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Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM can only be modified slowly, with difficulty, or not at all, so it is mainly used to store firmware (software that is closely tied to specific hardware, and unlikely to need frequent updates) or application software in plug-in cartridges.

Strictly, read-only memory refers to memory that is hard-wired, such as diode matrix and the later mask ROM (MROM), which cannot be changed after manufacture. Although discrete circuits can be altered in principle, integrated circuits (ICs) cannot, and are useless if the data is bad or requires an update. That such memory can never be changed is a disadvantage in many applications, as bugs and security issues cannot be fixed, and new features cannot be added.

More recently, ROM has come to include memory that is read-only in normal operation, but can still be reprogrammed in some way. Erasable programmable read-only memory (EPROM) and electrically erasable programmable read-only memory (EEPROM) can be erased and re-programmed, but usually this can only be done at relatively slow speeds, may require special equipment to achieve, and is typically only possible a certain number of times.
https://en.wikipedia.org/wiki/Read-only_memory
There are five basic ROM types:
ROM - Read Only Memory
PROM - Programmable Read Only Memory
EPROM - Erasable Programmable Read Only Memory
EEPROM - Electrically Erasable Programmable Read Only Memory
Flash EEPROM memory
Each type has unique characteristics, but all types of ROM memory have two things in common:
Data stored in these chips is non-volatile -- it is not lost when power is removed.
Data stored in these chips is either unchangeable or requires a special operation to change.
http://www.electronics.dit.ie/staff/tscarff/memory/rom.htm
http://computer.howstuffworks.com/rom2.htm

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Pulse-width modulation(PWM)
Pulse-width modulation (PWM), or pulse-duration modulation (PDM), is a modulation technique used to encode a message into a pulsing signal. Although this modulation technique can be used to encode information for transmission, its main use is to allow the control of the power supplied to electrical devices, especially to inertial[definition needed] loads such as motors. In addition, PWM is one of the two principal algorithms used in photovoltaic solar battery chargers,[1] the other being maximum power point tracking.

The average value of voltage (and current) fed to the load is controlled by turning the switch between supply and load on and off at a fast rate. The longer the switch is on compared to the off periods, the higher the total power supplied to the load.

The PWM switching frequency has to be much higher than what would affect the load (the device that uses the power), which is to say that the resultant waveform perceived by the load must be as smooth as possible. The rate (or frequency) at which the power supply must switch can vary greatly depending on load and application, for example

Switching has to be done several times a minute in an electric stove; 120 Hz in a lamp dimmer; between a few kilohertz (kHz), to tens of kHz for a motor drive; and well into the tens or hundreds of kHz in audio amplifiers and computer power supplies.
The term duty cycle describes the proportion of 'on' time to the regular interval or 'period' of time; a low duty cycle corresponds to low power, because the power is off for most of the time. Duty cycle is expressed in percent, 100% being fully on.

The main advantage of PWM is that power loss in the switching devices is very low. When a switch is off there is practically no current, and when it is on and power is being transferred to the load, there is almost no voltage drop across the switch. Power loss, being the product of voltage and current, is thus in both cases close to zero. PWM also works well with digital controls, which, because of their on/off nature, can easily set the needed duty cycle.

PWM has also been used in certain communication systems where its duty cycle has been used to convey information over a communications channel.
https://en.wikipedia.org/wiki/Pulse-width_modulation
https://www.multisim.com/help/components/pulse-width-modulation-pwm-components/
http://www.electronics-tutorials.ws/blog/pulse-width-modulation.html
http://www.embedded.com/electronics-blogs/beginner-s-corner/4023833/Introduction-to-Pulse-Width-Modulation

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General-purpose input/output (GPIO) is a generic pin on an integrated circuit or computer board whose behavior—including whether it is an input or output pin—is controllable by the user at run time.

