STM32F103ZET6 board comes with 3.2 inches graphical LCD which features an ILI9320 controller. Equipped LCD is capable of displaying 252144 colors when driven in 18-bit mode. We are going to run it in 16-bit mode, so we are limiting it to 65K colors. LCD driver is based on the existing code found on the internet, originally developed for the STM3210E board. Only minor modifications were needed, like assigning the proper control pins.
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 their library functions that allow sending formatted data streams. Arm GCC toolchain comes with the newlib C library from Redhat, so it isn’t specially designed for the 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, so 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. I named it newlib_stubs.c and placed it in the startup directory. Among system functions implementations like _write(), _fstat(), etc., there are also USARTs assigned to standard streams:
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 an advanced interrupt system that is pretty easily manageable. All interrupts are controlled inside Nested Vectored Interrupt Controller (NVIC), 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 the 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 a button library. This is a modest implementation that initializes port pins and then reads their status. The Development board is equipped with four user-programmable buttons named WAKEUP, TAMPER, USER1, and USER2. We will not care about the meaning of names; use them as general-purpose buttons for now.
The LPC2148 development board is a mighty board with an ATM7TDMI series microcontroller considered an old guy compared to Cortex ones. But still, these are widely used and are powerful. The Development board has some handy features installed. 12MHz crustal allowing to run the processor at full 60Mhz speed. Couple RS232 ports, VGA connector, PS/2 connector for keyboard or mouse, 20-pin JTAG, SD/MMC slot, USB B-type, 8 LEDs driven with a serial-in parallel-out shift register, 2×16 LCD, buzzer, audio jack with an amplifier, two programmable buttons, and 256Kb of I2C interfaced EEPROM. The microcontroller itself has 512KB of internal flash and 32+8KB of RAM. All ports are accessible, and any external hardware can be disconnected with jumpers. This is a great board for prototyping and end application.
I found some time to play with the STM32F103ZET6 development board and decided to set up a simple project for it. The trickiest part of this is to set up a project environment that would serve as a template for the following developments. Many ARM developers chose the 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, the code will work on both. Let’s stick to CodeSourcery. Just download it and install it on 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.
AVR microcontrollers aren’t the best choice to run the FreeRTOS scheduler due to low on-chip RAM. Atmega128 has only 4K of RAM, so this limits the FreeRTOS functionality to very basic. This problem can be solved by adding extra RAM, which may be connected to an external memory interface. We have already built an external memory block of 8K previously to test it with FreeRTOS applications. Let’s continue with our previous code, which runs several simple tasks (button state reading, LCD output, and LED flash), and we can add more to it. We are going to set up an external RAM for storing heaps. This will allow the storage of large data buffers without worrying too much about heap and stack overlaps.
The latest development board has just arrived. I thought it would be nice to push things more towards the ARM cortex-M3 playground. This an STMicroelectronics STM32F103ZET6 ARM Cortex – M3 MCU based development board with a 3.2” Touch LCD screen. This is a high – density performance line 32 -bit MCU featuring internal 512K of FLASH memory, 64K of RAM. It is rich in peripherals like USB, CAB, 11 timers, 3ADC, and many communication interfaces. The microcontroller seems to be powerful enough to run quite intensive tasks, but more memory is populated on board. Additionally, there are:
In the previous post, we just run a single task. Running RTOS with a single task has no meaning at all. This can be quickly done with a conventional program. But what if we need to have more separate functions. To execute them at exact timing would require a separate timer or interrupt. But microcontroller cannot guarantee an interruption for every task. This way, it is hard to make code modular, and testing can be painful. Using RTOS solves this kind of problem. It allows programming each task as an endless loop. Kernel scheduler takes care of assuring each task gets its chunk of processing time. Additionally, it does bearing the priority systems – more critical tasks are executed before less important ones. Let us go further with our example code and add more tasks to our FreeRTOS engine. We already have an LED flashing task that toggles LED every second. Additionally, we are going to create another task that checks the button state. Also, we are going to send some information to the LCD. As always, let’s take care of drivers for all of them.
FreeRTOS is known as Real-Time Operating System. It would probably be too dare to call it real-time-os, preferably a real-time scheduler where applications can be split into independent tasks that share complete processor resources by switching them rapidly. It. It looks like all functions are executed in parallel. This feature is called multitasking. There are many debates on using RTOS on AVR microcontrollers as they are arguably too small for the running scheduler. The main limitation is a small amount of ram and increased power usage. If you use lots of tasks in the application, you will probably run out of RAM to save context when switching between tasks. Consider FreeRTOS only if you use larger scale AVRs like Atmega128 or Atmega256. Indeed you can find smaller schedulers that are specially designed for smaller microcontrollers, even tiny series. On the other hand, if you master FreeRTOS, it can be used with multiple microcontrollers like ARM Cortex, PIC, and various compilers, including IAR, GCC, and Keil Rowley, Attolic. And the main reason to keep an eye on it – it is free. Probably it would take lots of time and space to go through RTOS theory. Some great information can be…