diff --git a/gateware/logic/iir.py b/gateware/logic/iir.py
index 2380dd7..60bfeb7 100644
--- a/gateware/logic/iir.py
+++ b/gateware/logic/iir.py
@@ -89,7 +89,7 @@ class Iir(Filter):
                 zr = Signal.like(z)
                 self.sync += zr.eq(z)
                 z = Signal.like(zr)
-                self.comb += z.eq(zr + signal * c[coeff])
+                self.sync += z.eq(zr + signal * c[coeff])
             self.comb += y_next.eq(z)
             self.latency.value = Constant(order + 1)
             self.interval.value = Constant(1)