[Editor's Note] When people think of semiconductors, they usually think of familiar semiconductors like computer CPUs and memory. Conversely, MCUs (Micro Controller Units), the core semiconductors that power electronic devices, are widely used in virtually every electronic device we encounter, yet they remain relatively unfamiliar to the general public. Recently, MCUs have been making headlines due to the semiconductor shortage, drawing public attention. Therefore, this magazine has prepared a series of articles by Yuji Kawano, Manager at STMicroelectronics, a company specializing in MCU semiconductors, to provide an in-depth look at MCUs.


There are many different types of microcontroller units (MCUs). A closer look reveals different structures, as MCU manufacturers' design engineers incorporate a variety of unique features to improve their products, but the internals of typical MCUs are similar.


The MCU basically performs three functions: △reading the program code created by the programmer, △processing the read program, and △after processing the program, outputting the processing result outside the MCU.

Here, 'Output' refers to the operation performed by the MCU, and includes functions such as displaying data, generating infrared signals, retrieving data by operating a button, checking the time, or communicating with an external IC.

In order to perform these functions, the MCU must have a program to perform the operation, and the place where this program is stored is called memory. Before the MCU is used, the programmer must input the program (the instructions for the MCU to process) into the memory.

The ability to process this program is also essential, and this is handled by the central processing unit (CPU). If the program is very simple, a single CPU can process the entire program. However, if the program is complex, the CPU must be able to temporarily store some data while processing instructions.

Memory is also used for this very purpose.

The output of the MCU, including other commands mentioned and things like generating infrared signals, is processed by the peripheral module.

In summary, an MCU consists of a CPU, some memory, and auxiliary modules.





Figure 1 schematically depicts an MCU comprised of three large blocks. The memory and CPU exchange commands and data, and information about program processing results is exchanged between the CPU and auxiliary modules, which are external interfaces to the MCU.

Data is transferred between the three blocks above via a bus, which is an information transmission circuit, but to understand how the inside of an actual MCU works, more information is needed.





There are several types of memory. Flash ROM and EEPROM are designed to store data that doesn't change, so they primarily store programs that change very infrequently. CPU operational data, on the other hand, is accessed relatively frequently. This frequently accessed data is stored in random access memory (RAM).

A CPU is broadly divided into an arithmetic-logic unit (ALU), registers, and shifters. The ALU performs arithmetic and logic operations, while the shifters move data. Registers temporarily store data, only until the data is removed from memory and processed.

There are also many different types of auxiliary module devices. This section covers auxiliary modules related to communication (communication module), AD/DA conversion, timer functions, etc.

Communication functions comply with standards such as SPI, USART, and I2C.

Other standards include I2S and USB, which only deal with audio transmission, and interface communications that are familiar to many people.

The auxiliary module for AD/DA conversion consists of an AD converter that converts analog data into digital data (ADC in Figure 2) and a DA converter that converts digital data into analog data ('DAC' in Figure 2).

It also includes a comparator for comparing analog data. A timer is used to measure time. Some high-performance timers have input capture, toggle, or PWM functions.

Now, let's see how the above-mentioned functions work when the MCU is actually used.

Here, 'when the MCU is used' refers to the process between the creation of a program by a programmer and the first exchange of information (data) with the outside of the MCU.




Figure 3 describes the various processes handled by the MCU components.