BOARD_SUBwhich isn't documented and required diving into the source code, and also the fact the Pro Micro has 2 PIDs for each variant (5V and 3.3V).
Hope this will help someone in the same position as me. This makefile is meant for use on Debian Linux, with Arduino-MK installed via
apt-get install arduino-mk.
MONITOR_PORT = /dev/ttyACM0 # this should point to where the sparkfun board definitions are installed SPF_AVR_PATH = $(HOME)/.arduino15/packages/SparkFun/hardware/avr/1.1.3 # Use own arduino install which is way more advanced than the default 1.0 # installed by arduin-core ARDUINO_DIR = $(HOME)/arduino-1.6.7 USER_LIB_PATH := $(HOME)/Arduino/libraries # this is needed to stdint.h exposes needed #defines CPPFLAGS += -std=gnu++11 # this is needed so we don't run into __cxa_guard_acquire issues CPPFLAGS += -fno-threadsafe-statics BOARD_TAG = promicro BOARD_SUB = 8MHzatmega32U4 # also Arduino-mk isn't clever enough yet to properly extract PID from # boards.txt, so we specify it here for now USB_PID = 0x9204 ALTERNATE_CORE = promicro8 ALTERNATE_CORE_PATH = $(SPF_AVR_PATH) BOOTLOADER_PATH = caterina BOOTLOADER_FILE = Caterina-promicro8.hex ISP_PROG = usbasp include /usr/share/arduino/Arduino.mk