Features and Overview of uart

Features:

• Asynchronous receiver and transmitter

• Data-format compliant with RS-232 serial-data format

• Burst rates up to 6 Mbits/second

• Data framing consists of start, optional parity, and stop bits

• Optional interrupt on receive register full and/or transmit buffer empty

• Parity, overrun, and framing error detection

• High level transmit and receive functions

Overview of uart:

The UART User Module is an 8-bit Universal Asynchronous Receiver Transmitter that supports duplex RS-232-compliant, data format serial communications over two wires. Received and transmitted data format includes a start bit, optional parity, and a stop bit. Programmable clocking and selectable interrupt or polling style operation is supported. Application Programming Interface (API) firmware routines are provided to initialize, configure, and operate the UART. An additional high level API is also provided that supports background command receiving and string printing.

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Why is ZigBee needed ?

There are a multitude of standards like Bluetooth and WiFi that address mid to high data rates for voice, PC LANs, video, etc. However, up till now there hasn't been a wireless network standard that meets the unique needs of sensors and control devices. Sensors and controls don't need high bandwidth but they do need low latency and very low energy consumption for long battery lives and for large device arrays.

There are a multitude of proprietary wireless systems manufactured today to solve a multitude of problems that don't require high data rates but do require low cost and very low current drain. These proprietary systems were designed because there were no standards that met their application requirements. These legacy systems are creating significant interoperability problems with each other and with newer technologies.

The ZigBee Alliance is not pushing a technology; rather it is providing a standardized base set of solutions for sensor and control systems.

• The physical layer was designed to accommodate the need for a low cost yet allowing for high levels of integration. The use of direct sequence allows the analog circuitry to be very simple and very tolerant towards inexpensive implementations.
• The media access control (MAC) layer was designed to allow multiple topologies without complexity. The power management operation doesn't require multiple modes of operation. The MAC allows a reduced functionality device (RFD) that needn't have flash nor large amounts of ROM or RAM. The MAC was designed to handle large numbers of devices without requiring them to be "parked".
• The network layer has been designed to allow the network to spatially grow without requiring high power transmitters. The network layer also can handle large amounts of nodes with relatively low latencies.

ZigBee is poised to become the global control/sensor network standard. It has been designed to provide the following features:

• Low power consumption, simply implemented
• Users expect batteries to last many months to years! Consider that a typical single family house has about 6 smoke/CO detectors. If the batteries for each one only lasted six months, the home owner would be replacing batteries every month!
• In contrast to Bluetooth, which has many different modes and states depending upon your latency and power requirements, ZigBee/IEEE 802.15.4 has two major states: active (transmit/receive) or sleep. The application software needs to focus on the application, not on which power mode is optimum for each aspect of operation.

• Even mains powered equipment needs to be conscious of energy. ZigBee devices will be more ecological than their predecessors saving megawatts at it full deployment. Consider a future home that has 100 wireless control/sensor devices,
• Case 1: 802.11 Rx power is 667 mW (always on)@ 100 devices/home & 50,000 homes/city = 3.33 megawatts
• Case 2: 802.15.4 Rx power is 30 mW (always on)@ 100 devices/home & 50,000 homes/city = 150 kilowatts
• Case 3: 802.15.4 power cycled at .1% (typical duty cycle) = 150 watts
• Low cost to the users means low device cost, low installation cost and low maintenance.
• ZigBee devices allow batteries to last up to years using primary cells (low cost) without any chargers (low cost and easy installation). ZigBee's simplicity allows for inherent configuration and redundancy of network devices provides low maintenance.
• High density of nodes per network
• ZigBee's use of the IEEE 802.15.4 PHY and MAC allows networks to handle any number of devices. This attribute is critical for massive sensor arrays and control networks.
• Simple protocol, global implementation
• ZigBee's protocol code stack is estimated to be about 1/4th of Bluetooth's or 802.11's. Simplicity is essential to cost, interoperability, and maintenance. The IEEE 802.15.4 PHY adopted by ZigBee has been designed for the 868 MHz band in Europe, the 915 MHz band in N America, Australia, etc; and the 2.4 GHz band is now recognized to be a global band accepted in almost all countries.

ZigBee/IEEE 802.15.4 - General Characteristics

• Dual PHY (2.4GHz and 868/915 MHz)
• Data rates of 250 kbps (@2.4 GHz), 40 kbps (@ 915 MHz), and 20 kbps (@868 MHz)
• Optimized for low duty-cycle applications (<0.1%)>
• CSMA-CA channel access
  - Yields high throughput and low latency for low duty cycle devices like sensors and controls
• Low power (battery life multi-month to years)
• Multiple topologies: star, peer-to-peer, mesh
• Addressing space of up to:
  - 18,450,000,000,000,000,000 devices (64 bit IEEE address)
  - 65,535 networks
• Optional guaranteed time slot for applications requiring low latency
• Fully hand-shaked protocol for transfer reliability
• Range: 50m typical (5-500m based on environment)

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Remote Configuration Commands of xbee

The API firmware has provisions to send configuration commands to remote devices using the Remote Command Request API frame. This API frame can be used to send commands to a remote module to read or set command parameters. The API firmware has provisions to send configuration commands (set or read) to a remote module using the Remote Command Request API frame Remote commands can be issued to read or set command parameters on a remote device.

