diff --git a/nmigen/vendor/lattice_ice40.py b/nmigen/vendor/lattice_ice40.py
index 14026fa..d5e4dd6 100644
--- a/nmigen/vendor/lattice_ice40.py
+++ b/nmigen/vendor/lattice_ice40.py
@@ -379,7 +379,7 @@ class LatticeICE40Platform(TemplatedPlatform):
                 m.submodules += Instance("SB_HFOSC",
                                          i_CLKHFEN=1,
                                          i_CLKHFPU=1,
-                                         p_CLKHF_DIV="0b{0:b}".format(self.hfosc_div),
+                                         p_CLKHF_DIV="0b{0:02b}".format(self.hfosc_div),
                                          o_CLKHF=clk_i)
                 delay = int(100e-6 * self.default_clk_frequency)
             # Internal low-speed clock: 10 KHz.