Starts from this link https://os.mbed.com/users/hudakz/code/MapleMini_Hello/
Adding SIM800C module to this borad. https://os.mbed.com/users/champion0311/code/Sim800/file/50d54df4a4c2/main.cpp/
Connections: SIMC800 MapleMini PWR -> GND 5V -> 5V (VIN) TX -> RX RX -> TX V_TTL -> VDD