Microcontrollers

A microcontroller (also microcontroller unit, MCU or µC) is a small computer on a single integrated circuit consisting of a relatively simple CPU combined with support functions such as a crystal oscillator, timers, watchdog timer, serial and analog I/O etc. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM. Microcontrollers are designed for small or dedicated applications. Thus, in contrast to the microprocessors used in personal computers and other high-performance or general purpose applications, simplicity is emphasized. Some microcontrollers may operate at clock rate frequencies as low as 4 kHz, as this is adequate for many typical applications, enabling 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 (DSP), with higher clock speeds and power consumption.

Microcontrollers are used in automatically controlled products and devices, such as automobile engine control systems, 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.

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.

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, 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.

Embedded design

A microcontroller can be considered a self-contained system with a processor, memory and peripherals and can be used with an embedded system. (Only the software needs be added.)[1] The majority of computer systems in use today are embedded in other machinery, such as automobiles, telephones, appliances, and peripherals for computer systems. These are called embedded systems. 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, solenoids, LEDs, small or custom LCD 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, and may lack human interaction devices of any kind.

Higher integration

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 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 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.[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 microcontroller could have a single instruction to provide that commonly-required function.

Microcontrollers typically do not have a math coprocessor, so floating point arithmetic is performed by software.

Volumes

About 55% of all CPUs sold in the world are 8-bit 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.
 
A PIC 18F8720 microcontroller in an 80-pin TQFP package.

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), 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 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. In mid-2009, some consolidation is evident, with vendors pruning product lines.

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

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.
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Data

From the earliest microcontrollers to today, six-transistor SRAM 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[citation needed] have "self-programmable" flash memory[7].
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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,[8] 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.[7]

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

MRAM could potentially be used to store firmware.