Tutorial to ARM(STM32)[Effective method for ARM beginners]

Embedded systems are specialized computer systems with specialized functions that are built into various devices as opposed to general-purpose computer systems such as PCs and tablets. The following are just a few of the devices that come to mind with embedded systems

Almost all of today's electrical equipment, from the smallest to the largest, probably incorporates embedded technology. In fact, it is rare to find a device without embedded technology.

Here we have the concept of hardware and software. Hardware is the physical, tangible thing such as switches and electrical/electronic circuits that we can see and touch directly.

In an embedded system, software is a program inside the on-board MCU that configures various functions to control the hardware. It is not something that is physically visible.

The reason why people say that the threshold for embedded technology is high is because it requires the ability to put together a wide range of knowledge, including electrical and electronic engineering, mechanical engineering, and control engineering, in addition to software programming skills.

However, there is nothing to be afraid of, for example, knowledge of electrical and electronic engineering is not a highly specialized subject, but rather just understanding and designing a few practical circuits.

The important thing is to understand the fundamentals of each field, in other words, to understand the essence of each field, not to pursue specialization.

For example, control engineering (feedback control), which is very reassuring to learn in embedded technology, is good example. It is really difficult to understand what is written in specialized books because of the world of mathematics. If you don't have the basics, it is almost impossible to learn on your own. Even if you understand it, you will not be able to actually use it.

It is also sufficient to capture only the important essence and use only a portion of it for practical purposes. If you want to become more specialized, it is better to gain some experience and understand the basic principles.

So far, I have focused on the concept of MCU and specifics for MCU development. I will now cover the essentials of basic electrical and electronic knowledge that you need to know before you start learning about MCU in advance.

This is a very important area because it is the part where embedded technology is said to require knowledge of hardware as well as programming. For example, the circuitry of the IO port that exchanges signals with MCU, what kind of signals are handled, and what kind of structure is used.

Input signals are taken into MCU from hardware such as sensors through a circuit called an interface. After MCU acquires the signals, digitizes them, processes them in a program, and then outputs them again to the outside world via an interface for some kind of processing in an embedded system. For this reason, it is necessary to understand the specific circuits and peripheral hardware.

It is also important to have an understanding of analog and digital technology in order to understand the interface part as well as hardware knowledge such as circuits. I hope that you will always remember that you are dealing with concrete objects when they create programs.

MCU contains a CPU, ROM, RAM, I/O, and manufacturer-specific circuits called peripherals.

The CPU is called a central processing unit (CPU) and plays the role of a control tower, or brain, in a MCU. It executes commands sequentially according to a program, reads data from the input port, and sends signals to the output port at intervals measured by a timer while displaying a monitor display using the communication function of the peripheral functions.

MCUs thus incorporate timers, communications, AD/DA converters, and other peripheral functions that are essential for home appliances, mobile phones, and other electrical and communications devices. The types of built-in peripheral functions depend on MCU, but basically, timers, AD converters, serial communication UARTs, and general-purpose input/output ports (GPIOs) are important functions that are built into any MCU, so it is easy to configure a small system if you can master these functions. The following is an example of a product that can be used in a variety of applications.

In the absence of MCUs, all of these peripheral functions were composed of hardware (analog and digital circuits), so building a single system was time-consuming, labor-intensive, and difficult. Nowadays, all of the peripheral functions are built into the MCU, so they can all be configured using software (programs) alone.

If the system is designed in anticipation of future specification changes, it will be easy to change various functions later with software modifications. This is an unimaginable convenience from the days when peripheral circuits were composed of hardware.

In this chapter, I will finally explain the details of each peripheral in the STM32F1xx series (Cortex-M3).

Although the STM32 MCU is a sophisticated one, it is not necessary to understand or master all of its functions. It is sufficient to use the functions correctly as needed. However, to do so, it is necessary to understand the peripheral functions (peripherals) of the STM32 to some extent.

