Microcontrollers Overview

Microcontrollers (MCUMicrocontroller Unit) are designed for monitoring and/or control tasks that can operate as a stand-alone device or can be interfaced to another device such as a computer. Microcontrollers are a self contained micro-processing system that includes memory (RAMRandom Access Memory, Flash), timers, interrupts, ADCAnalog-to-Digital Converter (ADC, A/D, or A-to-D), and I/OInput/Output ports. This integrated system allows programming the MCUMicrocontroller Unit to control interfacing with external electronic devices.

This overview of microcontrollers covers its usage, MCU components, CPU, Memory, I/O Ports, Timers, Interrupts, and the System Bus.

Microcontrollers come packaged in stand-alone single ICIntegrated Circuit chips (e.g., Atmega328P, RP2040) where the number of pins, size, structure, and architecture vary depending on the model. Microcontrollers are also available in development boards (e.g., Arduino Uno Rev3, Raspberry Pi Pico) with the MCUMicrocontroller Unit built onto a single PCBPrinted Circuit Board along with the power conditioning circuit, reset, clock circuit, USBUniversal Serial Bus chip and connector, and I/OInput/Output connectors. Development boards utilize the MCUMicrocontroller Unit in a way that is easy to setup and interface to: plugging in power, USBUniversal Serial Bus to computer connection, loading software, and connecting I/OInput/Output.

Usage

You will find microcontrollers used in all kinds of devices these days from consumer products to electronic test equipment. Any device that measures, stores, controls, and/or displays data will probably have a microcontroller in it. Microcontrollers are programmable, small in size, and consume relatively low power, making them an ideal choice for many applications.

Suppose you want to periodically measure the temperature of a room and capture that data on your computer. A microcontroller can be programmed to read in the measured data from a temperature sensor at specific times and transfer the data (wired/wireless) to your computer. In general, a microcontroller can be setup to read in digital or analog signals in real-time and process, store, and transfer the data to other devices such as a computer and/or LCDLiquid Crystal Display screen for visualizing the data.

Microcontrollers are not suitable for all applications. For example, microcontrollers do not have the processing power for video streaming with real-time compression and transmission: they don't have enough speed, memory, and bandwidth. A Single Board Computer (SBCSingle Board Computer), like a Raspberry Pi or BeagleBone, is more suitable for video applications.

MCU Components

The basic structure of a microcontroller with its main internal components is shown in the figure below.

CPUCentral Processing Unit:

The CPUCentral Processing Unit is the main processor of the microcontroller responsible for executing instructions from programs stored in memory, data transfer operations, and coordinates activities with the peripherals. The CPUCentral Processing Unit reads, decodes, and executing instructions to perform Arithmetic, Logic, and Data Transfer operations.

Memory:

Used to store program code and data. The program memory is used to store instructions the CPUCentral Processing Unit executes. Data memory is required to store data while executing the instructions.

I/OInput/Output Ports:

Used to interface with external devices. Input devices, like switches and sensors transfer their signals through the inputs ports to provide information to the CPUCentral Processing Unit in the form of binary data. The output ports receive data from the CPUCentral Processing Unit and provides the appropriate response to external devices like LEDLight Emitting Diodes and LCDLiquid Crystal Displays.

Serial Ports:

Used for serial communication with external devices (e.g., UARTUniversal Asynchronous Receiver-Transmitter, I2CInter-Integrated Circuit. Also referred to as IIC or I²C., SPISerial Peripheral Interface).

ADCAnalog-to-Digital Converter (ADC, A/D, or A-to-D):

Input for converting an analog signal to digital. Many sensors are analog devices and their signal must be converted to digital data for the CPUCentral Processing Unit.

Timers and Counters:

Used in timing external events, delays, modulation, clock functions, frequency measurements, sampling ADCAnalog-to-Digital Converter (ADC, A/D, or A-to-D) data, I/OInput/Output timing, serial communication, etc. Timers increment on the CPUCentral Processing Unit clock pulse, whereas counters increment on external pulses.

Interrupt Control:

Used to provide an interrupt for a working program based on events (internal/external, hardware/software). Interrupts cause the processor to immediately respond to the event by executing a specific set of instructions.

System Bus:

A bus is a collection of parallel conductors that have a similar function. The system bus in a microcontroller connects the CPUCentral Processing Unit with other peripherals (memory, I/OInput/Output ports, timers, etc.).

