Single-board microcontroller - AbsoluteAstronomy.com

A single-board microcontroller is a microcontroller

Microcontroller

A microcontroller 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...

built onto a single printed circuit board

Printed circuit board

A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...

. This board provides all of the circuitry necessary for a useful control task: microprocessor

Microprocessor

A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...

, I/O circuits

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...

, clock generator

Clock generator

A clock generator is a circuit that produces a timing signal for use in synchronizing a circuit's operation. The signal can range from a simple symmetrical square wave to more complex arrangements...

, RAM

Ram

-Animals:*Ram, an uncastrated male sheep*Ram cichlid, a species of freshwater fish endemic to Colombia and Venezuela-Military:*Battering ram*Ramming, a military tactic in which one vehicle runs into another...

, stored program memory and any support IC

Integrated circuit

An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

s necessary. The intention is that the board is immediately useful to an application developer, without needing to spend time and effort in developing the controller hardware.

As they are usually low-cost hardware, and have an especially low capital cost for development, single-board microcontrollers have long been popular in education. They are also a popular means for developers to gain hands-on experience with a new processor family.

Origins

Single-board microcontrollers appeared in the late 1970s when the first generations of microprocessors, such as the 6502

MOS Technology 6502

The MOS Technology 6502 is an 8-bit microprocessor that was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975. When it was introduced, it was the least expensive full-featured microprocessor on the market by a considerable margin, costing less than one-sixth the price of...

and the Z80

Zilog Z80

The Zilog Z80 is an 8-bit microprocessor designed by Zilog and sold from July 1976 onwards. It was widely used both in desktop and embedded computer designs as well as for military purposes...

, made it practical to build an entire controller on a single board, and affordable to dedicate a computer to a relatively minor task.

In March 1976, Intel announced a single-board computer product that integrated all the support components required for their 8080 microprocessor, along with 1 kbytes of RAM, 4 kbytes of user-programmable ROM, and 48 lines of parallel digital I/O with line drivers. The board also offered expansion through a bus connector, but could be used without an expansion card cage where applications didn't require additional hardware. Software development for this system was hosted on Intel's Intellec MDS microcomputer development system; this provided assembler and PL/M support, and permitted in-circuit emulation for debugging.

Processors of this era required a number of support chips in addition. RAM

Ram

and EPROM

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...

were separate, often requiring memory management or refresh circuitry for dynamic memory

Dynamic random access memory

Dynamic random-access memory is a type of random-access memory that stores each bit of data in a separate capacitor within an integrated circuit. The capacitor can be either charged or discharged; these two states are taken to represent the two values of a bit, conventionally called 0 and 1...

as well. I/O processing might be carried out by a single chip such as the 8255, but frequently required several more chips.

A single-board microcontroller differs from a single-board computer

Single-board computer

A single-board computer is a complete computer built on a single circuit board, with microprocessor, memory, input/output and other features required of a functional computer. Unlike a typical personal computer, an SBC may not include slots into which accessory cards may be plugged...

in that it lacks the general purpose user interface and mass storage interfaces that a more general-purpose computer would have. Compared to a microprocessor development board

Microprocessor development board

A microprocessor development board is a printed circuit board containing a microprocessor and the minimal support logic needed for an engineer to become acquainted with the microprocessor on the board, and to learn to program it...

, a microcontroller board would emphasize digital and analog control interconnections to some controlled system, where a development board might by comparison have only a few or no discrete or analog input/output devices. The development board exists to showcase or to train on some particular processor family and this internal implementation is more important than the external function.

Internal bus

The bus of the early single-board devices, such as the Z80 and 6502, was universally a Von Neumann architecture

Von Neumann architecture

The term Von Neumann architecture, aka the Von Neumann model, derives from a computer architecture proposal by the mathematician and early computer scientist John von Neumann and others, dated June 30, 1945, entitled First Draft of a Report on the EDVAC...

. Program and data memory were accessed by the same shared bus, even though they were stored in fundamentally different types of memory: ROM

Read-only memory

Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...

for programs and RAM

Ram

for data. This bus architecture was needed to economise on the number of pins needed from the limited 40 available for the processor's ubiquitous dual-in-line IC package.

It was common to offer the internal bus through an expansion connector, or at least the space for such a connector to be soldered on. This was a low-cost option and offered the potential for expansion, even if it was rarely made use of. Typical expansions would be I/O devices, or memory expansion. It was unusual to add peripheral devices such as tape or disk storage, or even a CRT display

When single-chip microcontrollers, such as the 8048, became available later on, the bus no longer needed to be exposed outside the package as all the necessary memory could be provided within the chip package. This generation of processors used a Harvard architecture

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...

of separate program and data buses, both internal to the chip. Many of these processors used a modified Harvard architecture

Modified Harvard architecture

The Modified Harvard Architecture is a variation of the Harvard computer architecture that allows the contents of the instruction memory to be accessed as if it were data...

