In many situations when working with STM32 microcontrollers, you will want to output text strings. There is no need to write specialized functions that output specially formatted strings as it is hard to keep up with various cases. It is convenient to use standard I/O streams and its library functions that allows sending formatted data streams. Arm GCC toolchain comes with newlib C library from Redhat, and so it isn’t specially designed for embedded toolchain. To use stdio functions we have to take care of several syscals so-called “stub functions.” These functions usually are provided by operating systems like you would write C programs in Windows or Linux. In our case, we aren’t using any OS, os to avoid error messages while compiling we have to provide these function declarations where most of them are dummy implementations. It’s not something new pick one that you find on the internet. I noticed that it was written for STM32 Discovery.
In the previous example, we implemented a simple demo program that reads buttons by continually checking their status in the main program loop. This isn’t an efficient and convenient way to do that. Imagine your application has to do lots of tasks, and in between, you also need to check button status – mission becomes impossible unless you use interrupts. In this part, we briefly introduce to STM32F10x interrupt system and write example code where LEDs and buttons are serviced within interrupts. ARM Cortex-M3 microcontrollers have advanced interrupt system that is pretty easily manageable. All interrupts are controlled inside Nested Vectored Interrupt Controller (NVIC) which is close to the Cortex core to ensure low latency and robust performance. Main features of NVIC include:
Last time we have made a good starting point with setting up a project template for STM32F103ZET6 development board using GNU tools. Using the same project template, we can move forward and start programming other elements. This time a quick note about adding button library. This is a modest implementation which initializes port pins and then reads their status. Development board is equipped with four user programmable buttons named WAKEUP, TAMPER, USER1, and USER2. We are not going to care about the meaning of names use them as general purpose buttons for now.
I found some time to play with the STM32F103ZET6 development board and decided to set up a simple project for it. Probably the trickiest part of this is to set up a project environment that would serve as a template for following developments. Many ARM developers chose CodeSourcery Lite edition toolchain. It has full command line functionality – this is what we usually need. If you want some alternative – you can select gnu yagarto ARM toolchain which is also great and free. No matter which tool you select code will work on both. Let’s stick to CodeSourcery. Just download it and install to your PC. As we said Lite version supports only command line tools – we need an interface for it. Eclipse IDE is one of the favorite choices so that we will grab this one too. Yagarto website has an excellent tutorial on how to set up the Eclipse IDE in a step-by-step manner. We won’t go into details with this.