diff --git a/artiq/gateware/rtio/core.py b/artiq/gateware/rtio/core.py
index 69babebac..a248632da 100644
--- a/artiq/gateware/rtio/core.py
+++ b/artiq/gateware/rtio/core.py
@@ -24,10 +24,8 @@ class _GrayCodeTransfer(Module):
         self.sync.rtio += value_gray_rtio.eq(self.i ^ self.i[1:])
         # transfer to system clock domain
         value_gray_sys = Signal(width)
-        self.specials += [
-            NoRetiming(value_gray_rtio),
-            MultiReg(value_gray_rtio, value_gray_sys)
-        ]
+        value_gray_rtio.attr.add("no_retiming")
+        self.specials += MultiReg(value_gray_rtio, value_gray_sys)
         # convert back to binary
         value_sys = Signal(width)
         self.comb += value_sys[-1].eq(value_gray_sys[-1])
diff --git a/artiq/gateware/targets/kc705.py b/artiq/gateware/targets/kc705.py
index 7209a2b3b..960623e8f 100755
--- a/artiq/gateware/targets/kc705.py
+++ b/artiq/gateware/targets/kc705.py
@@ -8,7 +8,6 @@ from migen.genlib.cdc import MultiReg
 from migen.build.generic_platform import *
 from migen.build.xilinx.vivado import XilinxVivadoToolchain
 from migen.build.xilinx.ise import XilinxISEToolchain
-from migen.fhdl.specials import Keep
 
 from misoc.interconnect.csr import *
 from misoc.interconnect import wishbone
@@ -147,12 +146,10 @@ class _NIST_Ions(MiniSoC, AMPSoC):
         self.submodules.rtio_moninj = rtio.MonInj(rtio_channels)
         self.csr_devices.append("rtio_moninj")
 
-        self.specials += [
-            Keep(self.rtio.cd_rsys.clk),
-            Keep(self.rtio_crg.cd_rtio.clk),
-            Keep(self.ethphy.crg.cd_eth_rx.clk),
-            Keep(self.ethphy.crg.cd_eth_tx.clk),
-        ]
+        self.rtio.cd_rsys.clk.attr.add("keep")
+        self.rtio_crg.cd_rtio.clk.attr.add("keep")
+        self.ethphy.crg.cd_eth_rx.clk.attr.add("keep")
+        self.ethphy.crg.cd_eth_tx.clk.attr.add("keep")
 
         self.platform.add_period_constraint(self.rtio.cd_rsys.clk, 8.)
         self.platform.add_period_constraint(self.rtio_crg.cd_rtio.clk, 8.)