MCUs Series - Part 5 - Basics of Microcontrollers
A microcontroller unit (MCU) is an intelligent and versatile semiconductor IC (integrated circuit) that can be programmed to control a wide range of electronic hardware. The primary component of an MCU is the processor unit, responsible for the calculation and control tasks. Supporting the processor unit are memory modules, communication interfaces, and various other peripherals. In modern electronics, microcontrollers are ubiquitous and function as the brains for a wide range of devices, including robots, appliances, drones, industrial machinery, vehicles, and game controllers.
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2. Objectives
Upon completion of this module, you will be able to:
- Understand the basic concepts and operation of MCUs
- Describe the components that make up a typical MCU
- Discuss the architecture of an MCU, using a Toshiba MCU as an example
- Explain the basics of software development with an MCU
3. History
In 1971, Intel Corporation developed a 4-bit microcontroller called the i4004 that was originally designed for use in calculators manufactured by a company called BUSICOM. The i4004 was later sold as a general purpose microcontroller. Intel followed this with 8-bit microcontrollers, such as the i8008, i8080A, and i8085. The 8086, a 16-bit microcontroller, was later released in 1978.
Around the same time period in 1973, Toshiba developed the TLCS-12, a 12-bit microcontroller that found its way into Ford vehicles as the in-vehicle engine controller. Since then, Toshiba has expanded its range of microcontrollers, covering 4-bit to 32-bit designs and higher.
4. Basic Concepts
What Is an MCU?
An MCU is essentially a brain that controls the hardware of an electronic device. The MCU receives input signals from a variety of sources: buttons, sensors, communications data such as Ethernet, and more. Software in the MCU interprets the input signals and responds with commands that control the peripheral circuitry, such as motors or displays.
What’s Inside an MCU?
An MCU is comprised of several pieces, the primary of which is the central processing unit (CPU). The CPU handles all of the logic and calculations.
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What Is a Large Scale Integrated Circuit? A Large Scale Integrated Circuit (LSI) is an integrated circuit (IC) that contains 1,000 or more elements. A microcontroller is an example of an LSI. |
In order to store data and code, an MCU has onboard memory that the CPU can write to and read from. There are two types of memory: RAM (Random Access Memory), which can read and write data, and ROM (Read Only Memory), which mainly stores program code and, as its name suggests, is read-only. Memory is either volatile or non-volatile. Volatile memory loses its stored data when power is turned off; non-volatile memory does not. RAM is considered volatile memory, and is typically used to store program calculations and for the runtime of software. ROM is non-volatile memory, and is suitable for storing data that doesn’t change, such as program code and configuration. Hard drives are also considered non-volatile memory.
| RAM (Volatile) | |
| SRAM (Static RAM) | Bipolar type |
| Stores information by flip-flop circuit | |
| Used for registers and cache memory | |
| High speed, low integration, large capacity, and low power consumption | |
| DRAM (Dynamic RAM) | MOS type |
| Stores information by capacitor | |
| Needs refresher at regular intervals | |
| Used for main storage | |
| Low speed, high integration | |
| ROM (Non-Volatile) | |
| Mask ROM | |
| Write information in manufacturing | |
| Impossible to write by users | |
| PROM | OTP |
| Programmable ROM | |
| Possible to write only once by users | |
| EPROM | |
| Write electrically | |
| Possible to erase by ultraviolet light | |
| EEPROM | |
| Electrically writable and erasable | |
| Flash | |
| Electrically writable and erasable | |
| Electrically writable and erasable with the block unit | |
Table 1: Memory type (RAM and ROM)
In order to interface with the outside world, an MCU must have an input and an output (I/O). I/O allows external devices to exchange data and control signals with the microcontroller. The I/Os on an MCU are referred to as ports, and their addresses are called port addresses.
The bus line, or system bus, is the wiring that connects the CPU, the memory, and the I/O together. There are three types of bus lines: data buses, address buses, and control buses.
