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DS1921G Datasheet(PDF) 21 Page - Maxim Integrated Products |
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DS1921G Datasheet(HTML) 21 Page - Maxim Integrated Products |
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21 / 42 page  Read Scratchpad [AAh] This command is used to verify scratchpad data and target addresses. After issuing the Read Scratchpad command, the master begins reading. The first 2 bytes are the target address. The next byte is the ending off- set/data status byte (E/S) followed by the scratchpad data beginning at the byte offset T[4:0], as shown in Figure 9. Regardless of the actual ending offset, the master can read data until the end of the scratchpad after which it receives an inverted CRC-16 of the com- mand code, target addresses TA1 and TA2, the E/S byte, and the scratchpad data starting at the target address. After the CRC is read, the bus master reads logical “1”s from the DS1921G until a reset pulse is issued. Copy Scratchpad [55h] This command is used to copy data from the scratch- pad to the writable memory sections. Applying a Copy Scratchpad command to the Sample Rate register can start a mission provided that several preconditions are met. See the Mission Start and Logging Process sec- tion and the flowchart in Figure 11 for details. After issu- ing the Copy Scratchpad command, the master must provide a 3-byte authorization pattern, which can be obtained by reading the scratchpad for verification. This pattern must exactly match the data contained in the three address registers (TA1, TA2, E/S, in that order). If the pattern matches, the AA flag is set and the copy begins. A pattern of alternating “1”s and “0”s is transmitted after the data has been copied until the master issues a reset pulse. While the copy is in progress, any attempt to reset the part is ignored. Copy typically takes 2µs per byte. The data to be copied is determined by the three address registers. The scratchpad data from the begin- ning offset through the ending offset is copied, starting at the target address. Anywhere from 1 to 32 bytes can be copied to memory with this command. The AA flag remains at logic 1 until it is cleared by the next Write Scratchpad command. Note that the Copy Scratchpad command, when applied to the address range 200h to 213h during a mission, ends the mission. Read Memory [F0h] The Read Memory command can be used to read the entire memory. After issuing the command, the master must provide the 2-byte target address. After the 2 bytes, the master reads data beginning from the target address and can continue until the end of memory, at which point logic “0”s are read. It is important to realize that the target address registers contain the address provided. The ending offset/data status byte is unaf- fected. The hardware of the DS1921G provides a means to accomplish error-free writing to the memory section. To safeguard data in the 1-Wire environment when read- ing and to simultaneously speed up data transfers, it is recommended to packetize data into data packets of the size of one memory page each. Such a packet would typically store a 16-bit CRC with each page of data to ensure rapid, error-free data transfers that elim- inate having to read a page multiple times to verify if the received data is correct (refer to Application Note 114: 1-Wire File Structure for the recommended file structure). Thermochron iButton ______________________________________________________________________________________ 21 |
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