Clock Generator:

Governs the speed at which the processor executes instructions and the synchronization of commands and data. The clock source can be either internal or external (e.g., crystal).

The following sections will cover each of these components in more detail.

CPU

The Central Processing Unit (CPUCentral Processing Unit) in a microcontroller is a complex circuit whose primary function is to execute programs that are stored in permanent non-volatile memory. The programs are created by the user on a PCPersonal Computer and downloaded onto the MCUMicrocontroller Units memory so it becomes a stand-alone processing system.

After the program is installed and the microcontroller is reset, it will fetch the program instructions from memory, decode its contents, and execute the instructions (usually repeatedly in a loop). The CPUCentral Processing Unit will then call upon the different components (I/OInput/Output, store/retrieve data from memory, set timers, serial communication, ADCAnalog-to-Digital Converter (ADC, A/D, or A-to-D), etc.) to execute their tasks according to the instructions in the program.

The CPUCentral Processing Unit performs the following operations.

• Fetch Instructions: the CPUCentral Processing Unit reads instructions from memory
• Interpret Instructions: the instruction is decoded. The decoder also restores the encoded instructions to their original form.
• Fetch Data: needed if the instruction requires reading data from memory or I/OInput/Output
• Process Data: performing arithmetic and/or logical operations in the ALUArithmetic Logic Unit
• Write Data: if the instructions include writing data to memory of I/OInput/Output

The figure below shows typical components commonly found in a microcontrollers CPUCentral Processing Unit.

Arithmetic and Logic Unit (ALUArithmetic Logic Unit):

The ALUArithmetic Logic Unit performs arithmetic (addition, multiplication, etc.) and logical operations (AND, OR, etc.) within the processor. The ALUArithmetic Logic Unit takes two inputs and returns the result of the operation as its output along with some info about the result in the status register (result is zero, result is a negative value, operation produced an overflow/carry, etc.).

Data Path:

The data path executes instructions. It consists of the ALUArithmetic Logic Unit, set of registers, and CPUCentral Processing Unit's internal buses that allow data to flow between them.

Control Unit:

The control unit directs the operation of the processor by telling the data path what to do. It determines which operation in the program instructions should be executed next and to configure the data path accordingly.

CPU Internal Registers:

The CPUCentral Processing Unit contains a small internal storage area of internal registers for transient information. They are used for storing the values (operands) and results of arithmetic and logic operations by the ALUArithmetic Logic Unit and can keep track of where the next instruction is supposed to come from memory. This reduces the number of reads/writes to external memory which is slower. Memory contains a large amount of data and it takes time to access them by the bus. Registers are a quick source of data within the ALUArithmetic Logic Unit that can be directly accessed, which significantly improves the speed of the operations.

Memory

Microcontrollers usually contain volatile memory, such as RAMRandom Access Memory, and non-volatile memory, such as Flash and/or EEPROMElectrically Erasable Programmable Read-Only Memory. The contents of volatile memory are lost if the microcontroller loses power, whereas the contents of non-volatile memory persists between power cycles. A description of the different types of memory in microcontrollers is given below.

RAMRandom Access Memory or SRAMStatic Random Access Memory:

Volatile memory that is the working data space for storing transient data, such as variables, the heap, and the stack, and can be written to and read from during program execution.

Flash:

Non-volatile memory used to store programs (code) and also any constant data values. It can be read, erased, and written to.

EEPROMElectrically Erasable Programmable Read-Only Memory:

Non-volatile memory to hold data that needs to persist between software changes and power cycles. It can be erased and written to, making it useful for logging system malfunctions and fault data during program execution. It is also useful for storing special data or configuration data that must be retained during a power failure but might need to be changed periodically.

In most microcontrollers, RAM is usually smaller in size than flash memory because it costs more. RAM has lower latency (faster read/write) than flash memory. Some microcontrollers, like the RPiRaspberry Pi Pico, do not have EEPROM and rely entirely on Flash memory to store permanent data. While other microcontrollers, like the Arduino Uno Rev3, use the Flash memory to store programs and rely on EEPROM for storing permanent data.

ATmega328P Memory in the Arduino Uno Rev3, Nano, and Mini

The Arduino Uno Rev3, Nano, and Mini ATmega328P microcontrollers have 32KB (0.5KB is used for Bootloader) of flash memory, 2KB of SRAM, and 1KB of EEPROM.

