In contrast to general-purpose CPUs, microcontrollers may not implement an external address or data bus as they integrate RAM and non-volatile memory on the same chip as the CPU. Using fewer pins, the chip can be placed in a much smaller, cheaper package.
Integrating the memory and other peripherals on a single chip and testing them as a unit increases the cost of that chip, but often results in decreased net cost of the embedded system as a whole. Even if the cost of a CPU that has integrated peripherals is slightly more than the cost of a CPU + external peripherals, having fewer chips typically allows a smaller and cheaper circuit board, and reduces the labor required to assemble and test the circuit board.
A microcontroller is a single integrated circuit, commonly with the following features:
* central processing unit - ranging from small and simple 4-bit processors to complex 32- or 64-bit processors
* discrete input and output bits, allowing control or detection of the logic state of an individual package pin
* serial input/output such as serial ports (UARTs)
* other serial communications interfaces like I²C, Serial Peripheral Interface and Controller Area Network for system interconnect
* peripherals such as timers, event counters, PWM generators, and watchdog
* volatile memory (RAM) for data storage
* ROM, EPROM, EEPROM or Flash memory for program and operating parameter storage
* clock generator - often an oscillator for a quartz timing crystal, resonator or RC circuit
* many include analog-to-digital converters
* in-circuit programming and debugging support
This integration drastically reduces the number of chips and the amount of wiring and circuit board space that would be needed to produce equivalent systems using separate chips. Furthermore, and on low pin count devices in particular, each pin may interface to several internal peripherals, with the pin function selected by software. This allows a part to be used in a wider variety of applications than if pins had dedicated functions. Microcontrollers have proved to be highly popular in embedded systems since their introduction in the 1970s.
Some microcontrollers use a Harvard architecture: separate memory buses for instructions and data, allowing accesses to take place concurrently. Where a Harvard architecture is used, instruction words for the processor may be a different bit size than the length of internal memory and registers; for example: 12-bit instructions used with 8-bit data registers.
The decision of which peripheral to integrate is often difficult. The microcontroller vendors often trade operating frequencies and system design flexibility against time-to-market requirements from their customers and overall lower system cost. Manufacturers have to balance the need to minimize the chip size against additional functionality.
Microcontroller architectures vary widely. Some designs include general-purpose microprocessor cores, with one or more ROM, RAM, or I/O functions integrated onto the package. Other designs are purpose built for control applications. A microcontroller instruction set usually has many instructions intended for bit-wise operations to make control programs more compact. For example, a general purpose processor might require several instructions to test a bit in a register and branch if the bit is set, where a microcontroller could have a single instruction to provide that commonly-required function.
Microcontroller typically do not have a math coprocessor, so fixed point or floating point arithmetic are performed by program code.