When we need some feedback from the microcontroller, usually we use USART. It allows to output messages and debug information to the terminal screen. Also, data can be sent to MCU same way. For this purpose, STM32 microcontrollers have more than one USART interface allowing to have multiple streams of data output and input. USART interface is designed to be very versatile, allowing to have lots of modes including LIN, IrDA, Smart card emulation, DMA based transmissions. But for now, let’s focus on standard USART communications we could send and receive messages from the terminal window.
Probably one of the key features of any microcontroller is the interrupt system. ARM Cortex-M3 microcontrollers may have up to 256 interrupted sources. The first 15 interrupt sources are called system exceptions. These exceptions arise within Cortex core like reset, NMI, hard fault and error, debug, and SystTick timer interrupt. In the exception table, they start from address 0x00000004 and are numbered from 1 to 15. There is no 0 number exception (FYI – the very top of exception table address is used to store the starting point of stack pointer): Each exception vector holds the four-byte address of the service routine that is called when an exception occurs. Exception table usually is located in startup code like this:
Previously we learned how to compile STM32VL Discovery projects that were included in the package. But to understand how to write our own programs, we need to get to some basics. I think the best place to start is the input and output system (I/O). Before we begin to write some code, let’s go through what’s inside STM32 ports. If you look into the STM32 reference manual, you’ll find that the I/O system is pretty flexible. Port pins can work in several modes: Input floating; Input pull-up; Input pull-down; Analog; Output open drain; Output push-pull; Alternate function push-pull; Alternate function open drain. Pins are organized as 16-bit ports that have their names like PORTA, PORTB, PORC, PORTD… Ports are 16-bit wide; they are controlled with 32-bit words. Individually each port pin can be configured to one of these functions. Additionally, each pin’s maximum speed can be set to one of the values: 2MHz, 10MHz, and 50MHz. STM32 I/Os are 5V tolerant. Anyway, the proper design should use 5 to 3.3V level converters.
In this tutorial, we will set up a simple template for programming ST32 -Discovery board. For this, we will use the latest Code Sourcery and Eclipse IDE. To make things simpler, we will use ARM-based 32-bit MCU STM32F10x Standard Peripheral Library v3.5.0, that can be downloaded from ST site. Also, we will use STM32VLDiscovery firmware package for example, files. And why write our linker, startup, and make scripts. For this, we will use Michael Fischer’s example project (STM32Test.zip) for yagarto. We only need slight modifications to fit our needs. In this stage, we assume that you have set up Eclipse and Code Sourcery, and we can go further. First of all, create new C project in Eclipse File->New: Enter the project name, select the path where the project will be stored, and select Makefile project->Empty project in the Project type list. In Toolchain, list select Other Toolchain. This will create an empty project that will run make to compile the project.
Startup code is executed just after microcontroller is reset before the main program. As linker script, startup code usually is implemented as universal code for all the same microcontroller type. So often you don’t need to write one from scratch. Anyway, it is good to know what happens there anyway. As we said, linker script has to go along with the startup code. This means that it will initialize MCU automatically according to data defined in the linker. Startup code initializes variables, copies defined variables from Flash to RAM, initializes stack and then gives resources to the main program. You will find that startup codes usually are written in assembly language, but this can also be done in C, which is easier to read and modify if necessary. First of all, in linker script we have pointed entry point to startup with the following command: So in startup code, this will be the first function called.
Previously we went through setting up a development environment for ARM Cortex-M3 microcontroller. We decided that two equal choices will do the same job – either CodeSourcery G++ Lite or Yagarto. Both use the same base of GNU tool-set. Developing with GCC tools To get a working binary, there is a series of tools involved during code development. Several tools are necessary to compile simple applications. These are compiler, assembler, linker and binary generator. Each of them does its task in a chain process. When you start compiling your project, a linker is usually invoked, which with correct parameters links libraries and object files. Once executable is generated, the binary image generator creates a binary image (.bin or .hex) uploaded to MCU. We won’t go into details right now as this will be more convenient to do in later code examples. Let us get directly to the code writing part, which is very important for linker and tasks before the main() routine.