C programming of a modbus RTU to MQTT gateway on Atmel ATXmega and Quectel hardware

The skills required for this project are a good knowledge of C programming, modbus RTU, MQTT, AT commands and Atmel microcontrollers. The hardware is based on an ATXmega(-A4-128) and a Quectel BG95 (or BG96) witch has a built-in MQTT protocol stack. The board will be a modbus RTU slave device with some registers witch are synchronised with the corresponding MQTT variable. The BG95 has a built in GNSS receiver and the time, latitude, longitude and status will be written in modbus registers. An anti-theft function can be activated witch send an alarm in case of a displacement of the system. Alarming will be done through MQTT and SMS. Programming must be done on CodeVision ([login to view URL]) and if necessary a license will be provided for the duration of the development. We want a well documented, interrupt based and without "wait" and loop programming. All code must be accessible and modifiable, so the use of libraries without sources is prohibited. A base software with modbus RTU slave protocol, a timed structure and a GPS decoder already exist and will be provided, so the most important thing to do is the interfacing with the BG95. The AT commands to initialize and make a communication with the MQTT broker have been tested and will be provided. The big job is to correctly handle the communication with the BG95 in all situations. The freelancer must have the experience of the communication with such "modem" and be able to evaluate all the possible case of bad situations such as lost of network, crashes or unsolicited messages. A detailed specification document is provided as attachment.