A microcontroller (sometimes abbreviated µC or uC) is a small computer on a single integrated circuit In electronics, an integrated circuit is a miniaturized electronic circuit (consisting mainly of semiconductor devices, as well as passive components) that has been manufactured in the surface of a thin substrate of semiconductor material. Integrated circuits are used in almost all electronic equipment in use today and have revolutionized the containing a processor core, memory, and programmable input/output In computing, input/output, or I/O, refers to the communication between an information processing system , and the outside world possibly a human, or another information processing system. Inputs are the signals or data received by the system, and outputs are the signals or data sent from it. The term can also be used as part of an action; to & peripherals. Program memory in the form of NOR flash or OTP ROM A programmable read-only memory or field programmable read-only memory (FPROM) is a form of digital memory where the setting of each bit is locked by a fuse or antifuse. Such PROMs are used to store programs permanently. The key difference from a strict ROM is that the programming is applied after the device is constructed. They are frequently is also often included on chip, as well as a typically small amount of RAM. Microcontrollers are designed for embedded applications, in contrast to the microprocessors A microprocessor incorporates most or all of the functions of a computer's central processing unit on a single integrated circuit (IC, or microchip). The first microprocessors emerged in the early 1970s and were used for electronic calculators, using binary-coded decimal (BCD) arithmetic in 4-bit words. Other embedded uses of 4-bit and 8-bit used in personal computers A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end user, with no intervening computer operator. This is in contrast to the batch processing or time-sharing models which allowed large expensive mainframe or other general purpose applications.

Microcontrollers are used in automatically controlled products and devices, such as automobile engine control systems, implantable medical devices, remote controls, office machines, appliances, power tools, and toys. By reducing the size and cost compared to a design that uses a separate microprocessor, memory, and input/output devices, microcontrollers make it economical to digitally control even more devices and processes. Mixed signal microcontrollers are common, integrating analog components needed to control non-digital electronic systems.

Some microcontrollers may use four-bit words and operate at clock rate The clock rate is the rate in cycles per second for the frequency of the clock in any synchronous circuit (such as a CPU). For example, a crystal oscillator frequency reference typically is synonymous with a fixed sinusoidal waveform, a clock rate is that frequency reference translated by electronic circuitry (AD Converter) into a corresponding frequencies as low as 4 kHz, for low power consumption (milliwatts or microwatts). They will generally have the ability to retain functionality while waiting for an event such as a button press or other interrupt; power consumption while sleeping (CPU clock and most peripherals off) may be just nanowatts, making many of them well suited for long lasting battery applications. Other microcontrollers may serve performance-critical roles, where they may need to act more like a digital signal processor A digital signal processor is a specialized microprocessor with an optimized architecture for the fast operational needs of digital signal processing (DSP), with higher clock speeds and power consumption.

Embedded design

A microcontroller can be considered a self-contained system with a processor, memory and peripherals and can be used as an embedded system An embedded system is a computer system designed to perform one or a few dedicated functions often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal computer , is designed to be flexible and to meet a wide.^[1] The majority of microcontrollers in use today are embedded in other machinery, such as automobiles, telephones, appliances, and peripherals for computer systems. These are called embedded systems An embedded system is a computer system designed to perform one or a few dedicated functions often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal computer , is designed to be flexible and to meet a wide. While some embedded systems are very sophisticated, many have minimal requirements for memory and program length, with no operating system, and low software complexity. Typical input and output devices include switches, relays A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism, but other operating principles are also used. Relays find applications where it is necessary to control a circuit by a low-power signal, or where several circuits must be controlled by one signal. The first relays were used in long, solenoids A solenoid[nb 1] is a three-dimensional coil wound into a tightly packed helix. In physics, the term solenoid refers to a loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it. Solenoids are important because they can create controlled magnetic fields and can be used as, LEDs A light-emitting diode (pronounced /ˌɛl iː ˈdiː/) is a semiconductor light source. LEDs are used as indicator lamps in many devices, and are increasingly used for lighting. Introduced as a practical electronic component in 1962, early LEDs emitted low-intensity red light, but modern versions are available across the visible, ultraviolet and, small or custom LCD A liquid crystal display is a thin, flat electronic visual display that uses the light modulating properties of liquid crystals (LCs). LCs do not emit light directly displays, radio frequency devices, and sensors for data such as temperature, humidity, light level etc. Embedded systems usually have no keyboard, screen, disks, printers, or other recognizable I/O devices of a personal computer A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end user, with no intervening computer operator. This is in contrast to the batch processing or time-sharing models which allowed large expensive mainframe, and may lack human interaction devices of any kind.

