self):
  38         yield self.i
  39         yield self.o
  40
  41     def ports(self):
  42         return list(self)