Figure 1: Bus line type
The data bus enables the exchange of data between the CPU and memory, as well as the CPU and I/O. This data includes calculation variables, calculation results, and input and output signals for external devices.
The address bus is a signal line that carries the addresses of the various parts of the MCU. This data contains the addresses specifying the location of the data when reading, and where data should be saved when writing.
The control bus is a signal line carrying the command regarding whether to “read” or “write” to the location specified by the address bus. When instructed to “write” by the control bus, the memory and I/O specified on the address bus receive the data that was sent by the data bus. When instructed to “read,” the data is output to the data bus.
Programming a Microcontroller
An MCU is a versatile device, because it is capable of being programmed to perform a wide range of functions, including logic and arithmetic calculations, receiving inputs from sensors, and controlling external devices. Its programmability is what makes an MCU one of the most powerful tools in modern engineering.
The software of a typical MCU runs sequentially; that is, the CPU will read an instruction at a specified address and execute it before moving on to the next address. This is in contrast to parallel processing, where instructions are executed at the same time, or in parallel. Field Programmable Gate Arrays (FPGAs) are capable of parallel processing, as are MCUs in which the processor has multiple cores.
MCUs are typically programmed with a high level language, such as C or Java, where the code is intuitive enough to be read and understood by a human. The grammar of high level languages is very close to the English language. Languages like C or Java are standardized and can be used with most different types of MCUs; there is no need to learn a new programming language when changing between MCUs. Once the code is written, it is compiled into the machine language that the processor can understand.
if(key1 == 1) {
led1 = 1;
} else {
led1 = 0;
}
}
The code above shows a simple example in C (turning an LED on or off depending on the status of a switch).
Some MCUs can be programmed in Assembly language, a lower level language that uses symbolic words with a one-to-one correspondence to machine language. Assembly language usually has different grammar and notation, depending on the MCU. When switching between MCUs, Assembly developers must learn the Assembly language of the new MCU.
LD B, 0x33
ADD A, B
LD (0x100), A
JP 0x9000
The code above shows an example of Assembly language.
- The first command (LD A, 0x55) tells the processor to load the data in the 0x55 address to a register A.
- The next command (LD B, 0x33) loads the data in 0x33 to register B.
- The third command (ADD A, B) adds the data in A and B together, storing the result in the first register (A).
- The fourth command (LD (0x100), A) loads the data in register A to address 0x100.
- The last command (JP 0x9000) tells the processor to jump to address 0x9000 and run the command stored at that address.
Microcontroller Architecture
This section describes the architecture of an MCU, using the Toshiba TLCS-870/C1 as an example. Other MCUs will differ in configuration and architecture; however, the design principles are generally similar.
Figure 2: Functional Block Diagram of an 8-bit Microcontroller
Figure 2 illustrates the architecture of an 8-bit MCU. The CPU core includes an arithmetic unit that performs calculations and three separate storage circuits storing the data, the read instructions, and the memory addresses of the read instructions.
The different units shown in Figure 2 are as follows:
- PSW (Program Status Word): A register that keeps track of the current operation. It holds the operation results and the state of the instruction execution result.
- General-purpose Register: A storage unit for the data. General-purpose registers are named based on their location, and have names like W, A, B, C, etc.
- Accumulator: The place that calculation results are stored. In the TLCS-870/C1, ten registers, W, A, B, C, D, E, H, L, IX, and IY, have the accumulator role.
- Program Counter (PC): The register that stores the memory address of the next instruction that should be read.
- ALU (Arithmetic Logic Unit): The arithmetic circuit that performs calculations.
- Instruction Register: A register that temporarily stores the read instructions.
- Instruction Decoder: The unit that decodes the instruction stored in the instruction register, then sends it to the control unit.
- Interrupt Control Circuit: The circuit that controls the interrupt function. An interrupt occurs when the program being executed is suspended in order to execute another one.
Each of these units performs complex operations in support of the MCU’s functions. The next section details these operations, referencing the Toshiba TLCS-870/C1.
