is done using an Atmega16, The AVR bit-bangs VGA signals through its SPI port. I modified the AVR VGA/PAL Terminal example by Vassilis Serasidis to drive the VGA monitor. Since the code running on the Atmega16 was critically timed so that not much more functions could be handled by it without interrupting the VGA out. So I choose another microcontroller, an Atmega8 ( a AtTiny would be enough, but I had a spare Atmega8 with me) to handle the GSM message part. The entire circuit is shown below:
The Atmega8 configures the GSM modem, receives messages, and after authentication, sends it to the display controller built around Atmega16 through a software serial port (pin4). The GSM modem is interfaced through the standard AT commands. The hardware UART of Atmega8 is used to send the received message after formatting to display to the display controller. The GSM modem is based on the popular SIM900 architecture and was bought from rhydolabz. It has TTL outputs (5v &3.3V) so that no level converters are needed in between the GSM module and microcontroller.
All the codes are available in my github.