diff --git a/artiq/gateware/wrpll/si549.py b/artiq/gateware/wrpll/si549.py
index fd87c2cfd..652e3f0b5 100644
--- a/artiq/gateware/wrpll/si549.py
+++ b/artiq/gateware/wrpll/si549.py
@@ -5,6 +5,148 @@ from migen.genlib.cdc import MultiReg, PulseSynchronizer, BlindTransfer
 from misoc.interconnect.csr import *
 
 
+class I2CClockGen(Module):
+    def __init__(self, width):
+        self.load  = Signal(width)
+        self.clk2x = Signal()
+
+        cnt = Signal.like(self.load)
+        self.comb += [
+            self.clk2x.eq(cnt == 0),
+        ]
+        self.sync += [
+            If(self.clk2x,
+                cnt.eq(self.load),
+            ).Else(
+                cnt.eq(cnt - 1),
+            )
+        ]
+
+
+class I2CMasterMachine(Module):
+    def __init__(self, clock_width):
+        self.scl   = Signal(reset=1)
+        self.sda_o = Signal(reset=1)
+        self.sda_i = Signal()
+
+        self.submodules.cg  = CEInserter()(I2CClockGen(clock_width))
+        self.idle  = Signal()
+        self.start = Signal()
+        self.stop  = Signal()
+        self.write = Signal()
+        self.read  = Signal()
+        self.ack   = Signal()
+        self.data  = Signal(8)
+
+        ###
+
+        busy = Signal()
+        bits = Signal(4)
+
+        fsm = CEInserter()(FSM("IDLE"))
+        self.submodules += fsm
+
+        fsm.act("IDLE",
+            If(self.start,
+                NextState("START0"),
+            ).Elif(self.stop & self.start,
+                NextState("RESTART0"),
+            ).Elif(self.stop,
+                NextState("STOP0"),
+            ).Elif(self.write,
+                NextValue(bits, 8),
+                NextState("WRITE0"),
+            ).Elif(self.read,
+                NextValue(bits, 8),
+                NextState("READ0"),
+            )
+        )
+
+        fsm.act("START0",
+            NextValue(self.scl, 1),
+            NextState("START1"))
+        fsm.act("START1",
+            NextValue(self.sda_o, 0),
+            NextState("IDLE"))
+
+        fsm.act("RESTART0",
+            NextValue(self.scl, 0),
+            NextState("RESTART1"))
+        fsm.act("RESTART1",
+            NextValue(self.sda_o, 1),
+            NextState("START0"))
+
+        fsm.act("STOP0",
+            NextValue(self.scl, 0),
+            NextState("STOP1"))
+        fsm.act("STOP1",
+            NextValue(self.scl, 1),
+            NextValue(self.sda_o, 0),
+            NextState("STOP2"))
+        fsm.act("STOP2",
+            NextValue(self.sda_o, 1),
+            NextState("IDLE"))
+
+        fsm.act("WRITE0",
+            NextValue(self.scl, 0),
+            If(bits == 0,
+                NextValue(self.sda_o, 1),
+                NextState("READACK0"),
+            ).Else(
+                NextValue(self.sda_o, self.data[7]),
+                NextState("WRITE1"),
+            )
+        )
+        fsm.act("WRITE1",
+            NextValue(self.scl, 1),
+            NextValue(self.data[1:], self.data[:-1]),
+            NextValue(bits, bits - 1),
+            NextState("WRITE0"),
+        )
+        fsm.act("READACK0",
+            NextValue(self.scl, 1),
+            NextState("READACK1"),
+        )
+        fsm.act("READACK1",
+            NextValue(self.ack, ~self.sda_i),
+            NextState("IDLE")
+        )
+
+        fsm.act("READ0",
+            NextValue(self.scl, 0),
+            NextState("READ1"),
+        )
+        fsm.act("READ1",
+            NextValue(self.data[0], self.sda_i),
+            NextValue(self.scl, 0),
+            If(bits == 0,
+                NextValue(self.sda_o, ~self.ack),
+                NextState("WRITEACK0"),
+            ).Else(
+                NextValue(self.sda_o, 1),
+                NextState("READ2"),
+            )
+        )
+        fsm.act("READ2",
+            NextValue(self.scl, 1),
+            NextValue(self.data[:-1], self.data[1:]),
+            NextValue(bits, bits - 1),
+            NextState("READ1"),
+        )
+        fsm.act("WRITEACK0",
+            NextValue(self.scl, 1),
+            NextState("IDLE"),
+        )
+
+        run = Signal()
+        self.comb += [
+            run.eq(self.start | self.stop | self.write | self.read),
+            self.idle.eq(~run & fsm.ongoing("IDLE")),
+            self.cg.ce.eq(~self.idle),
+            fsm.ce.eq(run | self.cg.clk2x),
+        ]
+
+
 class ADPLLProgrammer(Module):
     def __init__(self):
         self.i2c_divider = Signal(16)
@@ -18,11 +160,21 @@ class ADPLLProgrammer(Module):
         self.scl = Signal()
         self.sda_i = Signal()
         self.sda_o = Signal()
-        self.sda_oe = Signal()
 
         self.scl.attr.add("no_retiming")
         self.sda_o.attr.add("no_retiming")
-        self.sda_oe.attr.add("no_retiming")
+
+        # # #
+
+        master = I2CMasterMachine(16)
+        self.submodules += master
+
+        self.comb += [
+            master.cg.load.eq(self.i2c_divider.storage),
+            self.scl.eq(master.scl),
+            master.sda_i.eq(self.sda_i),
+            self.sda_o.eq(master.sda_o)
+        ]
 
 
 class Si549(Module, AutoCSR):
@@ -56,6 +208,10 @@ class Si549(Module, AutoCSR):
             programmer.adpll_stb.eq(self.adpll_stb)
         ]
 
+        self.gpio_enable.storage.attr.add("no_retiming")
+        self.gpio_out.storage.attr.add("no_retiming")
+        self.gpio_oe.storage.attr.add("no_retiming")
+
         # SCL GPIO and mux
         ts_scl = TSTriple(1)
         self.specials += ts_scl.get_tristate(pads.scl)
@@ -64,7 +220,6 @@ class Si549(Module, AutoCSR):
         self.comb += self.gpio_in.status[0].eq(status)
 
         self.specials += MultiReg(ts_scl.i, status)
-        self.gpio_enable.storage.attr.add("no_retiming")
         self.comb += [
             If(self.gpio_enable.storage,
                 ts_scl.o.eq(self.gpio_out.storage[0]),
@@ -83,14 +238,13 @@ class Si549(Module, AutoCSR):
         self.comb += self.gpio_in.status[1].eq(status)
 
         self.specials += MultiReg(ts_sda.i, status)
-        self.gpio_enable.storage.attr.add("no_retiming")
         self.comb += [
             If(self.gpio_enable.storage,
                 ts_sda.o.eq(self.gpio_out.storage[1]),
                 ts_sda.oe.eq(self.gpio_oe.storage[1])
             ).Else(
-                ts_sda.o.eq(programmer.sda_o),
-                ts_sda.oe.eq(programmer.sda_oe)
+                ts_sda.o.eq(0),
+                ts_sda.oe.eq(~programmer.sda_o)
             )
         ]
         self.specials += MultiReg(ts_sda.i, programmer.sda_i, "helper")