A microcontroller (sometimes abbreviated µC, uC or MCU)
is a small computer on a single integrated
circuit containing a
processor core, memory, and programmable input/output peripherals.
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 embedded applications, in contrast to the microprocessors used in personal computers 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, toys and other embedded systems. 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 frequencies
as low as 4
kHz, for low power
consumption (single-digit 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.
History
The first microprocessor was the 4-bit Intel 4004 released
in 1971, with the Intel 8008 and other more capable microprocessors
becoming available over the next several years. However, both processors
required external chips to implement a working system, raising total system
cost, and making it impossible to economically computerize appliances.
The Smithsonian Institution says TI engineers
Gary Boone and Michael Cochran succeeded in creating the first microcontroller
in 1971. The result of their work was the TMS 1000,
which went commercial in 1974. It combined read-only memory, read/write memory,
processor and clock on one chip and was targeted at embedded systems.[1]
Partly in response to the existence of the single-chip TMS
1000,[2] Intel
developed a computer system on a chip optimized for control applications, the Intel 8048,
with commercial parts first shipping in 1977.[2] It
combined RAM and ROM on
the same chip. This chip would find its way into over one billion PC keyboards,
and other numerous applications. At that time Intel's President, Luke J.
Valenter, stated that the microcontroller was one of the most successful in the
company's history, and expanded the division's budget over 25%.
Most microcontrollers at this time had two variants. One had
an erasable EPROM program
memory, with a transparent quartz window in the lid of the package to allow it
to be erased by exposure to ultraviolet light.
The other was a PROM variant which was only
programmable once; sometimes this was signified with the designation OTP,
standing for "one-time programmable". The PROM was actually exactly
the same type of memory as the EPROM, but because there was no way to expose it
to ultraviolet light, it could not be erased. The erasable versions required
ceramic packages with quartz windows, making them significantly more expensive
than the OTP versions, which could be made in lower-cost opaque plastic
packages. For the erasable variants, quartz was required, instead of less
expensive glass, for its transparency to ultraviolet—glass is largely opaque to
UV—but the main cost differentiator was the ceramic package itself.
In 1993, the introduction of EEPROM memory
allowed microcontrollers (beginning with the Microchip PIC16x84)[citation needed] to be
electrically erased quickly without an expensive package as required for EPROM, allowing both
rapid prototyping, and In System Programming. (EEPROM technology had
been available prior to this time, but the earlier EEPROM was more expensive
and less durable, making it unsuitable for low-cost mass-produced
microcontrollers.) The same year, Atmel introduced the first microcontroller
using Flash memory, a special type of EEPROM.[3]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 USD in quantity
(thousands) in 2009,[citation needed] and some
32-bit microcontrollers around1 USD for similar quantities. The NXP-LPC810
is a 32 bit ARM CORTEX M0+ in an 8 pin DIP package and sells in single quantity
for £0.66 or $1.00 in 2013.
Nowadays microcontrollers are cheap 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.
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.[4] More
recently, Semico has claimed the MCU market grew 36.5% in 2010 and 12% in 2011.[5] 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 devices such as washing machines, microwave ovens, and
telephones.
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.[6]The
majority of microcontrollers in use today are embedded in other machinery, such
as automobiles, telephones, appliances, and peripherals for computer 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 LCDdisplays, 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.
Interrupts
Microcontrollers must provide real time (predictable, though not
necessarily fast) response to events in the embedded system they are
controlling. When certain events occur, an interrupt system
can signal the processor to suspend processing the current instruction sequence
and to begin aninterrupt 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
Typically 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 convert
high-level language and assembler language codes into a compact machine
code 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 that may be field-alterable flash
or erasable read-only memory.
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 EPROMversions 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.
A customizable microcontroller incorporates a block of
digital logic that can be personalized in order to provide additional
processing capability, peripherals and interfaces that are adapted to
the requirements of the application. For example, the AT91CAP from Atmel has a
block of logic that can be customized during manufacture according to user
requirements.