For example, among peripherals, timer/counters and AD converters have so many functions that it may be difficult for beginners to know which function to select. Therefore, this site explains each peripheral starting with the most basic functions that can be used in the most basic way. I think it is best to start by confirming that the function actually works, and then gradually develop the function as you go along.

Actual operation can be checked by following the sample programs in "Application Program samples used in practice".

The functions of each peripheral are not all introduced on this site, but are excerpts that can be used in most embedded systems if only these are kept in mind, so I encourage you to learn them. Of course, more advanced functions may be necessary depending on the required specifications, but these are not covered in this site because they are necessary for professionals far beyond the beginner's level.

The reference manual of the STM32 MCU provides detailed information on each peripheral. If you have become somewhat proficient with the MCU and wish to use more advanced functions, please refer to the manual to enhance the functionality.

To master peripherals is to master peripheral libraries, and this is true whether the library is a Standard Peripheral Library (SPL) or a Hardware Absolute Layer (HAL). In other words, if you understand the functions of the SPL library used in this site for the time being, you will understand the essence of MCU functions, and of course, porting to HAL in the future will be no problem. Of course, it does not matter if you can directly manipulate registers, because you do not need the library.

This may sound a bit complicated, but I am confident that beginners will be able to understand it if they follow the steps in order. Let's begin.

For relatively simple processes such as the sample programs for each peripheral introduced so far, it is sufficient to divide the contents of each process into blocks and execute them sequentially.

In a system consisting of inputs from operation switches, sensors, etc., and outputs to display screens, motors, etc., multiple processes using peripherals are combined to use these inputs and outputs. In order for the system to function efficiently, each of these processes must be executed simultaneously, and the execution frequency and priority must be set arbitrarily.

Therefore, an OS suitable for embedded systems is introduced to manage each process. This OS suitable for embedded systems is called a real-time OS (RTOS), which can easily realize so-called multitasking.

It is like a light add-on software, and once the pattern is configured according to the rules, it can be easily adopted for each application program. It is recommended to introduce it at an early stage.

This site provides explanations of real-time operating systems (RTOSs), especially those that are essential for practical use. Since the purpose of this site is to use RTOSs in actual applications, explanations are given in a programmatic manner without using too many technical terms. I hope that you will learn to use it as soon as possible and make full use of it.

So far, I have discussed peripheral interfaces and MCU peripherals for operating it. This is the part with many software elements that I will be focusing on programming.

In order for an idea to come to mind and take shape, it must be designed. This chapter explains the steps from the concept of design to the point where the final operation can be confirmed.

Knowledge and experience of hardware is especially necessary for embedded technology, especially in the design phase. Software can be utilized only with properly designed hardware, so please make sure to learn the basics of design.

The IoT is something like the Internet of Things, and until now the products functioned independently in various fields, but nowadays, the products are connected to the network regardless of whether they are wired or wireless, and a little remote operation is performed. It has become possible to manage products via the Internet, for example, on a smartphone.

The word IoT itself is very abstract and difficult to understand, but by connecting products to the network, it has development potential that was unthinkable in the past. It can be said that it is essential for future embedded products to support IoT.

This site introduces the one using the wireless module that can easily make the existing embedded system compatible with IoT. The wireless module itself has a built-in MCU, and the WiFi protocol stack and Bluetooth protocol can be used only by serial communication, so it is relatively easy.

Regarding wired LAN, it is quite complicated how to use the communication protocol stack consisting of software on the condition that the MCU itself is compatible with Ethernet, and STM32F1 does not support it. It can be achieved relatively easily by using an Ethernet-enabled MCU represented by STM32F4 series, an automatic code generation function, and a HAL library development environment, but it is still necessary to know the basic usage of the protocol stack such as socket communication.

In my experience, it was necessary to somehow make the existing system commercialized in the STM32F1 series compatible with Ethernet, then I used WIZnet's W5500 TCP/IP controller IC to make it compatible with Ethernet externally. This IC has a built-in TCP/IP protocol stack as hardware, so it can be used with relatively simple code. This can be achieved if the communication interface SPI can be used.