Simple Phone March 2020

Up to now I have used only a simple scheduling that is excellent for simple embedded applications where interrupts are used for time critical operations. I wished to understand also how more complex systems work therefore I have studied and implemented a real time OS on STM32 microcontroller. And a phone is an ideal application for OS implementation since it needs to control many operations concurrently.

The simple phone was named telefon and will have a keypad, LCD display, microphone, speaker, SIM card slot, SIM800C module and of course the main processing unit will be STM32L552. A block diagram can be seen in Figure 1.

Figure 1: Block diagram of simple phone

PCB design

A custom PCB was designed to fit all the components compactly on one board. Since the MCU has many IO pins, all the buttons were able to be connected to individual pins. A care was needed to make a SIM800C Altium library and appropriately connect the pins on schematic. When drawing a layout some high speed precautions needed to be considered: proper use of ground plane under MCU and SIM800C module, termination of GSM antenna, protection of input pins using resistors/capacitors/diodes... Schematic and PCB layout are shown in Figures 2 and 3.

Figure 2: Schematic of simple phone

Figure 3: Layout of simple phone

MCU programming

A very capable MCU in compact and easy to solder case was used for the project with 256kB or RAM, 512kB of flash and many peripherals. The project was initially meant to be written in C++, therefore high RAM/flash requirement, but it was later discovered that arm compiler does not fully support C++ standard.

RTOS was implemented using STM32CubeIDE with predefined tasks and peripherals as shown in Figure 4. SIM800C was also transferring data to MCU, therefore Direct Memory Access (DMA) was used to intercept the receiving messages and store them in a cyclic memory block. If DMA was not used, we would miss messages from SIM800C, because receiving task is not constantly pooling for data. Constant pooling is expensive, another solution is use of interrupt, but since we are working with RTOS, we would not like to disturb OS by random interrupts. So the best solution is to DMA the messages into appropriately sized cyclic container and read it with receive task. The code for main.c can be found HERE.

Figure 4: STM32cubeIDE RTOS tasks

Phone functionality

The simple phone has the following applications:

Start menu - used to select which application to open
Snake - a fun game where you need to eat the apple
Pong - a game you can play alone or with a friend
GMScom terminal - a terminal to make a call, receive a call, send SMS and other phone related things

Some tasks are constantly running in the background:

UART - connection STM32 with GSM module
DMA receive - in order to read all incoming messages from GSM module
LCD refresh - a crucial task which displays items on the screen
Keypad - reads and stores pressed keys
RND - a hardware random number generator

A few screenshots are shown in Figure 5.

Figure 5: Screenshots of simple phone's applications

Conclusion

A project was a success and strengthened my knowledge of real time task scheduling with the use of more advanced scheduling methods. Also programming and polishing Snake, Pong and GMS terminal was a fun challenge despite consuming quite a lot of time. Also, it was noticed when powering the phone using laboratory supply instead of battery, GSM module froze, because of common mode interference. To recap project a short Video 1 was put together :)

Video 1: Simple Phone - telefon