Interrupts

Microcontrollers must provide real time In computer science, real-time computing , or reactive computing, is the study of hardware and software systems that are subject to a "real-time constraint"—i.e., operational deadlines from event to system response. By contrast, a non-real-time system is one for which there is no deadline, even if fast response or high performance is (predictable, though not necessarily fast) response to events in the embedded system they are controlling. When certain events occur, an interrupt In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution system can signal the processor to suspend processing the current instruction sequence and to begin an interrupt service routine (ISR, or "interrupt handler"). The ISR will perform any processing required based on the source of the interrupt before returning to the original instruction sequence. Possible interrupt sources are device dependent, and often include events such as an internal timer overflow, completing an analog to digital conversion, a logic level change on an input such as from a button being pressed, and data received on a communication link. Where power consumption is important as in battery operated devices, interrupts may also wake a microcontroller from a low power sleep state where the processor is halted until required to do something by a peripheral event.

Programs

Microcontroller programs must fit in the available on-chip program memory, since it would be costly to provide a system with external, expandable, memory. Compilers and assemblers are used to turn high-level language and assembler language codes into a compact machine code Machine code or machine language is a system of instructions and data executed directly by a computer's central processing unit. Machine code may be regarded as a primitive programming language or as the lowest-level representation of a compiled and/or assembled computer program. Programs in interpreted languages are not represented by machine for storage in the microcontroller's memory. Depending on the device, the program memory may be permanent, read-only memory that can only be programmed at the factory, or program memory may be field-alterable flash or erasable read-only memory.

Other microcontroller features

Microcontrollers usually contain from several to dozens of general purpose input/output pins (GPIO). GPIO pins are software configurable to either an input or an output state. When GPIO pins are configured to an input state, they are often used to read sensors or external signals. Configured to the output state, GPIO pins can drive external devices such as LED's or motors.

Many embedded systems need to read sensors that produce analog signals. This is the purpose of the analog-to-digital converter An analog-to-digital converter is a device which converts continuous signals to discrete digital numbers. The reverse operation is performed by a digital-to-analog converter (DAC) (ADC). Since processors are built to interpret and process digital data, i.e. 1s and 0s, they won't be able to do anything with the analog signals that may be sent to it by a device. So the analog to digital converter is used to convert the incoming data into a form that the processor can recognize. A less common feature on some microcontrollers is a digital-to-analog converter In electronics, a digital-to-analog converter is a device that converts a digital (usually binary) code to an analog signal (current, voltage, or electric charge). An analog-to-digital converter (ADC) performs the reverse operation (DAC) that allows the processor to output analog signals or voltage levels.

In addition to the converters, many embedded microprocessors include a variety of timers as well. One of the most common types of timers is the Programmable Interval Timer (PIT). A PIT just counts down from some value to zero. Once it reaches zero, it sends an interrupt to the processor indicating that it has finished counting. This is useful for devices such as thermostats, which periodically test the temperature around them to see if they need to turn the air conditioner on, the heater on, etc.

Time Processing Unit (TPU) is a sophisticated timer. In addition to counting down, the TPU can detect input events, generate output events, and perform other useful operations.

A dedicated Pulse Width Modulation Pulse-width modulation is a very efficient way of providing intermediate amounts of electrical power between fully on and fully off. A simple power switch with a typical power source provides full power only, when switched on. PWM is a comparatively-recent technique, made practical by modern electronic power switches (PWM) block makes it possible for the CPU to control power converters A switched-mode power supply is an electronic power supply unit (PSU) that incorporates a switching regulator in order to provide the required output voltage, resistive The electrical resistance of an object is a measure of its attraction to the passage of a steady electric current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the material loads, motors An electric motor uses electrical energy to produce mechanical energy, very typically through the interaction of magnetic fields and current-carrying conductors. The reverse process, producing electrical energy from mechanical energy, is accomplished by a generator or dynamo. Many types of electric motors can be run as generators, and vice versa, etc., without using lots of CPU resources in tight timer loops In computer science, control flow refers to the order in which the individual statements, instructions, or function calls of an imperative or a declarative program are executed or evaluated.

