move priority picker from soc to nmutil

diff --git a/src/nmutil/picker.py b/src/nmutil/picker.py
new file mode 100644 (file)
index 0000000..d47f785
--- /dev/null
+++ b/src/nmutil/picker.py
@@ -0,0 +1,42 @@
+""" Priority Picker: optimised back-to-back PriorityEncoder and Decoder
+
+    The input is N bits, the output is N bits wide and only one is
+    enabled.
+"""
+
+from nmigen import Module, Signal, Cat, Elaboratable
+
+class PriorityPicker(Elaboratable):
+    """ implements a priority-picker.  input: N bits, output: N bits
+    """
+    def __init__(self, wid):
+        self.wid = wid
+        # inputs
+        self.i = Signal(wid, reset_less=True)
+        self.o = Signal(wid, reset_less=True)
+
+    def elaborate(self, platform):
+        m = Module()
+
+        res = []
+        ni = Signal(self.wid, reset_less = True)
+        m.d.comb += ni.eq(~self.i)
+        for i in range(0, self.wid):
+            t = Signal(reset_less = True)
+            res.append(t)
+            if i == 0:
+                m.d.comb += t.eq(self.i[i])
+            else:
+                m.d.comb += t.eq(~Cat(ni[i], *self.i[:i]).bool())
+
+        # we like Cat(*xxx).  turn lists into concatenated bits
+        m.d.comb += self.o.eq(Cat(*res))
+
+        return m
+
+    def __iter__(self):
+        yield self.i
+        yield self.o
+
+    def ports(self):
+        return list(self)