DS1921G datasheet(17/45 Pages) DALLAS

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DS1921G Thermochron iButton

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generator and then shifting in the command code (0Fh) of the Write Scratchpad command, the Target

Addresses TA1 and TA2 as supplied by the master and all the data bytes. The master may end the Write

Scratchpad command at any time. However, if the ending offset is 11111b, the master may send 16 read

time slots and will receive an inverted CRC16 generated by the DS1921G.

The range 200h to 213h of the register page is protected during a mission. See Figure 6, Register

Page Map, for the access type of the individual registers between and during missions.

Read Scratchpad Command [AAh]

This command is used to verify scratchpad data and target address. After issuing the Read Scratchpad

command, the master begins reading. The first 2 bytes will be the target address. The next byte will be the

ending offset/data status byte (E/S) followed by the scratchpad data beginning at the byte offset (T4:T0),

as shown in Figure 9. Regardless of the actual ending offset, the master may read data until the end of the

scratchpad after which it will receive an inverted CRC16 of the command 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 will read logical 1s from the DS1921G until a reset pulse is issued.

Copy Scratchpad [55h]

This command is used to copy data from the scratchpad to the writable memory sections. Applying Copy

Scratchpad to the Sample Rate Register can start a mission provided that several preconditions are met.

See Mission Start and Logging Process description and the flow chart in Figure 11 for details. After

issuing 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

(Authorization Accepted) flag will be set and the copy will begin. A pattern of alternating 1s and 0s will

be 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 will be 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 will be copied, starting at the target address. Anywhere from 1 to 32

bytes may be copied to memory with this command. The AA flag will remain at logic 1 until it is cleared

by the next Write Scratchpad command. Note that Copy Scratchpad when applied to the address range

200h to 213h during a mission will end the mission.

Read Memory [F0h]

The Read Memory command may 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 may continue until the end of memory, at which point logic 0s will be read. It is im-

portant to realize that the target address registers will contain the address provided. The ending offset/data

status byte is unaffected.

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 reading 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

eliminate having to read a page multiple times to verify whether if the received data is correct. (See

Application Note 114 for the recommended file structure.)

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