Universal Asynchronous Receiver/Transmitter (UART) block makes it possible to receive and transmit data over a serial line with very little load on the CPU. Dedicated on-chip hardware also often includes capabilities to communicate with other devices (chips) in digital formats such as I2C and Serial Peripheral Interface (SPI).

Higher integration

In contrast to general-purpose CPUs, micro-controllers 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 and 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 micro-controller is a single integrated circuit In electronics, an integrated circuit is a miniaturized electronic circuit (consisting mainly of semiconductor devices, as well as passive components) that has been manufactured in the surface of a thin substrate of semiconductor material. Integrated circuits are used in almost all electronic equipment in use today and have revolutionized the, commonly with the following features:

• central processing unit The Central Processing Unit or the processor is the portion of a computer system that carries out the instructions of a computer program, and is the primary element carrying out the computer's functions. This term has been in use in the computer industry at least since the early 1960s . The form, design and implementation of CPUs have changed - ranging from small and simple 4-bit A bit or binary digit is the basic unit of information in computing and telecommunications; it is the amount of information that can be stored by a digital device or other physical system that can usually exist in only two distinct states. These may be the two stable positions of an electrical switch, two distinct voltage or current levels allowed 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 In computing, input/output, or I/O, refers to the communication between an information processing system , and the outside world possibly a human, or another information processing system. Inputs are the signals or data received by the system, and outputs are the signals or data sent from it. The term can also be used as part of an action; to & such as serial ports In computing, a serial port is a serial communication physical interface through which information transfers in or out one bit at a time . Throughout most of the history of personal computers, data transfer through serial ports connected the computer to devices such as terminals and various peripherals (UARTs A universal asynchronous receiver/transmitter is a type of "asynchronous receiver/transmitter", a piece of computer hardware that translates data between parallel and serial forms. UARTs are commonly used in conjunction with other communication standards such as EIA RS-232)
• other serial communications In telecommunication and computer science, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent together, on a link with several parallel channels. Serial communication is used for all long-haul interfaces like I²C I²C (pronounced /ˈaɪ skwɛərd ˈsiː/ or /ˈaɪ tuː ˈsiː/) is a multi-master serial single-ended computer bus invented by Philips that is used to attach low-speed peripherals to a motherboard, embedded system, or cellphone. Since the mid 1990s several competitors (e.g. Siemens AG (later Infineon Technologies AG), NEC, Texas Instruments,, Serial Peripheral Interface and Controller Area Network Controller–area network is a vehicle bus standard designed to allow microcontrollers and devices to communicate with each other within a vehicle without a host computer for system interconnect
• peripherals A peripheral is a device attached to a host computer but not part of it, and is more or less dependent on the host. It expands the host's capabilities, but does not form part of the core computer architecture such as timers A timer is a specialized type of clock. A timer can be used to control the sequence of an event or process. Whereas a stopwatch counts upwards from zero for measuring elapsed time, a timer counts down from a specified time interval, like an hourglass. Timers can be mechanical, electromechanical, electronic , or even software as all modern, event counters, PWM generators Pulse-width modulation is a very efficient way of providing intermediate amounts of electrical power between fully on and fully off. A simple power switch with a typical power source provides full power only, when switched on. PWM is a comparatively-recent technique, made practical by modern electronic power switches, and watchdog A watchdog timer is a computer hardware timing device that triggers a system reset if the main program, due to some fault condition, such as a hang, neglects to regularly service the watchdog (writing a “service pulse” to it, also referred to as “petting the dog” or "feed the watchdog" or "waking the watchdog"). The
• volatile memory (RAM Random-access memory is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order (i.e., at random). "Random" refers to the idea that any piece of data can be returned in a constant time, regardless of its physical location and whether or not it is related to the) for data storage
• ROM Read-only memory is a class of storage media used in computers and other electronic devices. Because data stored in ROM cannot be modified (at least not very quickly or easily), it is mainly used to distribute firmware (software that is very closely tied to specific hardware, and unlikely to require frequent updates), EPROM An EPROM, or erasable programmable read only memory, is a type of memory chip that retains its data when its power supply is switched off. In other words, it is non-volatile. It is an array of floating-gate transistors individually programmed by an electronic device that supplies higher voltages than those normally used in digital circuits. Once, EEPROM EEPROM stands for Electrically Erasable Programmable Read-Only Memory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration tables or device configuration or Flash memory Flash memory is a non-volatile computer storage technology that can be electrically erased and reprogrammed. It is primarily used in memory cards, USB flash drives, and solid-state drives for general storage and transfer of data between computers and other digital products. It is a specific type of EEPROM that is erased and programmed in large for program A computer program is a sequence of instructions written to perform a specified task for a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute the instructions. The same program in its human- and operating parameter storage
• clock generator - often an oscillator for a quartz timing crystal, resonator or RC circuit A resistor–capacitor circuit , or RC filter or RC network, is an electric circuit composed of resistors and capacitors driven by a voltage or current source. The 1st order RC circuit composed of one resistor and one capacitor, is the simplest example of an 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 A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring board. A PCB populated with electronic components is a 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. Micro-controllers have proved to be highly popular in embedded systems An embedded system is a computer system designed to perform one or a few dedicated functions often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal computer , is designed to be flexible and to meet a wide since their introduction in the 1970s.

