Information about Microcontroller

A microcontroller (or MCU) is a computer-on-a-chip. It is a type of microprocessor emphasizing self-sufficiency and cost-effectiveness, in contrast to a general-purpose microprocessor (the kind used in a PC). In addition to all arithmetic and logic elements of a general purpose microprocessor, the microcontroller usually also integrates additional elements such as read-only and read-write memory, and input/output interfaces.

Microcontrollers are frequently used in automatically controlled products and devices, such as automobile engine control systems, office machines, appliances, power tools, and toys. By reducing the size, cost, and power consumption compared to a design using a separate microprocessor, memory, and input/output devices, microcontrollers make it economical to electronically control many more processes.

Embedded design

The majority of computer systems in use today are embedded in other machinery, such as telephones, clocks, appliances, and vehicles. An embedded system may have minimal requirements for memory and program length. Input and output devices may be discrete switches, relays, or solenoids. An embedded conroller may lack any human-readable interface devices at all. For example, embedded systems usually don't have keyboards, screens, disks, printers, or other recognizable I/O devices of a personal computer. Microcontrollers may control electric motors, relays or voltages, and may read switches, variable resistors or other electronic devices.

Higher Integration

In contrast to general-purpose CPUs, microcontrollers may not implement an external address or data bus, because they integrate RAM and non-volatile memory on the same chip as the CPU. Because they need 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 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

This integration drastically reduces the number of chips and the amount of wiring and PCB space that would be needed to produce equivalent systems using separate chips. 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 that would provide that commonly-required function.

Large Volumes

Microcontrollers take the largest share of sales in the wider microprocessor market. Over 50% are "simple" controllers, and another 20% are more specialized digital signal processors (DSPs) . A typical home in a developed country is likely to have only one or two general-purpose microprocessors but somewhere between one and two dozen microcontrollers. A typical mid range automobile has as many as 50 or more microcontrollers. They can also be found in almost any electrical device: washing machines, microwave ovens, telephones etc.

Manufacturers have often produced special versions of their microcontrollers in order to help the hardware and software development of the target system. These have included EPROM versions that have a "window" on the top of the device through which program memory can be erased by ultra violet light, ready for reprogramming after a programming ("burn") and test cycle.

Other versions may be available where the ROM is accessed as an external device rather than as internal memory. A simple EPROM programmer, rather than a more complex and expensive microcontroller programmer, may then be used, however there is a potential loss of functionality through pin outs being tied up with external memory addressing rather than for general input/output. These kind of devices usually carry a higher cost but if the target production quantities are small, certainly in the case of a hobbyist, they can be the most economical option compared with the set up charges involved in mask programmed devices.

A more rarely encountered development microcontroller is the "piggy back" version. This device has no internal ROM memory; instead pin outs on the top of the microcontroller form a socket into which a standard EPROM program memory device may be installed. The benefit of this approach is the release of microcontroller pins for input and output use rather than program memory. These kinds of devices are normally expensive and are impractical for anything but the development phase of a project or very small production quantities.

An economical option for intermediate levels of production (usually a few score to a few thousand parts) is a one-time programmable (OTP) microcontroller. This uses the same die as the UV EPROM version of the part, and is programmed on the same equipment, but the package does not include the expensive quartz window required to admit UV light on to the chip.

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 a large number of systems will be made (say, several thousand), the cost of a mask-programmed memory is amortized over all products sold. A simpler integrated circuit process is used, and the contents of the read-only memory are set in the last step of chip manufacture instead of after assembly and test. However, mask-programmed parts cannot be updated in the field. If product firmware updates are still contemplated, a socket may be used to hold the controller which can then be replaced by a service technician, if required.

Programming Environments

Originally, microcontrollers were only programmed in assembly language, or later in C code. Recent microcontrollers integrated with on-chip debug circuitry accessed by In-circuit emulator via JTAG enables a programmer to debug the software of an embedded system with a debugger.

Some microcontrollers have begun to include a built-in high-level programming language interpreter for greater ease of use. The Intel 8052 and Zilog Z8 were available with BASIC very early on, and BASIC is more recently used in the popular BASIC Stamp MCUs.

Some microcontrollers such as Analog Device's Blackfin processors can be programmed using LabVIEW, which is a high level programming language.

