diff --git a/nrf5/boards/microbit/mpconfigboard_s110.mk b/nrf5/boards/microbit/mpconfigboard_s110.mk
index 0cd554415f..fbe641cf86 100644
--- a/nrf5/boards/microbit/mpconfigboard_s110.mk
+++ b/nrf5/boards/microbit/mpconfigboard_s110.mk
@@ -3,3 +3,4 @@ MCU_VARIANT = nrf51
 MCU_SUB_VARIANT = nrf51822
 LD_FILE = boards/nrf51822_aa_s110.ld
 FLASHER = pyocd
+CFLAGS += -DBLUETOOTH_LFCLK_RC