Some microcontrollers use a Harvard architecture The Harvard architecture is a computer architecture with physically separate storage and signal pathways for instructions and data. The term originated from the Harvard Mark I relay-based computer, which stored instructions on punched tape and data in electro-mechanical counters. These early machines had limited data storage, entirely contained: 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 micro-controller instruction set usually has many instructions intended for bit-wise operations to make control programs more compact.^[2] 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 micro-controller could have a single instruction to provide that commonly-required function.

Microcontrollers typically do not have a math coprocessor, so floating point In computing, floating point describes a system for representing numbers that would be too large or too small to be represented as integers. Numbers are in general represented approximately to a fixed number of significant digits and scaled using an exponent. The base for the scaling is normally 2, 10 or 16. The typical number that can be arithmetic is performed by software.

Volumes

About 55% of all CPUs The Central Processing Unit or the processor is the portion of a computer system that carries out the instructions of a computer program, and is the primary element carrying out the computer's functions. This term has been in use in the computer industry at least since the early 1960s . The form, design and implementation of CPUs have changed sold in the world are 8-bit In computer architecture, 8-bit integers, memory addresses, or other data units are those that are at most 8 bits wide. Also, 8-bit CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size. 8-bit is also a term given to a generation of computers in which 8-bit processors were the norm microcontrollers and microprocessors. According to Semico, over four billion 8-bit microcontrollers were sold in 2006.^[3]

A typical home in a developed country is likely to have only four general-purpose microprocessors but around three dozen microcontrollers. A typical mid-range automobile has as many as 30 or more microcontrollers. They can also be found in many electrical device such as washing machines, microwave ovens, and telephones.

Manufacturers have often produced special versions of their microcontrollers in order to help the hardware and software development of the target system. Originally these included EPROM versions that have a "window" on the top of the device through which program memory can be erased by ultraviolet light, ready for reprogramming after a programming ("burn") and test cycle. Since 1998, EPROM versions are rare and have been replaced by EEPROM and flash, which are easier to use (can be erased electronically) and cheaper to manufacture.

Other versions may be available where the ROM is accessed as an external device rather than as internal memory, however these are becoming increasingly rare due to the widespread availability of cheap microcontroller programmers.

The use of field-programmable devices on a microcontroller may allow field update of the firmware or permit late factory revisions to products that have been assembled but not yet shipped. Programmable memory also reduces the lead time required for deployment of a new product.

Where hundreds of thousands of identical devices are required, using parts programmed at the time of manufacture can be an economical option. These 'mask programmed' parts have the program laid down in the same way as the logic of the chip, at the same time.

Programming environments

Microcontrollers were originally programmed only in assembly language, but various high-level programming languages are now also in common use to target microcontrollers. These languages are either designed specially for the purpose, or versions of general purpose languages such as the C programming language. Compilers for general purpose languages will typically have some restrictions as well as enhancements to better support the unique characteristics of microcontrollers. Some microcontrollers have environments to aid developing certain types of applications. Microcontroller vendors often make tools freely available to make it easier to adopt their hardware.

Many microcontrollers are so quirky that they effectively require their own non-standard dialects of C, such as SDCC for the 8051, which prevent using standard tools (such as code libraries or static analysis tools) even for code unrelated to hardware features. Interpreters are often used to hide such low level quirks.

Interpreter firmware is also available for some microcontrollers. For example, BASIC on the early microcontrollers Intel 8052^[4]; BASIC and FORTH on the Zilog Z8^[5] as well as some modern devices. Typically these interpreters support interactive programming.

