• Onboard Communication Interface refers to the different communication channels/buses for interconnecting the various integrated circuits and other peripherals within the embedded system. The various interfaces for onboard communication are as follows:

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• Inter Integrated Circuit (I2C) Bus
• Serial Peripheral Interface (SPI) Bus 
• Universal Asynchronous Receiver Transmitter (UART)
• 1-Wire Interface
• Parallel Interface

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Inter Integrated Circuit (I2C) Bus�

Inter Integrated Circuit Bus is a synchronous bi-directional half duplex two wire serial interface bus. The I2C bus comprise of two bus lines, namely; Serial Clock-SCL and Serial Data-SDA.

SCL line is responsible for generating synchronization clock pulses and

• SDA is responsible for transmitting the serial data across devices.
• Devices connected to the I2C bus can act as either ‘Master’ device or ‘Slave’

device.

• The ‘Master’ device is responsible for controlling the communication by initiating/terminating data transfer, sending data and generating necessary synchronization clock pulses.
• ‘Slave’ devices wait for the commands from the master and respond upon receiving the commands. ‘Master’ and ‘Slave’ devices can act as either transmitter or receiver. I2C supports multi masters on the same bus.
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I2C Bus Interfacing

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Serial Peripheral Interface (SPI) Bus�

The Serial Peripheral Interface Bus (SPI) is a synchronous bi- directional full duplex four-wire serial interface bus.

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• SPI is a single master multi-slave system.
• SPI requires four signal lines for communication. They are Master Out Slave In (MOSI), Master In Slave Out (MISO), Serial Clock (SCLK) and Slave Select (SS).

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• When compared to I2C, SPI bus is most suitable for applications requiring transfer of data in ‘streams’.

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SPI bus interfacing�

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Universal Asynchronous Receiver Transmitter ( UART)

• Universal Asynchronous Receiver Transmitter (UART) based data transmission is an asynchronous form of serial data transmission.
• UART based serial data transmission doesn’t require a clock signal to synchronize the transmitting end and receiving end for transmission.
• Instead it relies upon the pre-defined agreement between the transmitting device and receiving device. The serial communication settings (Baud rate, number of bits per byte, parity, number of start bits and stop bit and flow control) for both transmitter and receiver should be set as identical

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• The ‘start’ bit informs the receiver that a data byte is about to arrive. The receiver device starts polling its ‘receive line’ as per the baud rate settings. If the baud rate is ‘x’ bits per second, the time slot available for one bit is 1/x seconds. The receiver unit polls the receiver line at exactly half of the time slot available for the bit.
• If parity is enabled for communication, the UART of the transmitting device adds a parity bit (bit value is 1 for odd number of 1s in the transmitted bit stream and 0 for even number of 1s).
• The UART of the receiving device discards the ‘Start’, ‘Stop’ and ‘Parity’ bit from the received bit stream and converts the received serial bit data to a word

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1-Wire Interface

• 1-wire interface is an asynchronous half-duplex communication protocol
• developed by Maxim Dallas Semiconductor
• It makes use of only a single signal line (wire) called DQ for communication and follows the master-slave communication model. One of the key feature of 1-wire bus is that it allows power to be sent along the signal wire as well
• The 1-wire interface supports a single master and one or more slave devices on the bus

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• The sequence of operation for communicating with a 1-wire slave device is listed below.
• 1. The master device sends a ‘Reset’ pulse on the 1-wire bus.
• 2. The slave device(s) present on the bus respond with a ‘Presence’ pulse.
• 3. The master device sends a ROM command (Net Address Command followed by the 64bit address of the device). This addresses the slave device(s) to which it wants to initiate a communication.
• 4. The master device sends a read/write function command to read/write the internal memory or register of the slave device.
• 5. The master initiates a Read data/Write data from the device or to the device

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Parallel Interface

• The on-board parallel interface is normally used for communicating with peripheral devices which are memory mapped to the host of the system.
• The communication through the parallel bus is controlled by the control signal interface between the device and the host
• The ‘Control Signals’ for communication includes ‘Read/Write’ signal and device select signal. The device normally contains a device select line and the device becomes active only when this line is asserted by the host processor.
• The direction of data transfer (Host to Device or Device to Host) can be controlled through the control signal lines for ‘Read’ and ‘Write’.

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External Communication Interfaces�

The External Communication Interface refers to the different communication channels/buses used by the embedded system to communicate with the external world.

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The various interfaces for external communication are as follows

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• RS-232 C & RS-485
• Universal Serial Bus (USB)
• IEEE 1394 (Firewire)
• Infrared (IrDA)
• Bluetooth (BT)
• Wi-Fi
• ZigBee
• General Packet Radio Service (GPRS)

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RS-232 C & RS-485�

• RS-232C is a legacy, full duplex, wired, asynchronous serial communication interface. RS-232 supports two different types of connectors, namely; DB-9; 9-Pin connector and DB-25: 25-Pin connector. RS-232 supports only point-to-point communication and not suitable for multi-drop communication.

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• RS-485 is the enhanced version of RS-422 and it supports multi-drop

communication with up to 32 transmitting devices (drivers) and 32 receiving devices on the bus.

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DB-9 and DB-25 RS232 Connector Interface�

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Universal Serial Bus (USB)�

• Universal Serial Bus (USB) is a wired high speed serial bus for data communication. The USB host can support connections up to 127, including slave peripheral devices and other USB hosts.

