Normally, all imported Python modules in CircuitPython are loaded into RAM in compiled form, whether they start as .mpy or .py files. Especially on M0 boards, a user program can run out of RAM if too much code needs to be loaded.
To ameliorate this problem, a CircuitPython image can include compiled Python code that is stored in the image, in flash memory, and executed directly from there. These are "internal frozen modules". The circuitplayground_express and esp8266 builds use this technique.
If you would like to build a custom image that includes some frozen modules, you can imitate how it's done in the circuitplayground_express build. Look at boards/circuit_playground_express/mpconfigboard.mk:
USB_VID = 0x239A USB_PID = 0x8019 USB_PRODUCT = "CircuitPlayground Express" USB_MANUFACTURER = "Adafruit Industries LLC" CHIP_VARIANT = SAMD21G18A CHIP_FAMILY = samd21 SPI_FLASH_FILESYSTEM = 1 EXTERNAL_FLASH_DEVICE_COUNT = 2 EXTERNAL_FLASH_DEVICES = "S25FL216K, GD25Q16C" LONGINT_IMPL = MPZ # Make room for frozen libs. CIRCUITPY_DISPLAYIO = 0 CIRCUITPY_FREQUENCYIO = 0 CIRCUITPY_I2CSLAVE = 0 SUPEROPT_GC = 0 CFLAGS_INLINE_LIMIT = 55 # Include these Python libraries in firmware. FROZEN_MPY_DIRS += $(TOP)/frozen/Adafruit_CircuitPython_BusDevice FROZEN_MPY_DIRS += $(TOP)/frozen/Adafruit_CircuitPython_CircuitPlayground FROZEN_MPY_DIRS += $(TOP)/frozen/Adafruit_CircuitPython_HID FROZEN_MPY_DIRS += $(TOP)/frozen/Adafruit_CircuitPython_LIS3DH FROZEN_MPY_DIRS += $(TOP)/frozen/Adafruit_CircuitPython_NeoPixel FROZEN_MPY_DIRS += $(TOP)/frozen/Adafruit_CircuitPython_Thermistor
Notice the FROZEN_MPY_DIRS lines in the file. Pick the mpconfigboard.mk file for the board you are using, and add one or more similar lines. You will need to do add directories for the libraries you want to include. If these are existing libraries in GitHub, you can add them as submodules. For instance, suppose you want to add the Adafruit_CircuitPython_HID library to the feather_m0_express build. Add this line to boards/feather_m0_express/mpconfigboard.mk:
FROZEN_MPY_DIRS += $(TOP)/frozen/Adafruit_CircuitPython_HID
Then add the library as a submodule:
cd circuitpython/frozen git submodule add https://github.com/adafruit/Adafruit_CircuitPython_HID
When you add the submodule it will be cloned into the frozen/ directory.
Note that there is limited unused space available in the images, especially in the non-Express M0 builds, and you may not be able to fit all the libraries you want to freeze. You can of course try to simplify the library code if necessary to make it fit.