Simulators are available for some microcontrollers, such as in Microchip's MPLAB environment. These allow a developer to analyze what the behavior of the microcontroller and their program should be if they were using the actual part. A simulator will show the internal processor state and also that of the outputs, as well as allowing input signals to be generated. While on the one hand most simulators will be limited from being unable to simulate much other hardware in a system, they can exercise conditions that may otherwise be hard to reproduce at will in the physical implementation, and can be the quickest way to debug and analyze problems.

Recent microcontrollers are often integrated with on-chip debug circuitry that when accessed by an in-circuit emulator via JTAG, allow debugging of the firmware with a debugger.

Types of microcontrollers

As of 2008 there are several dozen microcontroller architectures and vendors including:

• 68HC11
• 8051
• ARM processors (from many vendors) using ARM7 or Cortex-M3 cores are generally microcontrollers
• STMicroelectronics STM8S (8-bit), ST10 (16-bit) and STM32 (32-bit)
• Atmel AVR (8-bit), AVR32 (32-bit), and AT91SAM
• Freescale ColdFire (32-bit) and S08 (8-bit)
• Hitachi H8, Hitachi SuperH
• Hyperstone E1/E2 (32-bit, First full integration of RISC and DSP on one processor core [1996] [1])
• MIPS (32-bit PIC32)
• NEC V850
• PIC (8-bit PIC16, PIC18, 16-bit dsPIC33 / PIC24)
• PowerPC ISE
• PSoC (Programmable System-on-Chip)
• Rabbit 2000
• Texas Instruments Microcontrollers MSP430 (16-bit), C2000 (32-bit), and Stellaris (32-bit)
• Toshiba TLCS-870
• Zilog eZ8, eZ80

and many others, some of which are used in very narrow range of applications or are more like applications processors than microcontrollers. The microcontroller market is extremely fragmented, with numerous vendors, technologies, and markets. Note that many vendors sell (or have sold) multiple architectures.

Interrupt latency

In contrast to general-purpose computers, microcontrollers used in embedded systems often seek to optimize interrupt latency over instruction throughput. Issues include both reducing the latency, and making it be more predictable (to support real-time control).

When an electronic device causes an interrupt, the intermediate results (registers) have to be saved before the software responsible for handling the interrupt can run. They must also be restored after that software is finished. If there are more registers, this saving and restoring process takes more time, increasing the latency. Ways to reduce such context/restore latency include having relatively few registers in their central processing units (undesirable because it slows down most non-interrupt processing substantially), or at least not having hardware save them all (hoping that the software doesn't then need to compensate by saving the rest "manually"). Another technique involves spending silicon gates on "shadow registers": one or more duplicate registers used only by the interrupt software, perhaps supporting a dedicated stack.

Other factors affecting interrupt latency include:

• Cycles needed to complete current CPU activities. To minimize those costs, microcontrollers tend to have short pipelines (often three instructions or less), small write buffers, and ensure that longer instructions are continuable or restartable. RISC design principles ensure that most instructions take the same number of cycles, helping avoid the need for most such continuation/restart logic.
• The length of any critical section that needs to be interrupted. Entry to a critical section restricts concurrent data structure access. When a data structure must be accessed by an interrupt handler, the critical section must block that interrupt. Accordingly, interrupt latency is increased by however long that interrupt is blocked. When there are hard external constraints on system latency, developers often need tools to measure interrupt latencies and track down which critical sections cause slowdowns.
  • One common technique just blocks all interrupts for the duration of the critical section. This is easy to implement, but sometimes critical sections get uncomfortably long.
  • A more complex technique just blocks the interrupts that may trigger access to that data structure. This often based on interrupt priorities, which tend to not correspond well to the relevant system data structures. Accordingly, this technique is used mostly in very constrained environments.
  • Processors may have hardware support for some critical sections. Examples include supporting atomic access to bits or bytes within a word, or other atomic access primitives like the LDREX/STREX exclusive access primitives introduced in the ARMv6 architecture.
• Interrupt nesting. Some microcontrollers allow higher priority interrupts to interrupt lower priority ones. This allows software to manage latency by giving time-critical interrupts higher priority (and thus lower and more predictable latency) than less-critical ones.
• Trigger rate. When interrupts occur back-to-back, microcontrollers may avoid an extra context save/restore cycle by a form of tail call optimization.

Lower end microcontrollers tend to support fewer interrupt latency controls than higher end ones.