The ATmega328P EEPROM is not only scarce with a limit of 1024 bytes, but also has the limitation that it can only save one byte: you can only store 8-bit numbers, which includes integer values between 0 and 255.

You might think to just use the 32KB Flash memory instead EEPROM to write your data to. Normally, the ATmega328P only allows you to write to flash memory when when it's in bootloader mode. The standard bootloader allows you to write to flash when a new program is being installed, but does not allow runtime programs to write to flash. However, there is a hack to the bootloader that does allow you to run functions that enable you to write to flash (check out this Hackaday post on the subject).

RP2040 Memory in the RPi Pico

The RPiRaspberry Pi Pico RP2040 microcontroller has 264KB of SRAM and 2MB of QSPIQuad Serial Peripheral Interface (QSPI) is a serial communication interface designed for talking to flash chips by using 4 data lines. Flash. The Pico does not have a separate EEPROMElectrically Erasable Programmable Read-Only Memory non-volatile memory like the Arduino. For its flash memory, QSPIQuad Serial Peripheral Interface (QSPI) is a serial communication interface designed for talking to flash chips by using 4 data lines. is faster than traditional SPISerial Peripheral Interface because it uses 4 data lines (I0, I1, I2, and I3) as opposed to just 2 data lines (MOSIMaster Out Slave In (MOSI) is the SPI data output line from the master device and MISOMaster In Slave Out (MISO) is the SPI data output line from the slave device) on the traditional SPISerial Peripheral Interface.

Note that the RP2040 datasheet states that it has 264KB of internal RAMRandom Access Memory, but when you check the RAMRandom Access Memory availability using commands (mem_info) on the RPiRaspberry Pi Pico, it outputs only about 192KB of RAMRandom Access Memory available for use. This is because some of the memory is used by the Python interpreter, stack, buffers, etc.

Besides the SRAMStatic Random Access Memory main memory, there are two other dedicated RAMRandom Access Memory blocks that may be used in some circumstances:

• If flash XIPExecute In Place (XIP) is a method of executing programs directly from long-term storage rather than copying it into RAM caching is disabled, the cache becomes available as a 16KB memory
• If the USBUniversal Serial Bus is not used, the USBUniversal Serial Bus data DPRAMDual-Ported RAM (DPRAM) is a type of random-access memory that allows multiple reads or writes to occur at the same time, or nearly the same time, unlike single-ported RAM which allows only one access at a time. can be used as a 4KB memory

The RP2040 also has 16KB of ROMRead-Only Memory, but this is used for the flash boot sequence, flash programming routines, USBUniversal Serial Bus mass storage device with UF2UF2 is a firmware file format developed by Microsoft that allows for flashing microcontrollers over USB. It appears as a Mass Storage Device (USB Drive) when attached to your computer, so you can copy firmware to a USB drive instead of having to burn it using other tools. support, and utility libraries such as fast floating point.

MCU Internal Memory Comparison

The table below provides a comparison of internal memory (RAM, Flash, and EEPROM) for different microcontrollers. Some microcontrollers, such as the RP2040, ESP8266, and ESP32, support external memory with an SPISerial Peripheral Interface Flash chip (2MB/4MB/16MB).

I/O Ports

The I/OInput/Output ports on a microcontroller are used to send and receive data to the outside world. These ports have registers that are connected to the CPUCentral Processing Unit using the data bus and to the outside world with physical pins on the side of the microcontroller. Three registers control the behavior of the pins.

Data Direction Register (DDR):

The input/output function of the pin is configured by a bit in the DDR. Each I/OInput/Output port has its own DDR that contains one bit for each pin of the port. Each pin of a port can be configured differently.

Port Register (PORT):

Controls the high/low voltage level of the output pins.

Port Input Register (PIN):

The PIN register contains the current state (high/low) of a pin, whether the pin is configured as an input or output.

Often the I/OInput/Output ports have alternative functions (serial communication, ADCAnalog-to-Digital Converter (ADC, A/D, or A-to-D), etc.) to reduce the number of pins on the microcontroller and the size of the chip.

Serial Ports

Microcontrollers usually contain several pins for serial communication with other devices, often with multiple instances of a particular interface with the purpose of communicating with more than one device.

