diff --git a/.travis.yml b/.travis.yml
index 55545c201f..c0a4bcaea7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -176,9 +176,9 @@ jobs:
       after_failure:
         - tests/run-tests --print-failures
 
-    # unix nanbox (and using Python 2 to check it can run the build scripts)
+    # unix nanbox/float (and using Python 2 to check it can run the build scripts)
     - stage: test
-      env: NAME="unix nanbox port build and tests"
+      env: NAME="unix nanbox/float port build and tests"
       install:
         - sudo apt-get install gcc-multilib libffi-dev:i386
       script:
@@ -186,19 +186,25 @@ jobs:
         - make ${MAKEOPTS} -C ports/unix submodules
         - make ${MAKEOPTS} -C ports/unix PYTHON=python2 deplibs
         - make ${MAKEOPTS} -C ports/unix PYTHON=python2 VARIANT=nanbox
-        - make ${MAKEOPTS} -C ports/unix PYTHON=python2 VARIANT=nanbox test_full
+        - make ${MAKEOPTS} -C ports/unix PYTHON=python2 VARIANT=nanbox test_full || travis_terminate 1
+        - make ${MAKEOPTS} -C ports/unix clean
+        - make ${MAKEOPTS} -C ports/unix CFLAGS_EXTRA="-DMICROPY_FLOAT_IMPL=MICROPY_FLOAT_IMPL_FLOAT"
+        - make ${MAKEOPTS} -C ports/unix test
       after_failure:
         - tests/run-tests --print-failures
 
-    # unix stackless
+    # unix stackless/float with clang
     - stage: test
-      env: NAME="unix stackless port build and tests with clang"
+      env: NAME="unix stackless/float port build and tests with clang"
       install:
         - sudo apt-get install clang
       script:
         - make ${MAKEOPTS} -C mpy-cross CC=clang
         - make ${MAKEOPTS} -C ports/unix submodules
         - make ${MAKEOPTS} -C ports/unix CC=clang CFLAGS_EXTRA="-DMICROPY_STACKLESS=1 -DMICROPY_STACKLESS_STRICT=1"
+        - make ${MAKEOPTS} -C ports/unix CC=clang test || travis_terminate 1
+        - make ${MAKEOPTS} -C ports/unix clean
+        - make ${MAKEOPTS} -C ports/unix CC=clang CFLAGS_EXTRA="-DMICROPY_FLOAT_IMPL=MICROPY_FLOAT_IMPL_FLOAT"
         - make ${MAKEOPTS} -C ports/unix CC=clang test
       after_failure:
         - tests/run-tests --print-failures
diff --git a/ports/esp32/Makefile b/ports/esp32/Makefile
index 0789455854..bd89e8b0c5 100644
--- a/ports/esp32/Makefile
+++ b/ports/esp32/Makefile
@@ -382,6 +382,9 @@ OBJ_MP += $(addprefix $(BUILD)/, $(EXTMOD_SRC_C:.c=.o))
 OBJ_MP += $(addprefix $(BUILD)/, $(LIB_SRC_C:.c=.o))
 OBJ_MP += $(addprefix $(BUILD)/, $(DRIVERS_SRC_C:.c=.o))
 
+# Only enable this for the MicroPython source: ignore warnings from esp-idf.
+$(OBJ_MP): CFLAGS += -Wdouble-promotion -Wfloat-conversion
+
 # List of sources for qstr extraction
 SRC_QSTR += $(SRC_C) $(EXTMOD_SRC_C) $(LIB_SRC_C) $(DRIVERS_SRC_C)
 # Append any auto-generated sources that are needed by sources listed in SRC_QSTR
diff --git a/ports/javascript/Makefile b/ports/javascript/Makefile
index 384253e544..4f9dd444be 100644
--- a/ports/javascript/Makefile
+++ b/ports/javascript/Makefile
@@ -19,7 +19,7 @@ ifdef EMSCRIPTEN
     CPP += -isystem $(EMSCRIPTEN)/system/include/libc -cxx-isystem $(EMSCRIPTEN)/system/include/libcxx
 endif
 
