1-Wire Transactions in Java

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A complete communication with a 1-Wire device is called a transaction A transaction is divided into 3 phases 1 Initialization 2 Addressing 3 Data exchange The initialization phase consists of the bus master transmitting a reset pulse After receiving the reset pulse, all attached devices generate a presence pulse At this point the master knows that at least one device is attached to the bus After the initialization phase all devices are in a reset state waiting for the master to transmit one of the address layer commands Typically, during the addressing phase, a speci c device is targeted by broadcasting its entire 64-bit address This causes all but the addressed device to drop off the bus by transitioning to a high-impedance idle state waiting for the master to begin a new transaction The device whose address was broadcast is selected The 1-Wire addressing commands and the addressing phase is covered in more detail in Section 413 After a device has been selected, it is ready to receive device-speci c commands that allow access to the services it provides Different devices have different capabilities The details of special function commands and associated protocols and data speci c to a device family are described in a data sheet associated with the particular device Most application software doesn t need to worry about these details because the 1-Wire API hides them through an abstraction called a container, described in Section 44 However, applications with strict performance requirements may wish to communicate directly with the device
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Table 41 details a complete 1-Wire transaction for performing a temperature conversion using a DS18S20 temperature sensor Table 41
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Master Transmit Receive Transmit Transmit Transmit N/A
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Data Reset Presence pulse Address match (0x55) DS18S20 s address Convert temperature command (0x44) None Comments Reset all 1-Wire devices All devices announce their presence on the bus Devices wait for an address to be broadcast All other devices idle Send special function command Master leaves bus high for ~75s to provide power for conversion
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After the DS18S20 has nished with the temperature conversion, a second transaction is required to read the result This transaction is detailed in Table 42 Note that both transactions begin with a reset followed by device selection using the DS18S20 s address This is true of almost all 1-Wire transactions Table 42
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Master Transmit Receive Transmit Transmit Transmit Receive
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Data Reset Presence pulse Address match (0x55) DS18S20 s address Read scratchpad command (0xbe) Scratchpad data Comments Reset all 1-Wire devices All devices announce their presence on the bus Devices wait for an address to be broadcast All other devices idle Results of conversion are stored in the scratchpad Read the temperature data
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All 1-Wire devices contain a unique 64-bit address This address consists of three distinct parts, as shown in Figure 42
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The family code is used to determine the type (or family) of the 1-Wire device and therefore the services it provides For example, the family id for the DS2406 Dual Addressable Switch is 0x124 After reading the 1-Wire address and extracting the family id, the application knows it has discovered a switch with two switched I/O channels and 128 bytes of EPROM The Device id portion of the address can be viewed simply as a large number used to ensure uniqueness The CRC (Cyclic Redundancy Check) byte is used to ensure the integrity of both the family and device id The use of CRCs to protect the address as well as other data is covered in detail in Section 45 There are two methods by which 1-Wire devices are addressed Device discovery Device selection
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When an application that uses a 1-Wire network is started, it doesn t need to know the number, types, or addresses of the attached devices Device discovery is also referred to as an address search and allows the master to use a process of elimination to discover the addresses of all the devices on the network Once the host knows a device s address, its type and therefore the services it provides are easily attained by examination of the family id portion of the address Device selection is also called an address match and is used to select a speci c device given its address After the host application has discovered the addresses of all of the devices on the network, it uses the selection process to initiate further 1-Wire transactions targeted at a speci c device Let s consider device discovery and device selection in more detail For the following discussion it is helpful to represent a 1-Wire address, A, as an array of bits The array has 64 elements labeled A0 to A63, where A0 is the 0th element of the array and least signi cant bit of the address It is also useful to imagine that both the master and all 1-Wire chips maintain an iteration variable
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4 A list of all device types and associated family ids can be found online (see http:// wwwdalsemicom/products/autoinfo/familieshtml)
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