diff --git a/artiq/gateware/rtio/cdc.py b/artiq/gateware/rtio/cdc.py
index cc0201969..bd0b11d37 100644
--- a/artiq/gateware/rtio/cdc.py
+++ b/artiq/gateware/rtio/cdc.py
@@ -15,7 +15,7 @@ class GrayCodeTransfer(Module):
 
         # convert to Gray code
         value_gray_rtio = Signal(width, reset_less=True)
-        self.sync += value_gray_rtio.eq(self.i ^ self.i[1:])
+        self.sync.rtio += value_gray_rtio.eq(self.i ^ self.i[1:])
         # transfer to system clock domain
         value_gray_sys = Signal(width)
         value_gray_rtio.attr.add("no_retiming")
diff --git a/artiq/gateware/rtio/core.py b/artiq/gateware/rtio/core.py
index 7b3b6d6fa..66144ceca 100644
--- a/artiq/gateware/rtio/core.py
+++ b/artiq/gateware/rtio/core.py
@@ -66,7 +66,7 @@ class Core(Module, AutoCSR):
             interface=self.cri)
         self.submodules += outputs
         self.comb += outputs.coarse_timestamp.eq(tsc.coarse_ts)
-        self.sync += outputs.minimum_coarse_timestamp.eq(tsc.coarse_ts_sys + 16)
+        self.sync += outputs.minimum_coarse_timestamp.eq(tsc.coarse_ts + 16)
 
         inputs = InputCollector(tsc, channels, "sync",
             quash_channels=quash_channels,
diff --git a/artiq/gateware/rtio/tsc.py b/artiq/gateware/rtio/tsc.py
index d12c69044..f2c316047 100644
--- a/artiq/gateware/rtio/tsc.py
+++ b/artiq/gateware/rtio/tsc.py
@@ -1,8 +1,5 @@
 from migen import *
 
-from artiq.gateware.rtio.cdc import GrayCodeTransfer
-
-
 class TSC(Module):
     def __init__(self, mode, glbl_fine_ts_width=0):
         self.glbl_fine_ts_width = glbl_fine_ts_width
@@ -11,22 +8,10 @@ class TSC(Module):
         self.coarse_ts = Signal(64 - glbl_fine_ts_width)
         self.full_ts = Signal(64)
         
-        # in sys domain
-        # monotonic, may lag behind the counter in the IO clock domain, but
-        # not be ahead of it.
-        self.coarse_ts_sys = Signal.like(self.coarse_ts)
-        self.full_ts_sys = Signal(64)
-
-        # in rtio domain
         self.load = Signal()
         self.load_value = Signal.like(self.coarse_ts)
 
-        if mode == "async":
-            self.full_ts_cri = self.full_ts_sys
-        elif mode == "sync":
-            self.full_ts_cri = self.full_ts
-        else:
-            raise ValueError
+        self.full_ts_cri = self.full_ts
 
         # # #
 
@@ -35,14 +20,7 @@ class TSC(Module):
             ).Else(
                 self.coarse_ts.eq(self.coarse_ts + 1)
             )
-        coarse_ts_cdc = GrayCodeTransfer(len(self.coarse_ts))  # from rtio to sys
-        self.submodules += coarse_ts_cdc
-        self.comb += [
-            coarse_ts_cdc.i.eq(self.coarse_ts),
-            self.coarse_ts_sys.eq(coarse_ts_cdc.o)
-        ]
 
         self.comb += [
             self.full_ts.eq(self.coarse_ts << glbl_fine_ts_width),
-            self.full_ts_sys.eq(self.coarse_ts_sys << glbl_fine_ts_width)
         ]