Specific Elements of AVR Architecture

AVR is an extension of the idea presented in Vegard Wollan and Alf-Egil Bogen's thesis. Together with Gaute Myklebust, they patented the architecture, and in 1996, Atmel Norway was established as the AVR microcontroller design centre. In 2016, Microchip acquired Atmel.

AVR architecture (Fig. 1) is a popular choice for microcontrollers due to its efficiency and versatility. Here are some specific elements that define the AVR architecture:

• RISC Architecture: AVR microcontrollers are based on the RISC (Reduced Instruction Set Computing) architecture, which allows them to execute instructions in a single clock cycle. This results in high performance and low power consumption.
• Harvard Architecture: AVR utilises the Harvard architecture, which features separate memory spaces for program code and data. This separation allows simultaneous access to both memories, increasing processing speed.
• On-chip Memory: AVR microcontrollers feature built-in flash memory for storing program code, as well as SRAM and EEPROM for data storage. This integration simplifies design and improves reliability.
• General-purpose Registers: They contain 32 x 8-bit general-purpose registers, facilitating efficient data manipulation and quick access during operations.
• Interrupts: AVR microcontrollers support internal and external interrupt sources.
• Real Time Counter (RTC): Timer/Counter Type 2 (TC2) general-purpose, dual-channel, 8-bit timer/counter module. This timer/counter allows clocking from an external 32 kHz watch crystal
• I/O Capabilities: AVR microcontrollers offer various programmable I/O lines, allowing flexible interfacing with external devices and peripherals.
• Watchdog Timer: A programmable watchdog timer enhances system reliability by resetting the microcontroller in the event of software failure.
• Clock Oscillator: The built-in RC oscillator provides the necessary clock signals for the microcontroller's operation.

• Interrupt Unit - Handles asynchronous events by temporarily halting the main program flow to execute interrupt service routines.
• SPI Unit - Implements the Serial Peripheral Interface protocol for high-speed synchronous data exchange with external devices.
• Watchdog Timer - A fail-safe timer that resets the system if the software becomes unresponsive or enters an infinite loop.
• Analogue Comparator - Compares two analogue input voltages and outputs a digital signal based on their relative magnitude.
• I/O Module - Interfaces with external peripherals, enabling digital input and output operations.
• Flash Program Memory - Non-volatile memory used to store the program code, retaining data even when power is lost.
• Instruction Register - Temporarily holds the current instruction fetched from program memory before decoding.
• Instruction Decoder - Translates the binary instruction into control signals that direct the operation of internal units.
• Control Lines - Carry control signals that coordinate data flow and operations across the microcontroller.
• Program Counter - Keeps track of the address of the next instruction to be executed, enabling sequential program flow.
• Status and Control - Stores flags and control bits that reflect the current state of the processor and influence execution.
• 32×8 General Purpose Register - A set of 32 registers, each 8 bits wide, used for temporary data storage and arithmetic operations.
• ALU (Arithmetic Logic Unit) - Performs arithmetic and logical operations on data from the general-purpose registers.
• Data SRAM - Volatile memory used for runtime data storage, such as variables and stack operations.
• EEPROM - Electrically erasable non-volatile memory for storing user data that must persist across power cycles.
• I/O Lines - Physical pins that connect the microcontroller to external circuits for input and output tasks.
• Data Bus - An 8-bit pathway that transfers data between internal components, enabling communication and processing.

There are other popular microcontrollers available on the market, which are the main competitors to AVR. We depict some of them shortly.

PIC

• Manufacturer: Microchip Technology
• Architecture: RISC
• PIC16: 8-bit microcontrollers, various versions with different peripheral sets, low power consumption
• PIC18: 8-bit microcontrollers with advanced features such as CAN FD
• PIC32: 32-bit microcontrollers, high performance, wide range of applications

ESP8266/ESP32

• Manufacturer: Espressif Systems
• Architecture: Tensilica Xtensa (extended version based on RISC)
• ESP8266: Single-core processor, 80 MHz (overclockable to 160 MHz), 32 KB RAM, 80 KB user RAM, Wi-Fi
• ESP32: Dual-core processor, 240 MHz, 520 KB SRAM, Wi-Fi, Bluetooth, more GPIO

TI MSP430

• Manufacturer: Texas Instruments
• Architecture: RISC
• MSP430: 16-bit microcontrollers, ultra-low power consumption, advanced analogue peripherals, fast switching between modes