A microcontroller (often abbreviated as MCU or µC) is essentially a single-chip computer. Microcontrollers share many similarities with microprocessors and virtually all microcontrollers available today have features that place them along a continuum of chips between the extremes of definitive microcontrollers and microprocessors. Nonetheless, there are a number of characteristics that commonly distinguish them.

One characteristic common to microcontrollers is finite on-chip memory (typically Flash) to store code for execution. Some microcontrollers execute code directly from this memory. However, it’s not uncommon to find microcontrollers with a small amount of RAM as well. On-chip, memory enables microcontrollers to begin executing code quickly after powering up. Although internal memories mean faster access times, memory available to a microcontroller is often limited. This limited memory is not generally a drawback however as microcontrollers are mostly used in applications lending themselves to small, purpose-built programs.

Another characteristic associated with microcontrollers is a single power rail. This simplified system means generally fewer external components are needed in microcontroller systems.

Most microcontrollers rely on serial communication buses (I2C, SPI, UART) to interface with external chips and devices. This allows for typically simpler designs and smaller PCBs. As with the single power rail, serial buses generally means fewer external discrete components are required to enable a microcontroller design.

Clock speeds most commonly associated with microcontrollers are under 200 MHz, but they work well in real time applications as they have little or no dependence on external memories[RS1] . Microcontrollers are also often architected with a focus on interrupt-based processing. They often have robust bit-level manipulation instructions further lending them to real-time, single purpose applications.

Although microcontrollers can be selected based on a number of criteria, the bus width expressed in bits in most often the starting point when selecting a microcontroller. Common options are 8-, 16-, and 32-bits. Wider buses allow for better performance in most applications.