diff --git a/boards/bbcmicrobit_v2.json b/boards/bbcmicrobit_v2.json
index b2005c3..805dd58 100644
--- a/boards/bbcmicrobit_v2.json
+++ b/boards/bbcmicrobit_v2.json
@@ -5,7 +5,7 @@
     },
     "core": "nRF5",
     "cpu": "cortex-m4",
-    "extra_flags": "-DARDUINO_BBC_MICROBIT_V2 -DNRF52833_XXAA",
+    "extra_flags": "-DARDUINO_BBC_MICROBIT_V2 -DNRF52833_XXAA -DUSE_LFSYNT",
     "f_cpu": "64000000L",
     "mcu": "nrf52833",
     "variant": "BBCmicrobitV2",
diff --git a/boards/calliopemini_v3.json b/boards/calliopemini_v3.json
index 8f54a01..e62309f 100644
--- a/boards/calliopemini_v3.json
+++ b/boards/calliopemini_v3.json
@@ -5,7 +5,7 @@
     },
     "core": "nRF5",
     "cpu": "cortex-m4",
-    "extra_flags": "-DNRF52833_XXAA -DARDUINO_CALLIOPE_MINI_V3",
+    "extra_flags": "-DNRF52833_XXAA -DARDUINO_CALLIOPE_MINI_V3 -DUSE_LFSYNT",
     "f_cpu": "64000000",
     "hwids": [
       [