Interrupt Latency

In contrast to general-purpose computers, microcontrollers used in embedded systems often seek to minimize interrupt latency over instruction throughput.

When an electronic device causes an interrupt, the intermediate results, the registers, have to be saved before the software responsible for handling the interrupt can run, and then must be put back after it is finished. If there are more registers, this saving and restoring process takes more time, increasing the latency.

Low-latency CPUs generally have relatively few registers in their central processing units, or they have "shadow registers" that are only used by the interrupt software. The Z80 is a well known (and early) microprocessor with "shadow registers".

History

First microcontroller was Intel 8048 released in 1976.

Development platforms for hobbyists

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For almost every manufacturer of bare microcontrollers, there are a dozen little companies repacking its products into more hobbyist-friendly packages. Their product is often an MCU preloaded with a BASIC or similar interpreter, soldered onto a Dual Inline Pin board along with a power regulator. PICmicros seem to be very popular here, possibly due to good static protection. More powerful examples (e.g. faster execution, more RAM and code space) seem to be based on Atmel AVR or Hitachi chips and now ARM.

Arduino

Arduino is an open-source physical computing platform based on a simple input/output board and a development environment that implements the Processing/Wiring language. Arduino can be used to develop stand-alone interactive objects or can be connected to software on your computer (e.g. Flash, Processing, MaxMSP). The boards can be assembled by hand or purchased preassembled; the open-source IDE can be downloaded for free. Arduino uses an ATmega8 or ATmega168 microcontroller from Atmel's Atmel AVR series.

Platforms from Parallax, Inc.

BASIC Stamp by Parallax, is the 'big name' in BASIC microcontrollers. They are Microchip PIC micros programmed with an interpreter that processes the program stored in an external EEPROM. Several different modules are available of varying processing speeds, RAM, and EEPROM sizes. Most popular is the original BASIC Stamp 2 module. The BASIC Stamp is used by Parallax as a platform for introductory programming and robotic kits.

SX-Key, Parallax's development tool for the SX line of microcontrollers, supporting every SX chip commercially available. Using free SX-Key software (Assembly language), or the SX/B Compiler (BASIC-style language) from Parallax, the SX-Key programming tool can program SX chips in-system and perform in-circuit source-level debugging.

Propeller, A multi-core microcontroller developed by Parallax, Inc. It features eight 32bit cores and 32 I/O pins in the currently released version. Each core operates independently at 80Mhz, it is programmed in a language named SPIN(tm) which was developed by Parallax to support this unique micro.

PICAXE

This PICAXE range of controllers from Revolution Education Limited^[1] are based upon Microchip PICmicro's programmed with a BASIC interpreter. Using internal EEPROM or Flash to store the user's program they deliver a single-chip solution and are quite inexpensive. A PICAXE programmer is simply a serial plug plus two resistors. Complete development software, comprehensive documentation and application notes are all available free of charge.

The BASIC-like programming language is almost identical to that used by Parallax's Basic Stamp 1 (BS1) but has been enhanced to support on-chip hardware and additional functionality. In common with the BS1 programming language, the PICAXE has support only for a limited number of variables, but allows access to internal RAM for storage which helps overcome that limitation.

The 5.0.X versions of the Visual IDE ( the Programming Editor ) introduced 'enhanced compilers' which support block-structured programming constructs plus conditional compilation and other directives.

Initially targeted at the UK educational sector, use of the PICAXE has spread to hobbyists, semi-professionals and it can also be found inside commercial products. With its user base in many countries, the PICAXE has steadily gained a good international reputation.

A-WIT Technologies, Inc.

A-WIT Technologies, Inc.^[2] has a microcontroller module named the C STAMP, along with support boards, kits, and software tools and infrastructure. The C STAMP is designed around a PIC microcontroller, and is programmed in a very user friendly subset of the standard C language called WC that is easy and powerful, because it relies on A-WIT's supplied software infrastructure. This microcontroller module is very affordable, and it has 48 pins, 35 KiB of memory, and runs at 40 MHz. The C STAMP also has a vast array of accessories and components, which are supported by A-WIT's software interfaces that enables seamless connectivity. This, in turn, enhances the ease of complete system development.

Comfile Technology Inc.