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USB Device connection Topology

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contd

• USB cable supports connection upto 5 metres
• USB standard uses two different types of cables for connecting USB peripheral device and host device
• Type A Connector (for up stream connection0
• Connection with host
• Type B Connector (for down stream connection)
• Connection with slave device

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Data transfers of USB

• USB supports 4 different types of data transfers:

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• Control Transfer (configure and issue commands to USB device)
• Bulk Transfer (Sending block of data to device)
• Isochronous Transfer (used for real time communication)
• Interrupt transfer (transferring small amounts of data)

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IEEE 1394 (Firewire)��

• IEEE 1394 (Firewire) is a wired, isochronous high speed serial communication bus. It is also known as High Performance Serial Bus (HPSB).

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• 1394 is a popular communication interface for connecting embedded devices like Digital Camera, Camcorder, Scanners to desktop computers for data transfer and storage.

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• Unlike USB interface, IEEE 1394 doesn’t require a host for communicating between devices. For example, you can directly connect a scanner with a printer for printing. The data rate supported by 1394 is far higher than the one supported by USB 2.0 interface. The 1394 implementation is much costlier than USB implemetation

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contd

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Infrared Data Association (IrDA)�

• Infrared (IrDA) is a serial, half duplex, line of sight based wireless technology for data communication between devices.
• It is in use from the olden days of communication and you may be very familiar with it. The remote control of your TV, VCD player, etc works on Infrared data communication principle.

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Bluetooth (BT) �

• Bluetooth is a low cost, low power, short range wireless technology
• for data and voice communication. Bluetooth supports point-to-point (device to device) and point-to-multipoint (device to multiple device broadcasting) wireless communication.
• A Bluetooth device can function as either master or slave. When a network is formed with one Bluetooth device as master and more than one device as slaves, it is called a Piconet. A Piconet supports a maximum of seven slave devices.
• Bluetooth is the favorite choice for short range data communication in handheld embedded devices. Bluetooth technology is very popular among cell phone users as they are the easiest communication channel for transferring ringtones, music files, pictures, media files, etc between neighboring Bluetooth enabled phones.
• It supports a data rate of up to 1 Mbps and a range of approximately 30 feet for data communication

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Wi-Fi �

• Wi-Fi or Wireless Fidelity is the popular wireless communication technique for networked communication of devices. Wi-Fi is intended for network communication and it supports Internet Protocol (IP) based communication. It is essential to have device identities in a multipoint communication to address specific devices for data communication.

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• Wi-Fi based communications require an intermediate agent called Wi-Fi router/Wireless access point to manage the communications. Wi-Fi supports data rates ranging from 1 Mbps to 150 Mbps and offers a range of 100 to 300 feet.

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ZigBee

• ZigBee is a low power, low cost, wireless network communication
• protocol based on the IEEE 802.15.4-2006 standard.
• ZigBee is targeted for low power, low data rate and secure applications for Wireless Personal Area Networking (WPAN).
• ZigBee operates worldwide at the unlicensed bands of Radio spectrum, mainly at 2.400 to 2.484 GHz, 902 to 928 MHz and 868.0 to 868.6 MHz.
• ZigBee supports an operating distance of up to 100 meters and a data rate of 20 to 250Kbps.

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• ZigBee device categories are as follows:
• ZigBee Coordinator (ZC)/Network Coordinator: The ZigBee coordinator acts as the root of the ZigBee network. The ZC is responsible for initiating the ZigBee network and it has the capability to store information about the network.
• ZigBee Router (ZR)/Full Function Device (FFD): Responsible for passing
• information from device to another device or to another ZR.
• ZigBee End Device (ZED)/Reduced Function Device (RFD): End device containing ZigBee functionality for data communication.

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General Packet Radio Service (GPRS)�

• GPRS is a communication technique for transferring data over a mobile communication network like GSM.
• GPRS supports a theoretical maximum transfer rate of 17.2 kbps.
• The GPRS communication divides the channel into 8 timeslots and transmits data over the available channel.

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Sensors and Actuators

• A sensor is a transducer device that converts energy from one form to another for any measurement or control purpose
• The changes in system environment or variables are detected by the sensors connected to the input port of the embedded system
• Actuator is a form of transducer device (mechanical or electrical) which converts signals to corresponding physical action (motion). Actuator acts as an output device.
•  If the embedded system is designed for any controlling purpose, the system will produce some changes in the controlling variabl
• It is achieved through an actuator connected to the output port of the embedded system.e to bring the controlled variable to the desired value

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Contd’

• If the embedded system is designed for monitoring purpose only, then there is no need for including an actuator in the system. 

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• For example, take the case of an ECG machine. It is designed to monitor the heart beat status of a patient and it cannot impose a control over the patient’s heart beat and its order. The sensors used here are the different electrode sets connected to the body of the patient. The variations are captured and presented to the user (may be a doctor) through a visual display or some printed chart

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I/O Subsystem 

• The I/O subsystem of the embedded system facilitates the interaction of the embedded system with the external world

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• The interaction happens through the sensors and actuators connected to the input and output ports respectively of the embedded system.  The sensors may not be directly interfaced to the input ports, instead they may be interfaced through signal conditioning and translating systems like ADC, optocouplers, etc

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contd

• Light Emitting Diode (LED)
• 7-Segment LED Display
• Optocoupler
• Stepper Motor
• Relay Piezo Buzzer
• Push Button Switch
• Keyboard
• Programmable Peripheral Interface (PPI)�

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