diff --git a/docs/develop/cmodules.rst b/docs/develop/cmodules.rst
index 143057f7f..1b3ba04da 100644
--- a/docs/develop/cmodules.rst
+++ b/docs/develop/cmodules.rst
@@ -49,9 +49,17 @@ A MicroPython user C module is a directory with the following files:
   in your ``micropython.mk`` to a local make variable,
   eg ``EXAMPLE_MOD_DIR := $(USERMOD_DIR)``
 
-  Your ``micropython.mk`` must add your modules source files relative to your
-  expanded copy of ``$(USERMOD_DIR)`` to ``SRC_USERMOD``, eg
-  ``SRC_USERMOD += $(EXAMPLE_MOD_DIR)/example.c``
+  Your ``micropython.mk`` must add your modules source files to the
+  ``SRC_USERMOD_C`` or ``SRC_USERMOD_LIB_C`` variables. The former will be
+  processed for ``MP_QSTR_`` and ``MP_REGISTER_MODULE`` definitions, the latter
+  will not (e.g. helpers and library code that isn't MicroPython-specific).
+  These paths should include your expaned copy of ``$(USERMOD_DIR)``, e.g.::
+
+    SRC_USERMOD_C += $(EXAMPLE_MOD_DIR)/modexample.c
+    SRC_USERMOD_LIB_C += $(EXAMPLE_MOD_DIR)/utils/algorithm.c
+
+  Similarly, use ``SRC_USERMOD_CXX`` and ``SRC_USERMOD_LIB_CXX`` for C++
+  source files.
 
   If you have custom compiler options (like ``-I`` to add directories to search
   for header files), these should be added to ``CFLAGS_USERMOD`` for C code
diff --git a/ports/samd/Makefile b/ports/samd/Makefile
index 735d25bf2..9386a8dac 100644
--- a/ports/samd/Makefile
+++ b/ports/samd/Makefile
@@ -81,7 +81,8 @@ endif
 # Flags for optional C++ source code
 CXXFLAGS += $(filter-out -std=c99,$(CFLAGS))
 CXXFLAGS += $(CXXFLAGS_MOD)
-ifneq ($(SRC_CXX)$(SRC_MOD_CXX),)
+# TODO make this common -- shouldn't be using these "private" vars from py.mk
+ifneq ($(SRC_CXX)$(SRC_USERMOD_CXX)$(SRC_USERMOD_LIB_CXX),)
 LIBSTDCPP_FILE_NAME = "$(shell $(CXX) $(CXXFLAGS) -print-file-name=libstdc++.a)"
 LDFLAGS += -L"$(shell dirname $(LIBSTDCPP_FILE_NAME))"
 endif
diff --git a/ports/stm32/Makefile b/ports/stm32/Makefile
index 403b68b24..4e594d19f 100644
--- a/ports/stm32/Makefile
+++ b/ports/stm32/Makefile
@@ -157,7 +157,8 @@ endif
 # Flags for optional C++ source code
 CXXFLAGS += $(filter-out -Wmissing-prototypes -Wold-style-definition -std=gnu99,$(CFLAGS))
 CXXFLAGS += $(CXXFLAGS_MOD)
-ifneq ($(SRC_CXX)$(SRC_MOD_CXX),)
+# TODO make this common -- shouldn't be using these "private" vars from py.mk
+ifneq ($(SRC_CXX)$(SRC_USERMOD_CXX)$(SRC_USERMOD_LIB_CXX),)
 LIBSTDCPP_FILE_NAME = "$(shell $(CXX) $(CXXFLAGS) -print-file-name=libstdc++.a)"
 LDFLAGS += -L"$(shell dirname $(LIBSTDCPP_FILE_NAME))"
 endif
diff --git a/py/py.mk b/py/py.mk
index 8aac460b4..ec69ca42d 100644
--- a/py/py.mk
+++ b/py/py.mk
@@ -32,22 +32,44 @@ endif
 ifneq ($(USER_C_MODULES),)
 # pre-define USERMOD variables as expanded so that variables are immediate
 # expanded as they're added to them
-SRC_USERMOD :=
+
+# C/C++ files that are included in the QSTR/module build
+SRC_USERMOD_C :=
 SRC_USERMOD_CXX :=
+# Other C/C++ files (e.g. libraries or helpers)
+SRC_USERMOD_LIB_C :=
+SRC_USERMOD_LIB_CXX :=
+# Optionally set flags
 CFLAGS_USERMOD :=
 CXXFLAGS_USERMOD :=
 LDFLAGS_USERMOD :=
+
+# Backwards compatibility with older user c modules that set SRC_USERMOD
+# added to SRC_USERMOD_C below
+SRC_USERMOD :=
+
 $(foreach module, $(wildcard $(USER_C_MODULES)/*/micropython.mk), \
     $(eval USERMOD_DIR = $(patsubst %/,%,$(dir $(module))))\
     $(info Including User C Module from $(USERMOD_DIR))\
 	$(eval include $(module))\
 )
 
-SRC_MOD += $(patsubst $(USER_C_MODULES)/%.c,%.c,$(SRC_USERMOD))
-SRC_MOD_CXX += $(patsubst $(USER_C_MODULES)/%.cpp,%.cpp,$(SRC_USERMOD_CXX))
-CFLAGS_MOD += $(CFLAGS_USERMOD)
-CXXFLAGS_MOD += $(CXXFLAGS_USERMOD)
-LDFLAGS_MOD += $(LDFLAGS_USERMOD)
+SRC_USERMOD_C += $(SRC_USERMOD)
+
+SRC_USERMOD_PATHFIX_C += $(patsubst $(USER_C_MODULES)/%.c,%.c,$(SRC_USERMOD_C))
+SRC_USERMOD_PATHFIX_CXX += $(patsubst $(USER_C_MODULES)/%.cpp,%.cpp,$(SRC_USERMOD_CXX))
+SRC_USERMOD_PATHFIX_LIB_C += $(patsubst $(USER_C_MODULES)/%.c,%.c,$(SRC_USERMOD_LIB_C))
+SRC_USERMOD_PATHFIX_LIB_CXX += $(patsubst $(USER_C_MODULES)/%.cpp,%.cpp,$(SRC_USERMOD_LIB_CXX))
+
+CFLAGS += $(CFLAGS_USERMOD)
+CXXFLAGS += $(CXXFLAGS_USERMOD)
+LDFLAGS += $(LDFLAGS_USERMOD)
+
+SRC_QSTR += $(SRC_USERMOD_PATHFIX_C) $(SRC_USERMOD_PATHFIX_CXX)
+PY_O += $(addprefix $(BUILD)/, $(SRC_USERMOD_PATHFIX_C:.c=.o))
+PY_O += $(addprefix $(BUILD)/, $(SRC_USERMOD_PATHFIX_CXX:.cpp=.o))
+PY_O += $(addprefix $(BUILD)/, $(SRC_USERMOD_PATHFIX_LIB_C:.c=.o))
+PY_O += $(addprefix $(BUILD)/, $(SRC_USERMOD_PATHFIX_LIB_CXX:.cpp=.o))
 endif
 
 # py object files