, where some write access was possible to the program data space, thus permitting in-circuit programming. None of these processors required, or supported, a Harvard bus across a single-board microcomntroller. Where they supported a bus for expansion of peripherals, this used a dedicated IO bus, such as I²C, One-wire or various serial buses.

External bus expansion

Some microcontroller boards using a general-purpose microprocessor can bring the address and data bus of the processor to an expansion connector, allowing additional memory or peripherals to be added. This would provide resources not already present on the single board system. Since not all systems require expansion, the connector may be an option, with a mounting position provided for the connector for installation by the user if desired.

Input and output

Microcontroller systems provide multiple forms of input and output signals to allow application software to control an external "real-world" system. Discrete digital I/O provides a single bit of data (on, or off). Analog signals, representing a continuously variable range such as temperature or pressure, can also be inputs and outputs for microcontrollers.

Discrete digital inputs and outputs might only be buffered from the microprocessor data bus by an addressable latch, or might be operated by a specialized input/output integrated circuit such as an Intel 8255

Intel 8255

The Intel 8255 Programmable Peripheral Interface chip is a peripheral chip originally developed for the Intel 8085 microprocessor, and as such is a member of a large array of such chips, known as the MCS-85 Family. This chip was later also used with the Intel 8086 and its descendants.It was later...

or Motorola 6821 parallel input/output adapter

Peripheral Interface Adapter

The Peripheral Interface Adapter is a peripheral integrated circuit providing parallel I/O interfacing capability for microprocessor systems. Common PIAs include the Motorola MC6820 and MC6821, and the MOS Technology MCS6520, all of which are functionally identical but have slightly different...

. Later single-chip micrcontrollers have input and output pins available. The input/output circuits usually do not provide enough current to directly operate such devices as lamps or motors, so solid-state relays are operated by the microcontroller digital outputs, and inputs are isolated by signal conditioning

Signal conditioning

In electronics, signal conditioning means manipulating an analog signal in such a way that it meets the requirements of the next stage for further processing...

level-shifting and protection circuits.

One or more analog inputs, with an analog multiplexer and common analog to digital converter, are found on some microcontroller boards. Analog outputs may use a digital-to-analog converter, or on some microcontrollers may be controlled by pulse-width modulation

Pulse-width modulation

Pulse-width modulation , or pulse-duration modulation , is a commonly used technique for controlling power to inertial electrical devices, made practical by modern electronic power switches....

. As for discrete inputs, external circuits may be required to scale inputs, or to provide such functions as bridge

Wheatstone bridge

A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. Its operation is similar to the original potentiometer. It was invented by Samuel Hunter Christie in 1833 and...

excitation or cold-junction compensation.

To control component costs, many boards were designed with extra hardware interface circuits but the components for these circuits weren't installed and the board was left bare. The circuit was only added as an option on delivery, or could be populated later.

It is common practice for boards to include "prototyping areas", areas of the board already laid out as a solderable breadboard area with the bus and power rails available, but without a defined circuit. Several controllers, particularly those intended for training, also included a pluggable re-usable breadboard

Breadboard

A breadboard is a construction base for prototyping of electronics. The term is commonly used to refer to solderless breadboard ....

for easy prototyping of extra I/O circuits that could be changed or removed for later projects.

Communications and user interfaces

Communications interfaces vary depending on the age of the microcontroller system. Early systems might implement a serial port

Serial port

In computing, a serial port is a serial communication physical interface through which information transfers in or out one bit at a time...

to provide RS 232 or current loop

Current loop

A current loop describes two different electrical signalling schemes.- Digital :For digital serial communications, a current loop is a communication interface that uses current instead of voltage for signaling...

. The serial port could be used by the application program, or could be used, in conjunction with a monitor ROM, to transfer programs into the microcontroller memory. Current microcontrollers may support USB, wireless network (WiFi

WIFI

WIFI is a radio station broadcasting a brokered format. Licensed to Florence, New Jersey, USA, the station is currently operated by Florence Broadcasting Partners, LLC.This station was previously owned by Real Life Broadcasting...

, ZigBee

ZigBee

ZigBee is a specification for a suite of high level communication protocols using small, low-power digital radios based on an IEEE 802 standard for personal area networks. Applications include wireless light switches, electrical meters with in-home-displays, and other consumer and industrial...

, or others) ports, or provide an Ethernet connection, and may support a TCP/IP protocol stack. Some devices have firmware available to implement a Web server, allowing an application developer to rapidly build a Web-enabled instrument or system.

Programming

Many of the earliest systems had no internal facility for programming at all, and relied on a separate "host" system. This programming was typically in assembly language

Assembly language

An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices. It implements a symbolic representation of the machine codes and other constants needed to program a given CPU architecture...