Program Counter (PC)
The program counter (PC) is the register that manages which instruction should be executed next. The PC stores the address of the next instruction that should be executed. Generally, the address stored is the next sequential address after the address currently being executed; however, in the case of an interrupt instruction, the jump destination address is stored.
The CPU reads the instruction stored in the address in the program counter and executes it in sequence.
Figure 3: Calling the next instruction storage address (Program Counter)
Figure 3 shows that 0x8020 is stored in the PC. This means the instruction stored at address 0x8020 will be the next instruction to be executed (unless an interrupt command is received).
General-Purpose Register
General-purpose registers store the data used by the CPU, including data used for calculations, as well as calculation results. In 8-bit mode, the TLCS-870/C1 has eight registers, W, A, B, C, D, E, H, and L. These can be paired to use as 16-bit registers, the names of which would become WA, BC, DE, and HL.
| General-purpose register of TLCS-870/C1 | |
Figure 4: 8-bit General-Purpose Registers in the Toshiba TLCS-870/C1
The TLCS-870/C1 also has two 16-bit registers, IX and IY, which are mainly used as index registers when accessing the memory.
Figure 5: 16-bit Registers in the Toshiba TLCS-870/C1
The TLCS-870/C1 has two banks of the 8-bit and 16-bit general purpose registers.
Program Status Word (PSW)
The Program Status Word (PSW) stores information on the status of the execution of a program. It typically contains the value of the program counter and overflow or carry bits that indicate the state of the ALU during runtime. The PSW stores the information necessary to enable the program to continue after handling an interrupt request.
Stack
The stack is the area in the memory registers for saving program status during interrupts. When an interrupt request occurs, the PSW, PC, and various other flags are saved in the stack. After the interrupt processing finishes, the information in the stack is restored and the program continues running.
The Stack Pointer (SP) is a register that manages the stack, and indicates the address where the status information is saved. The stack functions in a first-in-last-out (FILO) manner, where the data stored first is taken out last. A good way to visualize a stack is a vertical stack of coins; the last coin placed on the stack is always the first coin to be taken.
Figure 6: FILO Method (First-In-Last-Out)
Adding data to the stack is called a “Push,” while reading data from the stack is called a “Pop.” The Stack Pointer advances when there is a Push, and returns when there is a Pop.
Interrupt Processing
When the MCU receives an interrupt request, the program being executed is suspended and another program executed. After the interrupt processing is completed, the MCU returns to the original program. There are two types of interrupts: software and hardware. A software interrupt occurs when the running program executes a dedicated instruction. Hardware interrupts occur when a signal is received either externally, from a dedicated interrupt terminal, or internally, from a peripheral circuit built into the MCU.
Interrupts can be maskable or non-maskable, depending on how the interrupt signal is received. Maskable interrupts can only execute if the CPU is set to allow interrupts. If interrupts are not allowed, the interrupt request signal is stored until it either executes once interrupts are allowed or is deleted when the interrupt signal is cancelled. Non-maskable interrupts execute even when interrupts are not allowed. If the CPU receives a non-maskable interrupt request, it will perform the processing unconditionally. Non-maskable interrupts are used for emergency processing, such as data backups or shutdown processes during power outages.
Integrated Development Environment
Software development using an MCU takes place in an integrated development environment (IDE). Many manufacturers provide an IDE custom tailored for their MCU. An IDE provides the compiler for the programming language used to write the code, an editor, a build manager, and a debugger. Many IDEs provide automated testing, enabling the creation of custom test cases; these can run automatically, creating a list of passes and fails.
5. Glossary
- Microcontroller Unit (MCU): An integrated circuit with the functionality of a simple computer, used for controlling components in electronic systems.
- Non-volatile Memory: A type of memory where the data is retained when the system is powered off.
- Volatile Memory: A type of memory where the data is lost when the system is powered off.
- RAM (Random Access Memory): A type of volatile read/write memory that stores temporary data, including calculations and working data. Because of its design, access time for data in RAM is the same irrespective of its location in the chip.
