[Editor's Note] Generally, when people think of semiconductors, they tend to picture components familiar to the general public, such as computer CPUs and memory. On the other hand, the Micro Controller Unit (MCU), which serves as a core semiconductor for driving electronic products, is used in virtually every electronic device we encounter, yet remains an unfamiliar semiconductor to the general public. Recently, however, MCUs have begun to attract public attention as they have been frequently mentioned in the media due to the semiconductor shortage. Accordingly, this publication has organized a series of articles by Manager Yuji Kawano of STMicroelectronics, a company specializing in MCU semiconductors, to provide a professional look into MCUs.


■ MCU, controls the functions of mechanical components in electronic circuits and electronic devices

MCU stands for Micro Controller Unit and is commonly referred to as a microcontroller. It is a core component installed in various electronic devices that enables a wide range of 'control' or 'computational tasks' through programming. Just as the human brain controls the human body, the MCU plays the role of coordinating the functions of electronic circuits or mechanical components that make up electronic devices.

The origins of the MCU lie in the Integrated Circuits (ICs) embedded in the first calculators. Since then, as efforts were made to develop better calculators using fewer components, the MCU has also evolved and developed alongside them.

Examples of MCU applications
▷ Timer function of a rice cooker or TV recorder
▷ Remote control emits infrared signal
▷ Measures mobile phone battery voltage and displays remaining battery capacity
▷ Clock time display
Press the button to perform the function
▷ Data exchange with a computer via USB (higher-level applications)

For an MCU to implement a specific function, it must undergo a programming process. This means that if you develop a rice cooker with a reservation function and set it to start cooking within an hour, you must program the rice cooker's MCU to perform this task. Most electronic products today are equipped with an MCU.


Although it is not recommended to disassemble electronic devices, if you happen to have the opportunity, you will find a black object mounted on a printed circuit board (PCB) like the one in Figure 1.2, as shown in Figure 1.1. This black object is the MCU. As can be seen in Figure 1.3, MCUs can be manufactured in various shapes and sizes.

■ What does the MCU do

In a word, the possibilities are endless. It is no exaggeration to say that most of the functions we are familiar with in electronic products are implemented by MCUs.

○ Visual function

Visual functions are very important in that they allow users to check information regarding the operating status of electronic devices. This function allows users to check the status of the device while it is in operation and can also be used to check the time when it is not in operation. Text can be displayed on liquid crystal displays (LCDs) or light-emitting diodes (LEDs). In some home appliances, the MCU is used simply to turn on LED lights or for flashing purposes.

○ Auditory function

Many electronic devices, such as microwave ovens and fire alarms, offer speech synthesis functionality. They also inform users of the device's operating status through music or alarm sounds. Auditory features are also commonly found in health devices like thermometers and blood pressure monitors.

○ Motor/Valve Control

An MCU can output signals to control motors or valves. More specifically, it can output signals to change the direction or speed of a motor, or to open and close valves. The infrared beam signal from a TV remote control is also output by an MCU.

○ Electronic signal measurement and output

The MCU has the capability to directly measure the voltage of sensor components. For example, when directly connected to a temperature sensor such as a thermometer, the MCU can measure the temperature. Similarly, when connected to a light (brightness) sensor, the MCU can also measure the brightness of the light. It can also measure battery voltage to indicate when it is time to replace the battery. At the same time, the MCU can output a specific voltage. Because it is capable of outputting a constant voltage, it can also function as a regulator, providing a reference voltage to other electronic components.

○ Communication

As PC usage increased, USB (Universal Serial Bus) became the standard communication interface for connecting PCs with other devices. For this reason, MCUs also possess USB connectivity capabilities. Simple data exchange between MCUs is also possible. Automobiles are a good example. Today's cars are 'moving communication networks'. A single vehicle uses numerous MCUs, and these many MCUs support CAN (Controller Area Networks) and LIN (Local Interconnect Network), enabling the entire vehicle to operate as a single system.

○ Calculation

Computation is the original purpose for which an MCU was created. To handle specific tasks, the MCU integrates and coordinates the various functions mentioned above. For example, it converts voltage measured by a temperature sensor into temperature for display, or calculates motor usage to adjust the performance of the corresponding motor. Integrating various functions necessary to process a single task is one of the most important functions of an MCU.



> ■ Applications of MCU

The application fields of an MCU vary depending on how it is programmed. What is clear is that the range is very wide. Figures 2.1 and 2.2 illustrate examples of everyday household appliances equipped with an MCU.

The MCU controls the displays, timers, compressors, and motors of major home appliances such as refrigerators, washing machines, and air conditioners. For example, the MCU installed in a blood pressure monitor controls the motor, valve, and digital display, while the MCU in a thermometer measures the signal from the temperature sensor.

The electronic device where MCUs are most widely used is likely the remote control. Today's remotes control not only TVs and air conditioners but also the operation of lights and fans. Furthermore, it is the role of the MCU to detect button presses and control the infrared signals transmitted by the remote.