Serial communication sends data sequentially, one bit at a time, where synchronization between the transmitter and receiver must be maintained to allow both to be expecting transmission/reception at the same time. There are two different ways of maintaining synchronization.

Synchronous Communication:

Has data sent in a continuous stream using a separate line with a pulsed clock signal for synchronization. Data bits are sent and received on the rising or falling edge of the common clock's signal. Examples of synchronous communication are I2CInter-Integrated Circuit. Also referred to as IIC or I²C. and SPISerial Peripheral Interface.

Asynchronous Communication:

There is no clock line. Data is sent in the form of packets with start and stop bits added, with both the transmitter and receiver configured to the same baud rate. Data can be sent intermittently rather than a steady stream. Examples of asynchronous communication are UARTUniversal Asynchronous Receiver-Transmitter and RS232.

There are three different transmission modes.

Simplex Mode:

The communication is unidirectional one-way where only one of the two devices can transmit and the other can only receive. Examples that use the simplex mode of transferring data are a keyboard, mouse, and monitor.

Half-Duplex Mode:

each device can transmit or receive, but not at the same time. When one device is sending, the other device must be receiving. An example that uses half-duplex mode is a Walkie-Talkie where two parties take turns speaking.

Full-Duplex Mode:

Each device can transmit and receive at the same time. An example using full-duplex mode is a telephone line where both parties can talk at the same time.

The transmission speed is characterized by the baud rate.

Baud Rate:

Data transmission rates are typically specified as baud, which is the symbol rate. If the system has only 2 symbols (0 and 1) transferred, then the baud is equivalent to bits per second (bpsbits per second) (e.g., 9600 baud means that data is being transferred at 9600 bits per second).

UART

The Universal Asynchronous Receiver Transmitter (UARTUniversal Asynchronous Receiver-Transmitter) uses two data lines, one to transmit (TXTransmit) and another to receive (RXReceive), for half-duplex or full-duplex communication.

TXTransmit:

output transmitting data

RXReceive:

input receiving data

Since UARTUniversal Asynchronous Receiver-Transmitter is asynchronous and has no clock signal line, the transmission data must have bits that indicate the start and end of the message. Data is sent over the bus in the form of a packet, which consists of a start bit, data frame (5 to 9 bits), a parity bit, and stop bits. Also, both devices must be configured to operate at the same baud rate. The user must select a baud rate from a set of possible rates on the devices because they depend on the system clock, but are generally within the range of 9600bpsbits per second and 115,200bpsbits per second.

I2C

Inter-Integrated Circuit (I2CInter-Integrated Circuit. Also referred to as IIC or I²C.) is a synchronous bus that uses two wires, Serial Clock Line (SCLSerial Clock (SCL) is the output clock signal line from the master device. Also referred to as SCK, SCLK, or CLK.) and Serial Data Line (SDASerial Data Line), for half-duplex communication over a short distance. I2C is also referred to as a Two-Wire Interface (TWITwo-Wire Interface). It operates on the master-slave principle with the microcontroller as the master and peripheral devices as the slaves.

SDA (Serial Data Line):

used by the master or slave to transmit/receive data

SCL (Serial Line Clock):

used by the master to transmit the clock signal

Both the SDA and SCL lines have pull-up resistors (R_PU) that are pulled up to VDDVoltage Drain Drain (VDD) is the supply voltage at the Drain of a transistor. The double subscript notation of repeating letters "DD" is used to denote a power supply voltage that is relative to ground. , the logic level supply of the MCUMicrocontroller Unit and device. The I2C bus drivers are open drain drivers, which means they can drive drive their output low, but cannot drive it high. For the line to be able to go high there has to be pull-up resistors to VDD. Some MCUs and devices already have pull up resistors within them for the SDA and SCL pins, where external pull up resistors are not needed.

The original I2C bus had a maximum speed of 100kHz, now called the standard mode. Most applications still use this speed, since it is usually sufficient for transferring data with most modules, such as sensors, memory modules, and LCDLiquid Crystal Display displays. The I2C protocol specifies this standard mode and higher speed modes, as given in the table below, but not all I2C devices support these modes.

I2C Modes

Mode	Speed
Standard Mode (Sm)	100kbps
Fast Mode (Fm)	400kbps
Fast Mode Plus (Fm+)	1Mbsp
High-Speed Mode (Hs)	3.4Mbps
Ultra-Fast Mode (UFm)	5Mbps

One of the main advantages of I2C is the capability of adding as many peripheral devices as you want to the bus (SDA and SCL lines) as long as the maximum bus capacitance of 400 pF is not exceeded. This is illustrated in the figure below.