- ROM (Read Only Memory): A type of non-volatile memory that cannot easily be modified after the manufacture of the memory device. ROM is used to store firmware and other data that is rarely modified.
- Central Processing Unit (CPU): The unit that performs logic and calculations, and also executes control instructions.
- Bus Line (or System Bus): The pathway in an MCU for the rapid moving of data. The bus line is also known as the Front Side Bus, local bus, or memory bus. The bus line is made up of three busses: the data bus, the address bus, and the control bus.
- Field Programmable Gate Array (FPGA): A semiconductor IC that is designed to be easily configured or reprogrammed. FPGAs contain programmable logic blocks and reconfigurable interconnects that can always be modified after manufacturing.
- Parallel Processing: A method of computing where two or more processes can be run simultaneously.
- Assembly language: A low-level computer language that is intended to communicate directly with the CPU. Assembly language has a one-to-one relationship with machine code and can be directly translated.
- Register: A piece of memory that data can be read from or written to.
- ALU (Arithmetic Logic Unit): The component of a CPU that performs arithmetic and logic operations.
- PSW (Program Status Word): A collection of data that keeps track of the current execution state of the system.
- Program Counter (PC): A register that manages the memory address of the next instruction to be executed.
- Accumulator: A register that temporarily stores the intermediate results of calculations of arithmetic and logic data in the CPU.
- Interrupt: A request for the CPU to stop the currently executing code. Interrupts can be generated by internal or external sources.
- Maskable Interrupt: An interrupt that does not run if the CPU is not set to allow interrupts.
- Non-maskable Interrupt: An interrupt that processes even when the CPU is not set to allow interrupts.
- Stack: An area in the memory registers that stores program status data during an interrupt.
- Stack Pointer: The register that manages the stack, indicating the address where the information about the current program status is located.
- Integrated Development Environment (IDE): A software application that provides tools that simplify the writing of code. IDEs typically include an editor, build manager, and debugging tools.
*Special thanks to Toshiba for providing technical and historical background.
Toshiba TMP88FW44FG
The Toshiba TMP88FW44FG is a member of the TLCS-870/X Series of 8-bit Microcontrollers. It features 36 interrupt sources, 91 pins for I/O (24 with large current output), and low power consumption modes.
Toshiba TMPA910CRAXBG
The Toshiba TMPA910CRAXBG is a 32-bit RISC MCU from the TX09 Family. It features an ARM9
CPU core and 7-layer multi-bus system, with a maximum operating frequency of 200 MHz.
Raspberry Pi RP2040TR7
The RP2040 is the debut microcontroller from Raspberry Pi and features a Dual ARM® Cortex M0+ processor running at 133 MHz, 264kB on-chip SRAM, and 30 GPIO pins, 4 of which can be used as analog inputs.
NXP LPC822M101JDH20J
The NXP LPC82x Series is a low-cost ARM® Cortex M0+ based, 32-bit MCU family operating at CPU frequencies up to 30MHz.
Renesas R7FA6M1AD3CFM#AA0
The RA6 Series of 32-bit MCUs features ARM® Cortex-M4 core CPUs running at up to 120MHz and support for a variety of communication interfaces, including CAN, I2C, SPI, UART, and USB.
ST Microelectronics STM32L011F4P6
The STM32L011 is a 32-bit MCU that features an ARM® Cortex-M0+ core CPU, up to 16kB Flash and 2kB RAM, all of which runs on an ultra-low power platform.
Infineon XMC1402T038X0032AAXUMA1
The XMC1400 features ARM® Cortex-M0+ core CPUs and is designed for industrial applications, addressing the real-time control needs of motor control and digital power conversion. It also features peripherals for LED lighting applications and Human Machine Interfaces (HMIs).
Maxim MAX32660GWE+
The MAX32660 features the ARM® Cortex-M4 core with floating point unit (FPU). It is designed for battery-powered devices and features ultra-low power consumption in a tiny form factor.
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