, sometimes C

C (programming language)

C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....

or even PL/M

PL/M

The PL/M programming languageis a high-level language developed byGary Kildall in 1972 for Intel for its microprocessors....

, and then cross-assembled or cross-compiled on the host. Some single-board microcontrollers support a BASIC language system, allowing programs to be developed on the target hardware. Hosted development allows all the storage and peripherals of a desktop computer to be used, providing a more powerful development environment.

EPROM burning

Early microcontrollers relied on erasable programmaable read-only memory

EPROM

(EPROM) devices to hold the application program. The completed object code

Object code

Object code, or sometimes object module, is what a computer compiler produces. In a general sense object code is a sequence of statements in a computer language, usually a machine code language....

from a host system would be "burned" onto an EPROM

EPROM

with an EPROM programmer, this EPROM was then physically plugged into the board. As the EPROM would be removed and replaced many times during program development, it was usual to provide a ZIF socket to avoid wear or damage. Erasing an EPROM with a UV

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

eraser takes a considerable time, and so it was also usual for a developer to have several EPROMs in circulation at any one time.

Some microcontroller devices were available with on-board EPROM; these, too, would be programmed in a separate burner, then put into a socket on the target system.

The use of EPROM sockets allowed field update of the application program, eitehr to fix errors or to provide updated features.

Keypad monitors

Where the single-board controller formed the entire development environment (typically in education) the board might also be provided with a simple hexadecimal

Hexadecimal

In mathematics and computer science, hexadecimal is a positional numeral system with a radix, or base, of 16. It uses sixteen distinct symbols, most often the symbols 0–9 to represent values zero to nine, and A, B, C, D, E, F to represent values ten to fifteen...

keypad, calculator-style LED display and a "monitor" program set permanently in ROM. This monitor allowed machine code

Machine code

Machine code or machine language is a system of impartible instructions executed directly by a computer's central processing unit. Each instruction performs a very specific task, typically either an operation on a unit of data Machine code or machine language is a system of impartible instructions...

programs to be entered directly through the keyboard and held in RAM. These programs were in machine code, not even in assembly language, and were assembled by hand on paper first. It's arguable as to which process was more time-consuming and error prone: assembling by hand, or keying byte-by-byte.

Single-board "keypad and calculator display" microcontrollers of this type were very similar to some low-end microcomputers of the time, such as the KIM-1

KIM-1

The KIM-1, short for Keyboard Input Monitor, was a small 6502-based single-board computer developed and produced by MOS Technology, Inc. and launched in 1976...

or the Microprofessor I

Microprofessor I

The Micro-Professor MPF-I, introduced in 1981 by Multitech , was the first branded computer product from Multitech and probably one of the world's longest selling computers...

. Some of these microprocessor "trainer" systems are still in production today, as a very low-cost introduction to microprocessors at the hardware programming level.

Hosted development

When desktop personal computers appeared, initially CP/M

CP/M

CP/M was a mass-market operating system created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc...

or Apple II

Apple II

The Apple II is an 8-bit home computer, one of the first highly successful mass-produced microcomputer products, designed primarily by Steve Wozniak, manufactured by Apple Computer and introduced in 1977...

, then later the IBM PC

IBM PC

The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...

and compatibles, there was a shift to hosted development. Hardware was now cheaper and RAM capacity had expanded such that it was possible to download the program through the serial port and hold it in RAM. This massive reduction in the cycle time to test a new version of a program gave an equally large boost in development speed.

This program memory was still volatile

Volatile memory

Volatile memory, also known as volatile storage, is computer memory that requires power to maintain the stored information, unlike non-volatile memory which does not require a maintained power supply...

and would be lost if power was turned off. Flash memory

Flash memory

Flash memory is a non-volatile computer storage chip that can be electrically erased and reprogrammed. It was developed from EEPROM and must be erased in fairly large blocks before these can be rewritten with new data...

was not yet available at a viable price. As a completed controller project usually required to be non-volatile, the final step in a project was often to burn an EPROM again.

Single-chip microcontrollers

With the development of single-chip microcontrollers such as the 8748

Intel 8048

The MCS-48 microcontroller series, Intel's first microcontroller, was originally released in 1976. Its first members were 8048, 8035 and 8748....

, it became possible to combine most of the features of the previous board into a single IC package. Single-chip microcontrollers integrate their necessary memory (both RAM and ROM) on-package and so do not need to expose their bus through the IC package's pins. These pins are available for I/O lines. Both of these changes make the design of a single-board microcontroller simpler, but also less necessary. Examples of single-chip microcontrollers include:

8748
Intel 8048
The MCS-48 microcontroller series, Intel's first microcontroller, was originally released in 1976. Its first members were 8048, 8035 and 8748....
PIC
PIC microcontroller
PIC is a family of Harvard architecture microcontrollers made by Microchip Technology, derived from the PIC1650 originally developed by General Instrument's Microelectronics Division...
Atmel AVR
Atmel AVR
The AVR is a modified Harvard architecture 8-bit RISC single chip microcontroller which was developed by Atmel in 1996. The AVR was one of the first microcontroller families to use on-chip flash memory for program storage, as opposed to one-time programmable ROM, EPROM, or EEPROM used by other...