SPI

The Serial Peripheral Interface (SPISerial Peripheral Interface) provides synchronous full-duplex communication on the master-slave principle using four lines.

MOSI (Master Out, Slave In):

used by the slave to transmit data to the master

MISO (Master In, Slave Out):

used by the master to transmit the clock signal

SCLK (System Clock):

used by the master to transmit the clock signal. Also referred to as SCK, SCL or CLK.

SS (Slave Select):

used by the master to select a slave

SPI operates on the master-slave principle with the microcontroller as the master and peripheral devices as the slaves. Two different wiring configuration can be used for multiple slave devices: the independent slave configuration and daisy chain slave configuration.

In the independent slave configuration, the master uses different SSSlave Select (SS) is the SPI output line from the master device to indicate that data is being sent. Also referred to as Chip Select (CS). pins (using GPIOGeneral Purpose Input Output output pins on a microcontroller) for each slave device shown in the figure below. When the master wants to communicate with a particular slave, it pulls its SSSlave Select (SS) is the SPI output line from the master device to indicate that data is being sent. Also referred to as Chip Select (CS). pin LOW during the duration of the communication exchange.

In the daisy chain slave configuration, the master uses only one SSSlave Select (SS) is the SPI output line from the master device to indicate that data is being sent. Also referred to as Chip Select (CS). pin (a GPIOGeneral Purpose Input Output output pins on a microcontroller) connected to all the slave devices shown in the figure below. When the master wants to communicate with a particular slave, it pulls its SSSlave Select (SS) is the SPI output line from the master device to indicate that data is being sent. Also referred to as Chip Select (CS). pin LOW to initiate communication, sends data over its MOSIMaster Out Slave In (MOSI) is the SPI data output line from the master device to the first slave device in the chain, which then flows from one slave to the next in the chain.

SPI is the fastest protocol compared to UARTUniversal Asynchronous Receiver-Transmitter and I2CInter-Integrated Circuit. Also referred to as IIC or I²C.. Communication speeds depend on the clock frequency of the microcontroller master device using a frequency supported by slaves devices, which are typically up to a few MHz.

Analog I/O

Analog Input

Most microcontrollers come with an Analog-To-Digital Converter (ADCAnalog-to-Digital Converter (ADC, A/D, or A-to-D)) to read in a continuous analog signal from an input pin and translate it into a digital number proportional to the signal. Many sensors have an analog voltage output related to the physical property being sensed. For example, a TMP36 temperature sensor produces an output voltage linearly related to the ambient temperature.

Most microcontrollers include an internal ADC with a subset of GPIOGeneral Purpose Input Output pins that you can use to interface with the ADC. The input pins are typically labeled with an 'A' in front of their label. For example, the analog input pins for the Arduino Uno R3 are labeled A0 through A5, the Arduino Nano pins are labeled A0 through A7, and the RPi Pico pins are labeled ADC0 through ADC2.

The most common ADC in MCUMicrocontroller Units is a SARSuccessive Approximation Register where an internal sample and hold (S/H) capacitor is charged to the input voltage (sample) and then disconnected (hold) to measure the time in number of clock cycles it takes to discharge across an internal resistor and zoom in on the correct ADC output.

The resolution of MCU ADCs typically range from 8-bit to 12-bit. For instance, the Arduino Uno R3 and Nano has a 10-bit ADC, whereas the RPi Pico ADC is 12-bit. Both of these MCUs will be covered below.

ATmega328P ADC in Arduino Uno Rev3 and Nano

The ATmega328P MCU used in the Arduino Uno R3 and Nano has a built in 10-bit ADC. The specs for this ADC from ATmega328P Datasheet (PDF) are given in the table below.