GPIO pins have no predefined purpose, and go unused by default.[1][2] The idea is that sometimes a system integrator who is building a full system might need a handful of additional digital control lines—and having these available from a chip avoids having to arrange additional circuitry to provide them. For example, the Realtek ALC260 chips (audio codec) have 8 GPIO pins, which go unused by default. Some system integrators (Acer Inc. laptops) use the first GPIO (GPIO0) on the ALC260 to turn on the amplifier for the laptop's internal speakers and external headphone jack.
https://en.wikipedia.org/wiki/General-purpose_input/output

General Purpose I/O Key Benefits

Fully programmable features and voltage domain support
Optimal interface solutions for ARM based SoC to ensuring high flexibility in implementation for area, power and performance optimization
Higher margins delivered thru smaller die area, lower power and flexibility in packaging
Time To Market advantage leveraging broadest EDA views, silicon proven technology and manufacturing flexibility
Lowest risk solutions with a long history of high volume production backed by an experienced support team.
https://www.arm.com/products/physical-ip/interface-ip/general-purpose-io.php

A General Purpose Input/output (GPIO) is an interface available on most modernmicrocontrollers (MCU) to provide an ease of access to the devices internal properties.Generally there are multiple GPIO pins on a single MCU for the use of multipleinteraction so simultaneous application. The pins can be programmed as input, wheredata from some external source is being fed into the system to be manipulated at a desiredtime and location. Output can also be performed on GPIOs, where formatted date can betransmitted efficiently to outside devices, this provides a simple mechanism to program
and retransmit data depending on user desires through a single port interface. The pinsare usually arranged into groups of 8 pins where signals can be sent or received to andfrom other devices. In many applications, the GPIOs can be configured as interrupt linesfor a CPU to signal immediate processing of input lines. In many newer designs, theyalso have the ability to control and use Direct Memory Access (DMA) to transfer blocksof data in a more efficient manner. Essentially all ports can be tailored to fit specificdesign goals and provide reusability within applications.
https://www.google.by/url?sa=t&rct=j&q=&esrc=s&source=web&cd=6&cad=rja&uact=8&ved=0ahUKEwjO4rWuz5fVAhVnMZoKHUqwBykQFghJMAU&url=http%3A%2F%2Fwww.egr.msu.edu%2Fclasses%2Fece480%2Fgoodman%2Ffall09%2Fgroup03%2FAN_balachandran.pdf&usg=AFQjCNGirY0Oxej6oaubD4tqNUNnHOAUTQ

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UART (Universal Asynchronous Receiver/Transmitter)
A UART (Universal Asynchronous Receiver/Transmitter) is the microchip with programming that controls a computer's interface to its attached serial devices. Specifically, it provides the computer with the RS-232C Data Terminal Equipment ( DTE ) interface so that it can "talk" to and exchange data with modems and other serial devices. As part of this interface, the UART also:
1.Converts the bytes it receives from the computer along parallel circuits into a single serial bit stream for outbound transmission
2.On inbound transmission, converts the serial bit stream into the bytes that the computer handles
3.Adds a parity bit (if it's been selected) on outbound transmissions and checks the parity of incoming bytes (if selected) and discards the parity bit
4.Adds start and stop delineators on outbound and strips them from inbound transmissions
5.Handles interrupt s from the keyboard and mouse (which are serial devices with special port s)
6.May handle other kinds of interrupt and device management that require coordinating the computer's speed of operation with device speeds
http://whatis.techtarget.com/definition/UART-Universal-Asynchronous-Receiver-Transmitter
https://en.wikipedia.org/wiki/Universal_asynchronous_receiver/transmitter

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Controller Area Network (CAN bus)

A Controller Area Network (CAN bus) is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other in applications without a host computer. It is a message-based protocol, designed originally for multiplex electrical wiring within automobiles to save on copper, but is also used in many other contexts.