-CFLAGS = -m32 -Wall -Werror $(INC) -std=c99 $(COPT)
+CFLAGS = -m32 -Wall -Werror -Wdouble-promotion -Wfloat-conversion $(INC) -std=c99 $(COPT)
 LDFLAGS = -m32 -Wl,-Map=$@.map,--cref -Wl,--gc-sections
 
 CFLAGS += -O0 -DNDEBUG
diff --git a/ports/mimxrt/Makefile b/ports/mimxrt/Makefile
index 8f06c208de..3bf9916e7f 100644
--- a/ports/mimxrt/Makefile
+++ b/ports/mimxrt/Makefile
@@ -40,7 +40,7 @@ INC += -I$(TOP)/lib/tinyusb/hw
 INC += -I$(TOP)/lib/tinyusb/hw/bsp/teensy_40
 
 CFLAGS_MCU = -mtune=cortex-m7 -mcpu=cortex-m7 -mfloat-abi=hard -mfpu=fpv5-d16
-CFLAGS = $(INC) -Wall -Werror -std=c99 -nostdlib -mthumb $(CFLAGS_MCU)
+CFLAGS = $(INC) -Wall -Werror -Wdouble-promotion -Wfloat-conversion -std=c99 -nostdlib -mthumb $(CFLAGS_MCU)
 CFLAGS += -DCPU_$(MCU_SERIES) -DCPU_$(MCU_VARIANT)
 CFLAGS += -DXIP_EXTERNAL_FLASH=1 \
 	-DXIP_BOOT_HEADER_ENABLE=1 \
diff --git a/ports/minimal/Makefile b/ports/minimal/Makefile
index b44dda7ac8..5801d64087 100644
--- a/ports/minimal/Makefile
+++ b/ports/minimal/Makefile
@@ -22,12 +22,12 @@ INC += -I$(BUILD)
 ifeq ($(CROSS), 1)
 DFU = $(TOP)/tools/dfu.py
 PYDFU = $(TOP)/tools/pydfu.py
-CFLAGS_CORTEX_M4 = -mthumb -mtune=cortex-m4 -mcpu=cortex-m4 -msoft-float -fsingle-precision-constant -Wdouble-promotion
+CFLAGS_CORTEX_M4 = -mthumb -mtune=cortex-m4 -mcpu=cortex-m4 -msoft-float -fsingle-precision-constant -Wdouble-promotion -Wfloat-conversion
 CFLAGS = $(INC) -Wall -Werror -std=c99 -nostdlib $(CFLAGS_CORTEX_M4) $(COPT)
 LDFLAGS = -nostdlib -T stm32f405.ld -Map=$@.map --cref --gc-sections
 else
 LD = gcc
-CFLAGS = -m32 $(INC) -Wall -Werror -std=c99 $(COPT)
+CFLAGS = -m32 $(INC) -Wall -Werror -Wdouble-promotion -Wfloat-conversion -std=c99 $(COPT)
 LDFLAGS = -m32 -Wl,-Map=$@.map,--cref -Wl,--gc-sections
 endif
 
diff --git a/ports/powerpc/Makefile b/ports/powerpc/Makefile
index d869ebc4f1..7aa4f1449c 100644
--- a/ports/powerpc/Makefile
+++ b/ports/powerpc/Makefile
@@ -17,7 +17,7 @@ INC += -I.
 INC += -I$(TOP)
 INC += -I$(BUILD)
 
-CFLAGS = $(INC) -g -Wall -std=c99 $(COPT)
+CFLAGS = $(INC) -g -Wall -Wdouble-promotion -Wfloat-conversion -std=c99 $(COPT)
 CFLAGS += -mno-string -mno-multiple -mno-vsx -mno-altivec -nostdlib
 CFLAGS += -mlittle-endian -mstrict-align -msoft-float
 CFLAGS += -Os
diff --git a/ports/qemu-arm/Makefile b/ports/qemu-arm/Makefile
index 9641e24254..47a45af61d 100644
--- a/ports/qemu-arm/Makefile
+++ b/ports/qemu-arm/Makefile
@@ -40,7 +40,7 @@ INC += -I.
 INC += -I$(TOP)
 INC += -I$(BUILD)
 
