diff --git a/artiq/gateware/rtio/phy/ttl_simple.py b/artiq/gateware/rtio/phy/ttl_simple.py
index 3250a2659..8a5e6e432 100644
--- a/artiq/gateware/rtio/phy/ttl_simple.py
+++ b/artiq/gateware/rtio/phy/ttl_simple.py
@@ -75,3 +75,21 @@ class Inout(Module):
         ]
 
         self.probes += [i, ts.oe]
+
+
+class ClockGen(Module):
+    def __init__(self, pad, ftw_width=16):
+        self.rtlink = rtlink.Interface(
+            rtlink.OInterface(ftw_width, suppress_nop=False))
+
+        # # #
+
+        ftw = Signal(ftw_width)
+        acc = Signal(ftw_width)
+        self.sync.rio += If(self.rtlink.o.stb, ftw.eq(self.rtlink.o.data))
+        self.sync.rio_phy += [
+            acc.eq(acc + ftw),
+            # known phase on write: at rising edge
+            If(self.rtlink.o.stb, acc.eq(2**(ftw_width - 1))),
+            pad.eq(acc[-1])
+        ]
diff --git a/doc/manual/fpga_board_ports.rst b/doc/manual/fpga_board_ports.rst
index 442c3e691..52bd8dffe 100644
--- a/doc/manual/fpga_board_ports.rst
+++ b/doc/manual/fpga_board_ports.rst
@@ -20,7 +20,9 @@ When plugged to an adapter, the NIST QC1 hardware can be used. The TTL lines are
 +--------------+----------+------------+
 | 1            | PMT1     | Input      |
 +--------------+----------+------------+
-| 2-17         | TTL0-15  | Output     |
+| 2-16         | TTL0-14  | Output     |
++--------------+----------+------------+
+| 17           | TTL15    | Clock      |
 +--------------+----------+------------+
 | 18           | EXT_LED  | Output     |
 +--------------+----------+------------+
diff --git a/soc/targets/artiq_kc705.py b/soc/targets/artiq_kc705.py
index d6631ef87..5ed077b5c 100644
--- a/soc/targets/artiq_kc705.py
+++ b/soc/targets/artiq_kc705.py
@@ -95,7 +95,7 @@ class NIST_QC1(_NIST_QCx):
             phy = ttl_simple.Inout(platform.request("pmt", i))
             self.submodules += phy
             rtio_channels.append(rtio.Channel.from_phy(phy, ififo_depth=512))
-        for i in range(16):
+        for i in range(15):
             phy = ttl_simple.Output(platform.request("ttl", i))
             self.submodules += phy
             rtio_channels.append(rtio.Channel.from_phy(phy))
@@ -105,6 +105,10 @@ class NIST_QC1(_NIST_QCx):
         rtio_channels.append(rtio.Channel.from_phy(phy))
         self.add_constant("RTIO_TTL_COUNT", len(rtio_channels))
 
+        phy = ttl_simple.ClockGen(platform.request("ttl", 15))
+        self.submodules += phy
+        rtio_channels.append(rtio.Channel.from_phy(phy))
+
         self.add_constant("RTIO_DDS_CHANNEL", len(rtio_channels))
         self.add_constant("DDS_CHANNEL_COUNT", 8)
         self.add_constant("DDS_AD9858")
@@ -123,6 +127,9 @@ class NIST_QC2(_NIST_QCx):
 
         rtio_channels = []
         for i in range(16):
+            if i == 14:
+                # TTL14 is for the clock generator
+                break
             if i % 4 == 3:
                 phy = ttl_simple.Inout(platform.request("ttl", i))
                 self.submodules += phy
@@ -137,6 +144,10 @@ class NIST_QC2(_NIST_QCx):
         rtio_channels.append(rtio.Channel.from_phy(phy))
         self.add_constant("RTIO_TTL_COUNT", len(rtio_channels))
 
+        phy = ttl_simple.ClockGen(platform.request("ttl", 14))
+        self.submodules += phy
+        rtio_channels.append(rtio.Channel.from_phy(phy))
+
         self.add_constant("RTIO_DDS_CHANNEL", len(rtio_channels))
         self.add_constant("DDS_CHANNEL_COUNT", 11)
         self.add_constant("DDS_AD9914")
diff --git a/soc/targets/artiq_pipistrello.py b/soc/targets/artiq_pipistrello.py
index b3bc058c3..3ad05a591 100644
--- a/soc/targets/artiq_pipistrello.py
+++ b/soc/targets/artiq_pipistrello.py
@@ -100,7 +100,7 @@ trce -v 12 -fastpaths -tsi {build_name}.tsi -o {build_name}.twr {build_name}.ncd
             rtio_channels.append(rtio.Channel.from_phy(phy, ififo_depth=512,
                                                        ofifo_depth=4))
 
-        for i in range(16):
+        for i in range(15):
             phy = ttl_simple.Output(platform.request("ttl", i))
             self.submodules += phy
             rtio_channels.append(rtio.Channel.from_phy(phy, ofifo_depth=256))
@@ -115,6 +115,10 @@ trce -v 12 -fastpaths -tsi {build_name}.tsi -o {build_name}.twr {build_name}.ncd
 
         self.add_constant("RTIO_TTL_COUNT", len(rtio_channels))
 
+        phy = ttl_simple.ClockGen(platform.request("ttl", 15))
+        self.submodules += phy
+        rtio_channels.append(rtio.Channel.from_phy(phy))
+
         self.add_constant("RTIO_DDS_CHANNEL", len(rtio_channels))
         self.add_constant("DDS_CHANNEL_COUNT", 8)
         phy = dds.AD9858(platform.request("dds"), 8)