diff --git a/py/circuitpy_mpconfig.mk b/py/circuitpy_mpconfig.mk
index 90bf0ce9e8..f391128fd3 100644
--- a/py/circuitpy_mpconfig.mk
+++ b/py/circuitpy_mpconfig.mk
@@ -65,6 +65,9 @@ CFLAGS += -DCIRCUITPY_ALARM=$(CIRCUITPY_ALARM)
 CIRCUITPY_ANALOGIO ?= 1
 CFLAGS += -DCIRCUITPY_ANALOGIO=$(CIRCUITPY_ANALOGIO)
 
+CIRCUITPY_ANALOGFASTIN ?= 0
+CFLAGS += -DCIRCUITPY_ANALOGFASTIN=$(CIRCUITPY_ANALOGFASTIN)
+
 CIRCUITPY_ATEXIT ?= $(CIRCUITPY_FULL_BUILD)
 CFLAGS += -DCIRCUITPY_ATEXIT=$(CIRCUITPY_ATEXIT)