Program memory

For production use as embedded system

Embedded system

An embedded system is a computer system designed for specific control functions within a larger system. 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...

s, the on-board ROM

Read-only memory

would be either mask programmed

Programmable read-only memory

A programmable read-only memory or field programmable read-only memory or one-time programmable non-volatile memory 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...

at the chip factory or one-time programmed (OTP) by the developer as a PROM

PROM

PROM may refer to:*Phosphate rich organic manure*Premature rupture of membranes, an obstetric term*PROM-1 land mine*Programmable read-only memory, related to electronics...

. PROMs often used the same UV EPROM technology for the chip, but in a cheaper package without the transparent erasure window. During program development it was still necessary to burn EPROMs, this time the entire controller IC, and so ZIF sockets would be provided.

With the development of affordable EEPROM

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...

, EAROM and eventually flash memory

Flash memory

, it became practical to attach the controller permanently to the board and to download program code to it through a serial connection to a host computer. This was termed "in-circuit programming". Erasure of old programs was carried out by either over-writing them with a new download, or bulk erasing them electrically (for EEPROM

EEPROM

) which was slower, but could be carried out in-situ.

The main function of the controller board was now to carry the support circuits for this serial interface, or USB on later boards. As a further convenience feature during development, many boards also carried low-cost features like LED monitors of the I/O lines or reset switches mounted on-board.

Single-board microcontrollers today

Microcontrollers are now cheap and simple to design circuit boards for. Development host systems are also cheap, especially when using open source

Open source

The term open source describes practices in production and development that promote access to the end product's source materials. Some consider open source a philosophy, others consider it a pragmatic methodology...

software. Higher level programming languages abstract

Abstraction (computer science)

In computer science, abstraction is the process by which data and programs are defined with a representation similar to its pictorial meaning as rooted in the more complex realm of human life and language with their higher need of summarization and categorization , while hiding away the...

details of the hardware, making differences between specific processors less obvious to the application programmer. Rewritable flash memory has replaced slow programming cycles, at least during program development. Accordingly almost all development now is based on cross-compilation from personal computers and download to the controller board through a serial-like interface, usually appearing to the host as a USB device.

The original market demand of a simplified board implementation is no longer so relevant to microcontrollers. Single-board microcontrollers are still important, but have shifted their focus to:

Easily accessible platforms aimed at traditionally "non-programmer" groups, such as artists, designers, hobbyists, and others interested in creating interactive objects or environments. Some typical projects in 2011 included; the backup control of DMX stage lights and special effects, multi-camera control, autonomous fighting robots, controlling bluetooth projects from a computer or smart phone, LEDs and multiplexing, displays, audio, motors, mechanics, and power control. These controllers may be embedded to form part of a physical computing
Physical computing
Physical computing, in the broadest sense, means building interactive physical systems by the use of software and hardware that can sense and respond to the analog world. While this definition is broad enough to encompass things such as smart automotive traffic control systems or factory automation...

project. Popular choices for this work are the Arduino
Arduino
Arduino is an open-source single-board microcontroller, descendant of the open-source Wiring platform, designed to make the process of using electronics in multidisciplinary projects more accessible. The hardware consists of a simple open hardware design for the Arduino board with an Atmel AVR...

, Dwengo or the Wiring (development platform)
Wiring (development platform)
Wiring is an open source electronics prototyping platform composed of a programming language, an integrated development environment , single-board microcontroller documentation thoughtfully created with designers and artists in mind and a community where experts, intermediate and beginners from...

.
Technology demonstrator boards for innovative processors or peripheral features:
- AVR Butterfly
  AVR Butterfly
  The AVR Butterfly , about the size of a credit card, is the cheapest self-contained, battery-powered single board microcontroller available. It runs the ATMEL AVR ATmega169PV Microcontroller IC. The board includes a liquid crystal display , joystick, speaker, serial port, real-time clock , internal...
- Parallax Propeller
  Parallax Propeller
  The Parallax P8X32A Propeller chip, introduced in 2006, is a multi-core architecture parallel microcontroller with eight 32-bit RISC CPU cores....

Origins

Internal bus

External bus expansion

Input and output

Communications and user interfaces

Programming

EPROM burning

Keypad monitors

Hosted development

Single-chip microcontrollers

Program memory

Single-board microcontrollers today

See also