ATmega328P ADC Specs

Parameter	Spec
Voltage Supply (Vs)	1.8V to 5.5V
Resolution	10-Bit
Number of Channels	8 single-ended inputs
Sampling frequency @ 5V operation and 125KHz ADCAnalog-to-Digital Converter (ADC, A/D, or A-to-D) sampling clock	9.6kHz
Sampling frequency @ 5V operation and 200KHz ADCAnalog-to-Digital Converter (ADC, A/D, or A-to-D) sampling clock	15kHz
Conversion time	13 ADCAnalog-to-Digital Converter (ADC, A/D, or A-to-D) clock cycles, 65μs to 260μs
ADCAnalog-to-Digital Converter (ADC, A/D, or A-to-D) clock range for good results	50kHz to 200kHz
Absolute Accuracy (INLIntegral Non-Linearity + DNLDifferential Non-Linearity + quantization error + gain & offset error)	2 LSBThe Least Significant Bit (LSB) in a binary number is the bit that represents the smallest place value carrying the lowest weight. It is also sometimes referred to as the low-order bit or right-most bit due to the convention in positional notation of writing less significant digits further to the right.

Note that although the ATmega328P ADC has 8 channels, the Arduino Uno R3 only makes use of 6 channels (A0 - A5) to interface with, but the Arduino Nano board makes use of all the channels (A0 - A7).

RP2040 ADC in RPi Pico

The RPi Pico has internal 12-bit ADC with 4 channels and a sampling rate up to 500kspsKilo Samples Per Second (1000 samples/s) (using an independent 48MHz clock). The ADC specs and block diagram provided below are from the RP2040 Datasheet (PDF).

RP2040 ADC Specs

Parameter	Spec
Voltage Supply (Vs)	1.8V to 3.3V
Resolution	12-Bit with 8.7 ENOBEffective Number Of Bits is a measure of the dynamic range of an ADC
Number of Channels	4
Sampling Frequency	500kSPSKilo Samples Per Second (1000 samples/s)
Conversion time	96 clock cycles, 2μs
ADCAnalog-to-Digital Converter (ADC, A/D, or A-to-D) clock	48MHz
DNLDifferential Non-Linearity	8.9 LSBThe Least Significant Bit (LSB) in a binary number is the bit that represents the smallest place value carrying the lowest weight. It is also sometimes referred to as the low-order bit or right-most bit due to the convention in positional notation of writing less significant digits further to the right.
INLIntegral Non-Linearity	7.85 LSBThe Least Significant Bit (LSB) in a binary number is the bit that represents the smallest place value carrying the lowest weight. It is also sometimes referred to as the low-order bit or right-most bit due to the convention in positional notation of writing less significant digits further to the right.

MCU ADC Comparison

The table below provides a comparison of the different ADCs for common microcontrollers. The number of channels (nCH) listed in the table is the maximum the MCU provides, but some microcontroller boards may not break out of all the channels into pins or may use some of the channels for a different purpose (temperature sensor, system voltage, etc.).

MCU ADCs

MCU	Bits	nCH
ATmega328P	10-Bit	8
ATmega32U4	10-Bit	12
ATtiny85	10-Bit	4
ESP8266	10-Bit	1
ESP32	12-Bit	18
RP2040	12-Bit	4
SAMD21	12-Bit	20
SAMD51	12-Bit	16
STM32F103 (Blue Pill)	12-Bit	10
STM32F411 (Black Pill)	12-Bit	16
MIMXRT1062 ARM Cortex-M7 (Teensy 4.0/4.1)	12-Bit	20

Timers and Counters

One of the most useful features of a microcontroller is the capability of performing time related tasks. Microcontrollers can be setup to read inputs at specific times, switch an external device on/off at specific times, or generate pulsed output at the right frequencies and pulse width to control the speed of motors.

Timers and counters are used in timing external events, delays in a program or hardware components, clock functions, frequency measurements, sampling ADCAnalog-to-Digital Converter (ADC, A/D, or A-to-D) data, general I/OInput/Output timing and Pulse Width Modulation (PWMPulse-Width Modulation), serial communication timing, etc.

Timers are basically counters that can either increment/decrement on CPUCentral Processing Unit clock pulses for timing events or on external pulses for counting events (the direction up/down is either fixed or configurable). The goal of using a timer in a microcontroller is twofold: (1) detector and/or capture external timing event and (2) generate timing events to control external devices.

Timer Resolution

The timer resolution is specified by b-bits, where the count value is in the range [0, 2^b-1]. For example, the ATmega328P MCUMicrocontroller Unit in the Arduino Uno R3 has three timer/counters called Timer0, Timer1, and Timer2. These timers have two different resolutions: Timer0 and Timer2 are 8-bit timers, whereas Timer1 is a 16-bit timer. The 8-bits means 2⁸ = 256 values from 0 to 255, whereas 16-bit means 2¹⁶ = 65536 values from 0 - 65535 for higher resolution.

