diff --git a/ports/esp8266/Makefile b/ports/esp8266/Makefile
index 64116b139b..2162c72f07 100644
--- a/ports/esp8266/Makefile
+++ b/ports/esp8266/Makefile
@@ -59,6 +59,9 @@ COPT += -Os -DNDEBUG
 LDFLAGS += --gc-sections
 endif
 
+# Options for mpy-cross
+MPY_CROSS_FLAGS += -march=xtensa
+
 SRC_C = \
 	strtoll.c \
 	main.c \
diff --git a/ports/stm32/Makefile b/ports/stm32/Makefile
index b4c7a15c13..d0b90c7617 100644
--- a/ports/stm32/Makefile
+++ b/ports/stm32/Makefile
@@ -105,6 +105,9 @@ else
 COPT += -Os -DNDEBUG
 endif
 
+# Options for mpy-cross
+MPY_CROSS_FLAGS += -march=armv7m
+
 SRC_LIB = $(addprefix lib/,\
 	libc/string0.c \
 	oofatfs/ff.c \