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
LEDs or motors.
Many embedded systems need to read sensors that produce
analog signals. This is the purpose of the analog-to-digital converter (ADC).
Since processors are built to interpret and process digital data, i.e. 1s and
0s, they are not 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 (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 may either count down from some value to zero, or up to the capacity of
the count register, overflowing 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.
A dedicated Pulse Width Modulation (PWM) block
makes it possible for the CPU to control power converters, resistive loads, motors,
etc., without using lots of CPU resources in tight timerloops.
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
Die of a PIC12C508 8-bit, fully
static,EEPROM/EPROM/ROM-based CMOSmicrocontroller
manufactured by Microchip Technology using a 1200 nanometreprocess.
Die of a STM32F100C4T6B ARM
Cortex-M3 microcontroller with 16 kilobytes flash
memory, 24 MHz Central Processing Unit(CPU), motor
control and Consumer Electronics Control (CEC) functions. Manufactured
by STMicroelectronics.
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, in addition to tending to decrease the defect rate for the
finished assembly.
A micro-controller 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
·
volatile memory (RAM) for data storage
·
ROM, EPROM, EEPROM or Flash
memory for program and operating parameter storage
·
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
·
clock generator - often an oscillator for a
quartz timing crystal, resonator or RC circuit
·
many include analog-to-digital converters, some include
digital-to-analog 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, 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 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 micro-controller instruction set usually has many
instructions intended for bit-wise operations to make control programs more
compact.[7] 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 arithmetic is performed by
software.
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;[8] BASIC
and FORTH on the Zilog Z8[9] as
well as some modern devices. Typically these interpreters support interactive programming.
Simulators are available for some
microcontrollers. These allow a developer to analyze what the behavior of the
microcontroller and their program should be if they were using the actual part.
Asimulator 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[edit source]
See also: List of common microcontrollers
As of 2008 there are several dozen microcontroller
architectures and vendors including:
·
ARM core processors (many vendors)
·
ARM Cortex-M cores are specifically targeted
towards microcontroller applications
·
Cypress Semiconductor's M8C Core used in
their PSoC
(Programmable System-on-Chip)
·
Infineon: 8-bit XC800, 16-bit
XE166, 32-bit XMC4000 (ARM based Cortex M4F), 32-bit TriCore and, 32-bit Aurix Tricore
Bit microcontrollers[10]
·
MIPS
·
Microchip Technology PIC, (8-bit PIC16, PIC18, 16-bit dsPIC33 /
PIC24), (32-bit PIC32)
·
NXP Semiconductors LPC1000, LPC2000,
LPC3000, LPC4000 (32-bit), LPC900, LPC700 (8-bit)
·
PowerPC ISE
·
Rabbit 2000 (8-bit)
·
Silicon Laboratories Pipelined 8-bit 8051
Microcontrollers and mixed-signal ARM-based 32-bit microcontrollers
·
STMicroelectronics STM8 (8-bit), ST10 (16-bit) and STM32 (32-bit)
·
Texas Instruments TI MSP430 (16-bit) C2000 (32-bit)
·
Toshiba TLCS-870 (8-bit/16-bit).
Many others exist, 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[edit source]
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 having the hardware not save them all
(this fails if the software then needs 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. Reduced
instruction set computing/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 is 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 theLoad-link/store-conditional/LDREX/STREX exclusive
access primitives introduced in the ARMv6 architecture.
·
Interrupt nesting. Some micro controllers 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, micro
controllers may avoid an extra context save/restore cycle by a form of tail call optimization.
Lower end micro controllers 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.
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. MRAMcould potentially replace
it as it is 4 to 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 do not have any (or not
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.[3]
Firmware
The earliest microcontrollers used 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,[11] was
the first microcontroller to use EEPROM to
store firmware. In the same year, Atmel introduced the first microcontroller
using NOR Flash memory to store firmware.[3]
Reference:
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