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DS28E17 Datasheet(PDF) 9 Page - Maxim Integrated Products |
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DS28E17 Datasheet(HTML) 9 Page - Maxim Integrated Products |
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9 / 27 page  A multidrop bus consists of a 1-Wire bus with multiple slaves attached. The DS28E17 supports both a standard and overdrive communication speed of 15kbps (max) and 77kbps (max), respectively. The value of the pullup resis- tor primarily depends on the network size and load condi- tions. The DS28E17 requires a pullup resistor of 2.2kΩ (max) at any speed. The idle state for the 1-Wire bus is high. If for any reason a transaction needs to be suspended, the bus must be left in the idle state if the transaction is to resume. If this does not occur and the bus is left low, one or more devices on the bus could be reset. Transaction Sequence The protocol for accessing the DS28E17 through the 1-Wire port is as follows: ● Initialization ● ROM function command ● Device command ● Transaction/data Initialization All transactions on the 1-Wire bus begin with an initializa- tion sequence. The initialization sequence consists of a reset pulse transmitted by the bus master followed by presence pulse(s) transmitted by the slave(s). The pres- ence pulse lets the bus master know that the DS28E17 is on the bus and is ready to operate. 1-Wire ROM Function Commands Once the bus master has detected a presence, it can issue one of the seven ROM function commands that the DS28E17 supports. All ROM function commands are 8 bits long. A list of these commands follows. See the flow- chart in Figure 4a and Figure 4b. Read ROM [33h] The Read ROM command allows the bus master to read the DS28E17’s 8-bit family code, unique 48-bit serial number, and 8-bit CRC. This command can only be used if there is a single slave on the bus. If more than one slave is present on the bus, a data collision occurs when all slaves try to transmit at the same time (open drain produces a wired-AND result). The resultant family code and 48-bit serial number result in a mismatch of the CRC. Match ROM [55h] The Match ROM command, followed by a 64-bit ROM sequence, allows the bus master to address a specific DS28E17 on a multidrop bus. Only the DS28E17 that exactly matches the 64-bit ROM sequence responds to the subsequent Device command. All other slaves wait for a reset pulse. This command can be used with a single device or multiple devices on the bus. Search ROM [F0h] When a system is initially brought up, the bus master might not know the number of devices on the 1-Wire bus or their ROM ID numbers. By taking advantage of the wired-AND property of the bus, the master can use a pro- cess of elimination to identify the ID of all slave devices. For each bit in the ID number, starting with the least sig- nificant bit, the bus master issues a triplet of time slots. On the first slot, each slave device participating in the search outputs the true value of its ID number bit. On the second slot, each slave device participating in the search outputs the complemented value of its ID number bit. On the third slot, the master writes the true value of the bit to be selected. All slave devices that do not match the bit written by the master stop participating in the search. If both of the read bits are zero, the master knows that slave devices exist with both states of the bit. By choos- ing which state to write, the bus master branches in the search tree. After one complete pass, the bus master knows the ROM ID number of a single device. Additional passes identify the ID numbers of the remaining devices. Refer to Application Note 187: 1-Wire Search Algorithm for a detailed discussion, including an example. DS28E17 1-Wire-to-I2C Master Bridge www.maximintegrated.com Maxim Integrated │ 9 |
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