Once a counter reaches its maximum value, it will tick back to zero (this is called overflow). Timers generally raise an interrupt whenever they experience an overflow and a status flag is set to notify the processor of this event so rollover events may be counted.

The timer module usually provides mode bits which allow the user to select prescaler values (1, 8, 64, 256, 1024, ...) to scale down the input clock frequency and slow down the counter rate to measure a longer duration.

The prescaler effectively extends the range of the timer, but comes at the cost of coarser time step resolution.

Detecting External Events

Once the input timer is configured with the appropriate prescaler, a method for detecting the event to needs to be chosen to know when to increment the counter. There are two main methods for detecting input events: polling and interrupts.

Polling is where the microcontroller constantly polls a flag for a logic high signal when an event occurs on the physical pin. A more efficient use of resources is using interrupts to detect the event, which allow a microcontroller to perform other tasks until the interrupt event occurs. Most microcontrollers are equipped with built-in interrupt systems with their timer input modules.

Interrupts

Interrupts allow the microcontroller take over the flow of a program to respond to a higher priority event where a state change is detected. When an interrupt occurs, a signal is sent to the CPUCentral Processing Unit to begin executing different code that is written to respond to the interrupt after the end of the current instruction.

The code that executes for an interrupt is called the Interrupt Service Routine (ISRInterrupt Service Routine) that is programmed by the user to take specific actions. Once the ISR is complete, the program will resume where it left off.

There are usually different interrupts available on microcontrollers. Two main interrupts that are often used are the following.

Real Time Interrupt (RTIReal Time Interrupt):

Provides a regular, periodic interruption in the main program flow. This is useful for updating a real time clock or checking the battery level.

External Interrupt Request (IRQInterrupt Request (IRQ) pins are capable of receiving an external signal to interrupt a running program and alter its behavior.):

An external controller hardware pin that responds to voltage level state changes. When a state change occurs on the pin an interrupt is generated and the associated interrupt service routine is executed.

Microcontrollers usually have multiple sources of interrupts, like the input pins going high, timer overflow, ADC completion, etc.

System Bus

The system bus is a group of parallel wires that connect the CPUCentral Processing Unit with other peripherals (memory, I/OInput/Output ports, timers, etc.). Most microcontrollers are equipped with an address bus, a data bus, and a control bus. Together, these three make up the system bus.

Address Bus:

The address bus carries only the information regarding the address of data, the physical location of the data in memory, that the processor will read data from or write data to. The addressing signals pass from the CPUCentral Processing Unit to memory, I/OInput/Output (or peripherals), and other addressable devices around the processor.

Data Bus:

The data bus routes data to/from different peripherals within the microcontroller. Data moving in and out of the data bus is bi-directional, since the processor reads and writes data. The data bus "width" is the MCUMicrocontroller Unit bit size, which is typically 8-bits, 16-bits, 32-bits, or 64-bits.

Control Bus:

The control bus manages the information flow between components indicating whether the operation is a read or a write and ensuring that the operation happens at the right time.

Selecting A Microcontroller

Choosing a microcontroller depends on your application needs. There are plenty of options to choose from with different bit sizes, clock speeds, multiple CPUCentral Processing Unit cores, memory, operating voltage, power consumption, number of I/OInput/Output pins, interfaces for communication, and package size.

In addition to the microcontroller features, an important factor to consider is the amount of documentation and other resources available (tutorials, forums) to understand the device and troubleshoot any problems that occur.

Here are some common microcontroller board selections for different use cases.

• For something simple or for prototyping, the Arduino Uno R3 with an ATmega328P MCU is a good choice.
• If you need a mid range microcontroller, then the RP2040 in RPiRaspberry Pi Pico, SAMD21, or blue/black pill STM32 will do.
• If you need more processing power, the Teensy 4.0/4.1 or higher end STM32's are the fastest.
• For WiFi applications an ESP8266, ESP32, RPiRaspberry Pi Pico W, or Arduino Nano 33 IoT will provide network connectivity.
• For small form factor, the ATtiny85, Tiny2040, Seeed Studio XIAO, Adafruit QT Py, or DFRobot Beetle boards are about the size of a postage stamp or smaller.