Sending a Remote Command:

To send a remote command, the Remote Command Request frame should be populated with the 64-bit address and the 16-bit address (if known) of the remote device, the correct command options value, and the command and parameter data (optional). If a command response is desired, the Frame ID should be set to a non-zero value.

Applying Changes on Remote:

When remote commands are used to change command parameter settings on a remote device, parameter changes do not take effect until the changes are applied. For example, changing the BD parameter will not change the actual serial interface rate on the remote until the changes are applied.

Changes can be applied using remote commands in one of three ways:

• Set the apply changes option bit in the API frame

• Issue an AC command to the remote device

• Issue a WR + FR command to the remote device to save changes and reset the device.

Remote Command Responses:

If the remote device receives a remote command request transmission, and the API frame ID is non-zero, the remote will send a remote command response transmission back to the device that sent the remote command. When a remote command response transmission is received, a device sends a remote command response API frame out its UART. The remote command response indicates the status of the command (success, or reason for failure), and in the case of a command query, it will include the register value.

The device that sends a remote command will not receive a remote command response frame if:

• The destination device could not be reached

• The frame ID in the remote command request is set to 0.

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ZigBee Networks

Zigbee networks are called personal area networks (PAN). Each network contains a 16-bit identifier called a PAN ID.

ZigBee defines three different device types – coordinator, router, and end device.

Coordinator – Responsible for selecting the channel and PAN ID. The coordinator starts a new PAN. Once it has started a PAN, the coordinator can allow routers and end devices to join the PAN. The coordinator can transmit and receive RF data transmissions, and it can assist in routing data through the mesh network. Coordinators are not intended to be battery-powered devices. Since the coordinator must be able to allow joins and/or route data, it should be mains powered.

Router – A router must join a ZigBee PAN before it can operate. After joining a PAN, the router can allow other routers and end devices to join the PAN. The router can also transmit and receive RF data transmissions, and it can route data packets through the network. Since routers can allow joins and participate in routing data, routers cannot sleep and should be mains powered.

End Device – An end device must join a ZigBee PAN, similar to a router. The end device, however, cannot allow other devices to join the PAN, nor can it assist in routing data through the network. An end device can transmit or receive RF data transmissions. End devices are intended to be battery powered devices. Since the end device may sleep, the router or coordinator that allows the end device to join must collect all data packets intended for the end device, and buffer them until the end device wakes and is able to receive them. The router or coordinator that allowed the end device to join and that manages RF data on behalf of the end device is known as the end device’s parent. The end device is considered a child of its parent.

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API Operation in xbee

API operation is an alternative to transparent operation. The frame-based API extends the level to which a host application can interact with the networking capabilities of the module. When in API mode, all data entering and leaving the module is contained in frames that define operations or events within the module.

Transmit Data Frames (received through the DIN pin (pin 3)) include:

• RF Transmit Data Frame

• Command Frame (equivalent to AT commands)

Receive Data Frames (sent out the DOUT pin (pin 2)) include:

• RF-received data frame

• Command response

• Event notifications such as reset, associate, disassociate, etc.

The API provides alternative means of configuring modules and routing data at the host application layer. A host application can send data frames to the module that contain address and payload information instead of using command mode to modify addresses. The module will send data frames to the application containing status packets; as well as source, and payload information from received data packets.

The API operation option facilitates many operations such as the examples cited below:

->Transmitting data to multiple destinations without entering Command Mode

->Receive success/failure status of each transmitted RF packet

->Identify the source address of each received packet

RF modules that contain the following firmware versions will support API operation: 1.1xx (coordinator) and 1.3xx (router/end device).

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XBee

The XBee/XBee-PRO ZNet 2.5 OEM (formerly known as Series 2 and Series 2 PRO) RF Modules were engineered to operate within the ZigBee protocol and support the unique needs of low-cost, low-power wireless sensor networks. The modules require minimal power and provide reliable delivery of data between remote devices. The modules operate within the ISM 2.4 GHz frequency band and are compatible with the following:

• XBee RS-232 Adapter

• XBee RS-232 PH (Power Harvester) Adapter

• XBee RS-485 Adapter

• XBee Analog I/O Adapter

• XBee Digital I/O Adapter

• XBee Sensor Adapter

• XBee USB Adapter

• XStick

• ConnectPort X Gateways

• XBee Wall Router.

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RESISTORS

A resistor is a two-terminal electrical or electronic component that opposes as eletric current by producing a voltage drop between its terminals is accordance with ohm's law V=IR The electrical resistance is equal to the current through the resistor where the temperature remains the same. Resistors are used as part of electronic network and electronic circuit.




since the elecronics are moving under the influence of a constant force should be accelerated to higher and higher velocities their motion should contiued ecen after we switch off the electric field kepping the circuit complete as already told it does not happen the speed of the electrons does not increase continousely and the current stop as soon as the electric field is switched off.

thus the eletrons of a conductor during their motion face opposition to their free flow similar in many respect to meachanical friction is diffrent for diffrent material it also changes with the size of the conductor

CAUSE OF RESISTANCE:-

When a potential difference is applied across a conductor as electric field is set up across its two ends when it is free electrons gets accelerated during their motion. The motion opposed the opposition offered by the atoms as a result of which the electrons are showed down is termed as the resistance of the conductor. Thus resistance is the opposition to the flow of current through a conductor

LAWS OF RESISTANCE:-

• Varies directly as the length of the conductor
• It is inversely proportional to the area of cross section
• It depends upon the nature of material of the conductor

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