The architecture of a microcontroller defines its internal structure, the instruction set, the size of the data buses and registers, and the memory model. The instruction set contains the instructions that the processor can execute. The architecture also determines the support for peripherals and other embedded functions. Modern Intel and AMD based computers use x86-64 (x64 for short) architecture.
The simpler Von Neumann model uses shared memory to handle both data and instructions, so they can be accessed via a single bus system. This means more flexible memory usage and cheaper hardware, but also potentially slower operation. The Harvard model stores data and instructions in separate memory, so using two bus systems can make the processor faster.
Previously manufactured by Atmel, AVRs have been produced by Microchip since its acquisition in 2016. They use a reduced instruction set (RISC) and a modified Harvard architecture. The general-purpose registers are 8-bit, making them suitable for simpler tasks in embedded systems. An AVR microcontroller is used in the popular Arduino Uno R3.
ARM Ltd does not manufacture processors, but licenses its developments to manufacturers. Smartphones and modern Apple computers also contain ARM architecture processors. They also have a very large market share in microcontrollers, with Reneas, STMicroelectronics, Raspberry Pi and countless manufacturers using the ARM Cortex-M family. These are all 32-bit processors with varying memory models.
Unlike the instruction set architectures (ISA) used by ARM, Intel or AMD, RISC-V is an open standard. It is a modular, open source, customisable architecture. Its popularity is growing year on year, with the increasing adoption of RISC-V processors for smart devices, servers, and software support is also evolving.
A popular product from Esppressif Systems is the ESP32 family of microcontrollers. As the name suggests, it uses 32-bit cores. Some of their products use licensed processors from the ARM-like company Tensilica, while others contain RISC-V CPUs. Compatibility between the different models is maintained by ESP-IDF, Espressif's SDK, which can be integrated into Arduino's development environment or VS Code.