History

The first single-chip microprocessor was the 4-bit Intel 4004 released in 1971. With the Intel 8008 and more capable microprocessors available over the next several years.

These however all required external chip(s) to implement a working system, raising total system cost, and making it impossible to economically computerise appliances.

The first computer system on a chip optimised for control applications - microcontroller was the Intel 8048 released in 1975,^{[citation needed]} with both RAM and ROM on the same chip. This chip would find its way into over one billion PC keyboards, and other numerous applications. At this time Intels President, Luke J. Valenter, stated that the (Microcontroller) was one of the most successful in the companies history, and expanded the division's budget over 25%.

Most microcontrollers at this time had two variants. One had an erasable EEPROM program memory, which was significantly more expensive than the PROM variant which was only programmable once.

In 1993, the introduction of EEPROM memory allowed microcontrollers (beginning with the Microchip PIC16x84) [2]^{[citation needed]}) to be electrically erased quickly without an expensive package as required for EPROM, allowing both rapid prototyping, and In System Programming.

The same year, Atmel introduced the first microcontroller using Flash memory.^[6]

Other companies rapidly followed suit, with both memory types.

Cost has plummeted over time, with the cheapest 8-bit microcontrollers being available for under $0.25 in quantity (thousands) in 2009,^{[citation needed]} and some 32-bit microcontrollers around $1 for similar quantities.

Nowadays microcontrollers are low cost and readily available for hobbyists, with large online communities around certain processors.

In the future, MRAM could potentially be used in microcontrollers as it has infinite endurance and its incremental semiconductor wafer process cost is relatively low.

Microcontroller embedded memory technology

Since the emergence of microcontrollers, many different memory technologies have been used. Almost all microcontrollers have at least two different kinds of memory, a non-volatile memory for storing firmware and a read-write memory for temporary data.

Data

From the earliest microcontrollers to today, six-transistor SRAM is almost always used as the read/write working memory, with a few more transistors per bit used in the register file. MRAM could potentially replace it as it is 4-10 times denser which would make it more cost effective.

In addition to the SRAM, some microcontrollers also have internal EEPROM for data storage; and even ones that don't have any (or don't have enough) are often connected to external serial EEPROM chip (such as the BASIC Stamp) or external serial flash memory chip.

A few recent microcontrollers beginning in 2003 have "self-programmable" flash memory^[6].

Firmware

The earliest microcontrollers used hard-wired or mask ROM to store firmware. Later microcontrollers (such as the early versions of the Freescale 68HC11 and early PIC microcontrollers) had quartz windows that allowed ultraviolet light in to erase the EPROM.

The Microchip PIC16C84, introduced in 1993,^[7] was the first microcontroller to use EEPROM to store firmware.

Also in 1993, Atmel introduced the first microcontroller using NOR Flash memory to store firmware.^[6]

PSoC microcontrollers, introduced in 2002, store firmware in SONOS flash memory.

MRAM could potentially be used to store firmware.

See also

• In-circuit emulator
• List of common microcontrollers
• Microarchitecture
• Microbotics
• Programmable logic controller
• PSoC
• Single-board microcontroller

Notes

1. ^ Heath, Steve (2003). Embedded systems design. EDN series for design engineers (2 ed.). Newnes. pp. 11–12. ISBN 9780750655460. http://books.google.com/books?id=BjNZXwH7HlkC&pg=PA11.
2. ^ Easy Way to build a microcontroller project
3. ^ http://www.semico.com
4. ^ "8052-Basic Microcontrollers" by Jan Axelson 1994
5. ^ "Optimizing the Zilog Z8 Forth Microcontroller for Rapid Prototyping" by Robert Edwards 1987, page 3. http://www.ornl.gov/info/reports/1987/3445602791343.pdf
6. ^ ^{a b c} "Atmel’s Self-Programming Flash Microcontrollers" by Odd Jostein Svendsli 2003
7. ^ Microchip unveils PIC16C84, a reprogrammable EEPROM-based 8-bit microcontroller" 1993

External links

• Microcontroller at the Open Directory Project
• Microcontroller.com - Embedded Systems industry website with tutorials and dedicated resources
• Understanding DC Electrical Characteristics of Microcontrollers
• Cornell ECE476 final project designs
• Microcontroller based electronics projects
• Microcontroller Projects Every Day
• Embedded Systems Design magazine

Categories: Digital electronics | Embedded systems | Microcontrollers | Central processing unit

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