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Arduino Uno R3

Created:   07Apr2023   18:18:45   UTC 2023-04-07T18:18:45Z
Updated:   18Aug2024   02:33:48   UTC 2024-08-18T02:33:48Z
Rating:  (0 reviewsThis article has not been rated yet)

The Arduino Uno R3 is a microcontroller board based on the Atmega238P MCU that is designed for prototyping. This is a hardware overview of the board that covers:

• Specs
• Board Layout
• Different ways of powering the board and output power
• Digital I/O, Analog Inputs, and Communication Interfaces (UART/I2C/SPI)

Arduino Nano

Created:   20Nov2023   19:40:22   UTC 2023-11-20T19:40:22Z
Updated:   18Aug2024   04:04:37   UTC 2024-08-18T04:04:37Z
Rating:  (0 reviewsThis article has not been rated yet)

The Arduino Nano is a small breadboard-friendly microcontroller board based on the ATmega328 MCU. This is a hardware overview of the board that covers:

• Specs
• Board Layout
• Different ways of powering the board and output power
• Digital I/O, Analog Inputs, and Communication Interfaces (UART/I2C/SPI)
• Accessories

Raspberry Pi Pico

Created:   09Apr2023   02:58:19   UTC 2023-04-09T02:58:19Z
Updated:   18Aug2024   08:09:36   UTC 2024-08-18T08:09:36Z
Rating:  (0 reviewsThis article has not been rated yet)

Overview of the Raspberry Pi Pico microcontroller board that covers specs, board layout, and power.

Products

Microcontroller Board Products

ATmega328P

ATmega32U4

RP2040

SAMD21

SAMD51/SAME51

STM32F103 (Blue Pill)

STM32F401/405/411 (Black Pill)

ATtiny85

Prototyping Microcontroller Board Products

Uno R3 ATmega328P

Leonardo ATmega32U4

Adafruit M0 Express SAMD21

SparkFun Redboard Turbo SAMD21

Adafruit Metro M4 SAMD51

STM32 NUCLEO-F103RB

STM32 NUCLEO-F401RE

STM32 NUCLEO-F411RE

Compact Microcontroller Board Products

Nano ATmega328P

Pro Mini ATmega328P (5V 16MHz)

Pro Mini ATmega328P (3.3V 8MHz)

Adafruit ItsyBitsy ATmega32U4 (5V 16MHz)

Adafruit ItsyBitsy ATmega32U4 (3.3V 8MHz)

DFRobot Beetle ATmega32U4

RPi Pico RP2040

SparkFun Pro Micro RP2040

Adafruit QT Py RP2040

Pimoroni Tiny RP2040

Seeed Studio XIAO RP2040

Waveshare RP2040 Zero

Adafruit QT Py SAMD21

Adafruit Trinket M0 SAMD21

Adafruit ItsyBitsy M0 SAMD21

SparkFun Mini SAMD21

Seeed Studio XIAO SAMD21

Adafruit ItsyBitsy M4 Express SAMD51

STM32F103 Blue Pill

STM32F401 Green Pill

STM32F401 Black Pill

STM32F411 Black Pill

Adafruit Trinket ATtiny85 5V

Adafruit Trinket ATtiny85 3.3V

Digispark ATtiny85 (USB Type-A)

Digispark ATtiny85 (USB Micro-B)

WiFi Microcontroller Dev Board Products

ESP8266 NodeMCU

ESP8266 D1 Mini

Adafruit ESP8266 Feather Huzzah

DFRobot ESP8266 FireBeetle

ESP32-WROOM-32 NodeMCU

ESP32-WROOM-32U NodeMCU

ESP32-WROOM-32 D1 Mini

Arduino Nano 33 IoT

Arduino Nano RP2040 Connect

Arduino Nano ESP32

SparkFun ESP32 Thing

SparkFun Thing Plus ESP32 Micro-B

SparkFun Thing Plus ESP32 uFL

SparkFun Thing Plus ESP32 USB-C

Adafruit HUZZAH32 ESP32 Feather

DFRobot FireBeetle ESP32

Seeed Studio XIAO ESP32-C3

Seeed Studio XIAO ESP32-S3

SparkFun Qwiic Pocket ESP32-C6

RPi Pico W

Adafruit Feather M0ATWINC1500 (PCB ANT)

Adafruit Feather M0ATWINC1500 (ufl)

SparkFun Thing Plus DA16200