-CFLAGS += $(INC) -Wall -Wpointer-arith -Werror -std=gnu99 $(COPT) \
+CFLAGS += $(INC) -Wall -Wpointer-arith -Wdouble-promotion -Wfloat-conversion -Werror -std=gnu99 $(COPT) \
 	 -ffunction-sections -fdata-sections
 
 #Debugging/Optimization
diff --git a/ports/stm32/Makefile b/ports/stm32/Makefile
index 38c3491b1c..2ddebd1cff 100644
--- a/ports/stm32/Makefile
+++ b/ports/stm32/Makefile
@@ -93,7 +93,7 @@ CFLAGS_MCU_l4 = $(CFLAGS_CORTEX_M) -mtune=cortex-m4 -mcpu=cortex-m4
 CFLAGS_MCU_h7 = $(CFLAGS_CORTEX_M) -mtune=cortex-m7 -mcpu=cortex-m7
 CFLAGS_MCU_wb = $(CFLAGS_CORTEX_M) -mtune=cortex-m4 -mcpu=cortex-m4
 
-CFLAGS += $(INC) -Wall -Wpointer-arith -Werror -std=gnu99 -nostdlib $(CFLAGS_MOD) $(CFLAGS_EXTRA)
+CFLAGS += $(INC) -Wall -Wpointer-arith -Werror -Wdouble-promotion -Wfloat-conversion -std=gnu99 -nostdlib $(CFLAGS_MOD) $(CFLAGS_EXTRA)
 CFLAGS += -D$(CMSIS_MCU)
 CFLAGS += $(CFLAGS_MCU_$(MCU_SERIES))
 CFLAGS += $(COPT)
@@ -117,7 +117,7 @@ ifeq ($(MICROPY_FLOAT_IMPL),none)
 CFLAGS += -DMICROPY_FLOAT_IMPL=MICROPY_FLOAT_IMPL_NONE
 else
 CFLAGS += -DMICROPY_FLOAT_IMPL=MICROPY_FLOAT_IMPL_FLOAT
-CFLAGS += -fsingle-precision-constant -Wdouble-promotion
+CFLAGS += -fsingle-precision-constant
 endif
 endif
 
@@ -238,6 +238,13 @@ SRC_LIBM += lib/libm/ef_sqrt.c
 endif
 endif
 
+LIBM_O = $(addprefix $(BUILD)/, $(SRC_LIBM:.c=.o))
+
+# Too many warnings in libm_dbl, disable for now.
+ifeq ($(MICROPY_FLOAT_IMPL),double)
+$(LIBM_O): CFLAGS := $(filter-out -Wdouble-promotion -Wfloat-conversion, $(CFLAGS)) 
+endif
+
 EXTMOD_SRC_C = $(addprefix extmod/,\
 	modonewire.c \
         )
@@ -494,7 +501,7 @@ endif
 OBJ =
 OBJ += $(PY_O)
 OBJ += $(addprefix $(BUILD)/, $(SRC_LIB:.c=.o))
-OBJ += $(addprefix $(BUILD)/, $(SRC_LIBM:.c=.o))
+OBJ += $(LIBM_O)
 OBJ += $(addprefix $(BUILD)/, $(EXTMOD_SRC_C:.c=.o))
 OBJ += $(addprefix $(BUILD)/, $(DRIVERS_SRC_C:.c=.o))
 OBJ += $(addprefix $(BUILD)/, $(SRC_C:.c=.o))
diff --git a/ports/stm32/mboot/Makefile b/ports/stm32/mboot/Makefile
index 9e56733005..89fe97ab22 100755
--- a/ports/stm32/mboot/Makefile
+++ b/ports/stm32/mboot/Makefile
@@ -54,7 +54,7 @@ CFLAGS_MCU_f7 = $(CFLAGS_CORTEX_M) -mtune=cortex-m7 -mcpu=cortex-m7
 CFLAGS_MCU_h7 = $(CFLAGS_CORTEX_M) -mtune=cortex-m7 -mcpu=cortex-m7
 CFLAGS_MCU_l4 = $(CFLAGS_CORTEX_M) -mtune=cortex-m4 -mcpu=cortex-m4
 