CAN is a multi-master serial bus standard for connecting Electronic Control Units [ECUs] also known as nodes. Two or more nodes are required on the CAN network to communicate. The complexity of the node can range from a simple I/O device up to an embedded computer with a CAN interface and sophisticated software. The node may also be a gateway allowing a standard computer to communicate over a USB or Ethernet port to the devices on a CAN network.
https://en.wikipedia.org/wiki/CAN_bus

A controller area network (CAN) is ideally suited to the many high-level industrial protocols embracing CAN and ISO-11898:2003 as their physical layer. Its cost, performance, and upgradeability provide for tremendous flexibility in system design. This application report presents an introduction to the CAN fundamentals, operating principles, and the implementation of a basic CAN bus with TI's CAN transceivers and DSPs. The electrical layer requirements of a CAN bus are discussed along with the importance of the different features of a TI CAN transceiver

https://www.google.by/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwjSsPHvu-DVAhVLCpoKHZCUDBUQFghJMAI&url=http%3A%2F%2Fwww.ti.com%2Flit%2Fan%2Fsloa101b%2Fsloa101b.pdf&usg=AFQjCNFYrUHtj9niNEh8AxjghaazYkeKBw
http://www.ni.com/white-paper/2732/en/#toc1

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Serial Peripheral Interface bus (SPI)

The Serial Peripheral Interface bus (SPI) is a synchronous serial communication interface specification used for short distance communication, primarily in embedded systems. The interface was developed by Motorola in the late 1980s and has become a de facto standard. Typical applications include Secure Digital cards and liquid crystal displays.

SPI devices communicate in full duplex mode using a master-slave architecture with a single master. The master device originates the frame for reading and writing. Multiple slave devices are supported through selection with individual slave select (SS) lines.

Sometimes SPI is called a four-wire serial bus, contrasting with three-, two-, and one-wire serial buses. The SPI may be accurately described as a synchronous serial interface,[1] but it is different from the Synchronous Serial Interface (SSI) protocol, which is also a four-wire synchronous serial communication protocol. SSI Protocol employs differential signaling and provides only a single simplex communication channel.

https://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus

https://www.google.by/url?sa=t&rct=j&q=&esrc=s&source=web&cd=13&cad=rja&uact=8&ved=0ahUKEwiBhLve8vTVAhVGCpoKHV5lDGkQFgh3MAw&url=http%3A%2F%2Fwww.cypress.com%2Ffile%2F132066%2Fdownload&usg=AFQjCNHONc4c6Xj2gl_fDmRDYQZTRexNEA

[url]
https://www.allaboutcircuits.com/technical-articles/spi-serial-peripheral-interface/[/url]

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I2C (Inter-Integrated Circuit), pronounced I-squared-C or I-two-C, is a multi-master, multi-slave, packet switched, single-ended, serial computer bus invented by Philips Semiconductor (now NXP Semiconductors). It is typically used for attaching lower-speed peripheral ICs to processors and microcontrollers in short-distance, intra-board communication. Alternatively I2C is spelled I2C (pronounced I-two-C) or IIC (pronounced I-I-C).

Since October 10, 2006, no licensing fees are required to implement the I2C protocol. However, fees are required to obtain I2C slave addresses allocated by NXP.[1]

Several competitors, such as Siemens AG (later Infineon Technologies AG, now Intel mobile communications), NEC, Texas Instruments, STMicroelectronics (formerly SGS-Thomson), Motorola (later Freescale, now merged with NXP[2]), Nordic Semiconductor and Intersil, have introduced compatible I?C products to the market since the mid-1990s.

SMBus, defined by Intel in 1995, is a subset of I2C, defining a stricter usage. One purpose of SMBus is to promote robustness and interoperability. Accordingly, modern I2C systems incorporate some policies and rules from SMBus, sometimes supporting both I2C and SMBus, requiring only minimal reconfiguration either by commanding or output pin use.

https://en.wikipedia.org/wiki/I%C2%B2C#Physical_layer

In this tutorial, you will learn all about the I2C communication protocol, why you would want to use it, and how it’s implemented.

The Inter-integrated Circuit (I2C) Protocol is a protocol intended to allow multiple “slave” digital integrated circuits (“chips”) to communicate with one or more “master” chips. Like the Serial Peripheral Interface (SPI), it is only intended for short distance communications within a single device. Like Asynchronous Serial Interfaces (such as RS-232 or UARTs), it only requires two signal wires to exchange information.
https://learn.sparkfun.com/tutorials/i2c
https://www.google.by/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&cad=rja&uact=8&ved=0ahUKEwjeu67r2pbWAhXEaVAKHaSnA-sQFghGMAM&url=http%3A%2F%2Fwww.ti.com%2Flit%2Fan%2Fslva704%2Fslva704.pdf&usg=AFQjCNE_zKr4keWm7Q8lV4ZHUQcJQxDAfg

