dsp: add limits support to SatAddMixin

artiq/gateware/dsp/tools.py

@@ -29,21 +29,33 @@ def eqh(a, b):
 
 
 class SatAddMixin:
-    def sat_add(self, a):
+    """Signed saturating addition mixin"""
+    def sat_add(self, *a, limits=None, clipped=None):
         a = list(a)
         # assert all(value_bits_sign(ai)[1] for ai in a)
-        n = max(len(ai) for ai in a)
-        o = log2_int(len(a), need_pow2=False)
-        s = Signal((n + o, True))
-        s0 = Signal((n, True))
-        z = Signal((1, True))
+        length = max(len(ai) for ai in a)
+        carry = log2_int(len(a), need_pow2=False)
+        full = Signal((length + carry, True))
+        limited = Signal((length, True))
+        clip = Signal(2)
+        if clipped is not None:
+            clipped.eq(clip)
         self.comb += [
-            s.eq(reduce(add, a, z)),
-            s0[-1].eq(s[-1]),
-            If(s[-o-1:] == Replicate(s[-1], o + 1),
-                s0[:-1].eq(s[:n-1]),
+            full.eq(reduce(add, a)),
+        ]
+        if limits is None:
+            self.comb += [
+                If(full[-1-carry:] == Replicate(full[-1], carry + 1),
+                    limited.eq(full),
+                    clip.eq(0),
                 ).Else(
-                s0[:-1].eq(Replicate(~s[-1], n - 1)),
+                    limited.eq(Cat(Replicate(~full[-1], length - 1), full[-1])),
+                    clip.eq(Cat(full[-1], ~full[-1])),
                 )
             ]
-        return s0
+        else:
+            self.comb += [
+                clip.eq(Cat(full < limits[0], full > limits[1])),
+                limited.eq(Array([full, limits[0], limits[1], 0])[clip]),
+            ]
+        return limited