Comfile Technology Inc.^[3] produces a series of microcontrollers branded as CUBLOC and CuTOUCH, using the Atmel ATmega128 processor. They are very price competitive, being aimed at industrial applications, and include features such as Ladder Logic in addition to BASIC, a 80 kB program memory, and hardware pulse width modulation. Their focus is on developing industrial controllers which are fast, easy-to-use, and versatile. Comfile Technology's CuTOUCH is a visual Touch-screen controller that can be programmed in BASIC and Ladder Logic.

Coridium ARMexpress

ARMexpress^[4] is the first of a new family of DIP-24 (stamp-sized) controllers that combine a 60 MHz ARM CPU with a builtin BASIC compiler to achieve new levels of performance in this form factor. This combination makes this simple to use but very fast controller a good choice for the prototype builder or system integrator. 40K of code and 40K of data are available to the user, and code speed rivals that of programs written in C. The dialect of BASIC conforms more to Visual BASIC, but has hardware extensions like PBASIC.

ZX-24, ZX-40, ZX-44

The ZX series^[5] MCUs are based on the Atmel ATmega32 and ATmega644 processors. The devices run a field-upgradable Virtual Machine that features built-in multi-tasking, 32-bit floating point math and 1.5K to 3.5K of RAM for user's programs. Multi-tasking facilitates a more structured approach to coding for interface devices that require prompt service, e.g. serial devices, infrared remotes, etc.

The programming language for the ZX series is ZBasic, a modern dialect of Basic modeled after Microsoft's Visual Basic. The biggest improvement over the typical MCU Basic dialect is the availability of parameterized subroutines/functions that support local variables. Strong type checking is another improvement that aids in writing correct programs more quickly. User-defined types (structures) are also supported along with aliases, based variables, sub-byte data types (Bit and Nibble) and other advanced capabilities.

Errors of ATmega32

• Interrupts abort TWI Power-down
• TWI master does not Accept spikes on Bus Lines
• TECR Write operations Ignored when immediately repeated

1-Interrupts abort TWI Power-down: TWIpoer-down operation may wake up by other interrupts.If an interrupt (e.g.,INTO) occurs during TWI poower-down address watch and wakes up the CPU,the TWI aborts operation and returns t its idle state. if the interrupt occurs in the middle of a power-down address match,the received address will be lost and the slave will not return an ACN. problm fix/workaround ensure that the TWI Address match is the only enabled interrupt when entering power-down. The master can handle this by resending the request if NACH is received.

2- TWI master does not Accept spikes on Bus Lines: When the part operates as Master, and the bus is idle(SDA=1;SCL=1),generating a short spike on SDA (SDA=0 for a short interval), no interrupt is generated, and the status code is still $F8(idle). But when the software initiates a new start condition and clears TWINT, nothing happens on SDA or SCL, and TWINT is never set again. problm fix/workaround Either of the following: 1)Ensur no spikes occur on SDA or SCL lines. 2)Generate a valid START condition followed by a STOP condition on the bus. This provokes a bus error reported as a TWI interrupt with status code $00. 3)In a single-master system, the user should write the TWSTO bit immediately befor writing the TWSTA bit.

3-TWCR Write operations Ignored when immediately repeated: repeated write to RWCR must be delayed. If a write operation to TWCR is immediately followed by another write operation to TWCR, the first write operation may be ignored. problm fix/workaround ensure at least one instruction (e.g.,INTO) is executed between two writes to TWCR

See also

• Embedded system
• Microarchitecture
• In-circuit emulator (ICE)
• List of common microcontrollers
• Microbotics
• Contiki

Notes

External links

• Microcontroller at the Open Directory Project
• PIC Microcontroller Tutorial and Projects
• Introduction to the MAXQ Architecture
• Understanding DC Electrical Characteristics of Microcontrollers
• Cornell ECE476 final project designs
• Microcontroller Projects Every Day
• Embedded Systems Glossary
• Embedded Systems Design magazine

•  • [ e] CPU technologies
Microarchitecture, ISA • RISC, CISC, EPIC, VLIW • Pipelining, Superscalar, Out-of-order execution, Speculative execution, Multithreading, Multiprocessing • ALU, FPU, Vector processor, SIMD • 32-bit/64-bit, Registers, Cache • ASIC, FPGA, DSP, Microcontroller, ASIP, SoC