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In system programming, an interrupt is a signal to the processor emitted by hardware or software indicating an event that needs immediate attention. An interrupt alerts the processor to a high-priority condition requiring the interruption of the current code the processor is executing. The processor responds by suspending its current activities, saving its state, and executing a function called an interrupt handler (or an interrupt service routine, ISR) to deal with the event. This interruption is temporary, and, after the interrupt handler finishes, the processor resumes normal activities.[1] There are two types of interrupts: hardware interrupts and software interrupts.

Hardware interrupts are used by devices to communicate that they require attention from the operating system.[2] Internally, hardware interrupts are implemented using electronic alerting signals that are sent to the processor from an external device, which is either a part of the computer itself, such as a disk controller, or an external peripheral. For example, pressing a key on the keyboard or moving the mouse triggers hardware interrupts that cause the processor to read the keystroke or mouse position. Unlike the software type (described below), hardware interrupts are asynchronous and can occur in the middle of instruction execution, requiring additional care in programming. The act of initiating a hardware interrupt is referred to as an interrupt request (IRQ).

A software interrupt is caused either by an exceptional condition in the processor itself, or a special instruction in the instruction set which causes an interrupt when it is executed. The former is often called a trap or exception and is used for errors or events occurring during program execution that are exceptional enough that they cannot be handled within the program itself. For example, a divide-by-zero exception will be thrown if the processor's arithmetic logic unit is commanded to divide a number by zero as this instruction is an error and impossible. The operating system will catch this exception, and can choose to abort the instruction. Software interrupt instructions can function similarly to subroutine calls and are used for a variety of purposes, such as to request services from device drivers, like interrupts sent to and from a disk controller to request reading or writing of data to and from the disk.

Each interrupt has its own interrupt handler. The number of hardware interrupts is limited by the number of interrupt request (IRQ) lines to the processor, but there may be hundreds of different software interrupts. Interrupts are a commonly used technique for computer multitasking, especially in real-time computing. Such a system is said to be interrupt-driven.[3]

Interrupts are similar to signals, the difference being that signals are used for IPC, mediated by the kernel (possibly via system calls) and handled by processes, while interrupts are mediated by the processor and handled by the kernel. The kernel may pass an interrupt as a signal to the process that caused it (typical examples are SIGSEGV, SIGBUS, SIGILL and SIGFPE).
https://en.wikipedia.org/wiki/Interrupt

n general, there are hardware interrupts and software interrupts. A hardware interrupt occurs, for example, when an I/O operation is completed such as reading some data into the computer from a tape drive. A software interrupt occurs when an application program terminates or requests certain services from the operating system. In a personal computer, a hardware interrupt request (IRQ) has a value that associates it with a particular device.

Watch a brief tutorial on interrupts and I/O:
https://www.youtube.com/watch?time_continue=166&v=tn45bY-EA8Y
https://www.embeddedrelated.com/showarticle/469.php

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Microcontrollers and its applications in digital electronics

Introduction
1 The composition of the microcontroller
1.1 Basic structure
1.2 Peripherals
1.3 Instruction Set
2 Communication
2.1 Wireless communication between hardware
2.2 Hardware communication
3 The market’s mainstream microcontrollers
3.1 Intel MCS-51 series microcontrollers
3.2 Texas Instruments MSP430 series microcontrollers
3.3 STMicroelectronics STM32 series microcontrollers
3.4 Microship PIC series microcontrollers
3.5 ATMEL AVR series microcontrollers
3.6 Freescale Semiconductor series microcontrollers
4 Application of microcontrollers
4.1 Application domain
4.2 The application of STM32 microcontroller in a quadcopter
4.3 The application of AVR microcontroller in automotive air conditioner
4.4 The application of MSC-51 microcontroller in traffic light
4.5 The application of TC1782 microcontroller in motor drive system
5 Education
5.1 Online courses
5.2 Sites with documents
5.3 Books about microcontrollers
Conclusion