-CFLAGS = $(INC) -Wall -Wpointer-arith -Werror -std=gnu99 -nostdlib $(CFLAGS_MOD) $(CFLAGS_EXTRA)
+CFLAGS = $(INC) -Wall -Wpointer-arith -Wdouble-promotion -Wfloat-conversion -Werror -std=gnu99 -nostdlib $(CFLAGS_MOD) $(CFLAGS_EXTRA)
 CFLAGS += -D$(CMSIS_MCU)
 CFLAGS += $(CFLAGS_MCU_$(MCU_SERIES))
 CFLAGS += $(COPT)
diff --git a/ports/unix/Makefile b/ports/unix/Makefile
index 060b3a9842..8cb95d08f9 100644
--- a/ports/unix/Makefile
+++ b/ports/unix/Makefile
@@ -39,7 +39,7 @@ INC += -I$(BUILD)
 
 # compiler settings
 CWARN = -Wall -Werror
-CWARN += -Wpointer-arith -Wuninitialized
+CWARN += -Wpointer-arith -Wuninitialized -Wdouble-promotion -Wsign-compare -Wfloat-conversion
 CFLAGS += $(INC) $(CWARN) -std=gnu99 -DUNIX $(CFLAGS_MOD) $(COPT) -I$(VARIANT_DIR) $(CFLAGS_EXTRA)
 
 # Debugging/Optimization
diff --git a/ports/unix/mpconfigport.h b/ports/unix/mpconfigport.h
index 1d429bc422..301917a613 100644
--- a/ports/unix/mpconfigport.h
+++ b/ports/unix/mpconfigport.h
@@ -70,7 +70,9 @@
 #define MICROPY_REPL_AUTO_INDENT    (1)
 #define MICROPY_HELPER_LEXER_UNIX   (1)
 #define MICROPY_ENABLE_SOURCE_LINE  (1)
+#ifndef MICROPY_FLOAT_IMPL
 #define MICROPY_FLOAT_IMPL          (MICROPY_FLOAT_IMPL_DOUBLE)
+#endif
 #define MICROPY_LONGINT_IMPL        (MICROPY_LONGINT_IMPL_MPZ)
 #ifndef MICROPY_STREAMS_NON_BLOCK
 #define MICROPY_STREAMS_NON_BLOCK   (1)
diff --git a/ports/unix/variants/coverage/mpconfigvariant.mk b/ports/unix/variants/coverage/mpconfigvariant.mk
index d5c41a157c..b56ffcbbc0 100644
--- a/ports/unix/variants/coverage/mpconfigvariant.mk
+++ b/ports/unix/variants/coverage/mpconfigvariant.mk
@@ -5,7 +5,7 @@ DEBUG ?= 1
 
 CFLAGS += \
 	-fprofile-arcs -ftest-coverage \
-	-Wdouble-promotion -Wformat -Wmissing-declarations -Wmissing-prototypes -Wsign-compare \
+	-Wformat -Wmissing-declarations -Wmissing-prototypes \
 	-Wold-style-definition -Wpointer-arith -Wshadow -Wuninitialized -Wunused-parameter \
 	-DMICROPY_UNIX_COVERAGE
 
diff --git a/ports/windows/Makefile b/ports/windows/Makefile
index 595ce2036f..6c89635131 100644
--- a/ports/windows/Makefile
+++ b/ports/windows/Makefile
@@ -15,7 +15,7 @@ INC += -I$(TOP)
 INC += -I$(BUILD)
 
 # compiler settings
-CFLAGS = $(INC) -Wall -Wpointer-arith -Werror -std=gnu99 -DUNIX -D__USE_MINGW_ANSI_STDIO=1 $(CFLAGS_MOD) $(COPT) $(CFLAGS_EXTRA)
+CFLAGS = $(INC) -Wall -Wpointer-arith -Wdouble-promotion -Werror -std=gnu99 -DUNIX -D__USE_MINGW_ANSI_STDIO=1 $(CFLAGS_MOD) $(COPT) $(CFLAGS_EXTRA)
 LDFLAGS = $(LDFLAGS_MOD) -lm $(LDFLAGS_EXTRA)
 
 # Debugging/Optimization