It is also applied to camera equipment and hobby tools such as cycling. MCUs used in cameras control autofocus, shutter speed, and image stabilization, while in bicycles, they are utilized in speedometers and gear shifting. In the case of expensive bicycles, the MCU even controls the suspension.

Examples of MCU applications can also be easily found in office electronics. Not only industrial equipment, but also household power drills use MCUs to check motor speed and battery level. MCUs were originally developed for calculators. Therefore, their original purpose was to handle advanced calculation tasks. Although such high-level calculations are not largely necessary in daily life, they can be utilized in various fields.


As shown in Figure 3, STMicroelectronics' STM32 series MCUs are installed in the AIGAMO ROBOT. This small robot is a weeding robot developed by the Gifu Prefectural Research Institute of Information Technology on behalf of the Japanese Ministry of Economy, Trade and Industry. Additionally, the same MCUs were used as the gyro sensor and governor sensor in a portable sleep apnea testing device (Figure 4.1) and a model helicopter (Figure 4.2). As seen in the examples above, it can be seen that MCUs can be utilized extensively, ranging from simple devices to cutting-edge technologies, depending on programming.

■ The Role of MCUs in Home Appliances

Let's take a closer look at how MCUs are utilized in real-world home appliances through a few examples.

○ Blood pressure monitor

A single blood pressure monitor utilizes a significant number of MCU functions. Figure 5 shows an example of the electronic circuitry contained in a blood pressure monitor. The MCU processing operations shown below are the functions used when the blood pressure monitor is actually in operation. The processing sequence numbers correspond to the MCU circuit numbers in Figure 5.

The user presses the power switch to operate the blood pressure monitor.
▷ The user presses a button to input information about the person being examined. Here, the MCU identifies which button is pressed.
The user presses the start button to measure blood pressure.
The motor starts operating and inflates the blood pressure cuff with air. The MCU sends a signal to the driver (transistor), and the driver operates the motor.
The heart rate check sensor reads changes in heart rate. The signal read by the heart rate sensor is converted into voltage, and the MCU measures that voltage.
The blood pressure monitor air valve is adjusted according to the data from the heart rate sensor. The MCU sends a signal to the driver (transistor), and the driver adjusts the air valve.
The blood pressure monitor sounds an alarm when blood pressure measurement is completed. The MCU sends an alarm activation command signal.
▷ The measurement result is displayed on the LCD. The MCU turns on the LCD.
The measurement results are stored in a storage device called EEPROM (Electrically Erasable and Programmable Read-Only Memory). The MCU transmits data to the EEPROM using communication functions.
▷ The high-end product has the function of transmitting measurement results to a PC for data analysis.
▷ When not in operation, the blood pressure monitor displays the current time on the LCD screen. The MCU measures and displays the time.
Although not directly related to blood pressure measurement, the remaining battery capacity is also checked regularly. The MCU regularly measures the power supply voltage.

As can be seen from the items above, a single MCU performs numerous roles.

○ Remote control

Figure 6 shows the electronic circuit of the infrared remote control. The operations processed by the infrared remote control's MCU are as follows. The processing sequence numbers correspond to the MCU circuit numbers in Figure 6.



▷ The remote control has no power switch. This is because it is available for use as soon as the battery is installed. When the remote control is not in use, the MCU switches to standby mode and conserves battery power as much as possible until the power button is pressed.
▷ The user presses a button to execute it. The blocking function of the MCU identifies which button is pressed and switches from standby mode to normal operation mode. Subsequently, the MCU performs an operation according to the pressed button.
An infrared signal is transmitted according to the input execution information. The MCU outputs an infrared signal to turn on the infrared LED screen.
▷ Information about the input operation is displayed on the LCD. The MCU operates the LCD.
▷ If the remote control has a clock function, the LCD displays the current time when the remote control is not in operation. The MCU measures and displays the time.

■ Programming

MCUs can be utilized in various ways depending on how they are programmed. This also implies that an unprogrammed MCU is completely useless. Generally, programs related to the MCU are written using a Windows PC. Applications requiring specific MCU programming can be downloaded from the internet. While some applications are available for free, others have limited program sizes or are free for a set period only.

Once programming is complete, the program must be written to the MCU for execution, and a debugger and programmer are required for this task. Additionally, specialized debuggers and programmers are needed depending on the type of MCU.



Figure 7 shows a sample of the ST-Link debugger and programmer, STMicroelectronics products. In the figure above, the PC and ST-Link are connected via a USB cable, and the ST-Link is connected to the MCU on the PCB via a dedicated cable. At first glance, it may appear that there are many cables connecting the PCB and the ST-Link, but in reality, only four lines are connected (two lines for power supply monitoring, one reset line, and one control signal line). Users must fabricate the PCB, as the PCB is the most essential component for programming and using the MCU.