DS1925 datasheet(20/46 Pages) MAXIM

20 / 46 page

master does not need to continue reading; it can start

a new trial to write data to the scratchpad. Similarly, a

set AA flag indicates that the Write command was not

recognized by the device. If everything went correctly,

both flags are cleared and the ending offset indicates the

address of the last byte written to the scratchpad. Now

the master can continue verifying every data bit. After the

master has verified the data, it has to use the XPC Copy

Scratchpad command. This command sequence uses the

three address registers TA1, TA2, and E/S as the master

has read them verifying the scratchpad. As soon as the

XPC Copy Scratchpad starts, it copies the data to the

requested location beginning at the target address.

Memory and Control

Function Commands

Figure 9 describes the protocols necessary for access-

ing the memory and the special function registers

of the DS1925. See the Mission Example: Prepare

and Start a New Mission section for how to use these and

other functions to set up the DS1925 for a mission. The

communication between master and DS1925 takes place

either at standard speed (default, OD = 0) or at overdrive

speed (OD = 1). If not explicitly set into the overdrive

mode, the DS1925 assumes standard speed. Internal

memory access during a mission has priority over external

access through the 1-Wire interface. This affects sev-

eral of the commands described below. See the Memory

Access Conflicts section for details and remedies.

Write Scratchpad Command [0Fh]

After issuing the Write Scratchpad command, the master

must first provide the 2-byte target address, followed by

the data to be written to the scratchpad. The data is writ-

ten to the scratchpad starting at the byte offset (T[4:0]).

When executing the Write Scratchpad command, the

CRC generator inside the DS1925 (see Figure 15) calcu-

lates a CRC of the entire data stream, starting at the com-

mand code and ending at the last data byte sent by the

master. This CRC is generated using the CRC-16 poly-

nomial by first clearing the CRC generator and then shift-

ing 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. If the ending offset

is 11111b, the master may send 16 read time slots and

receive the inverted CRC-16 generated by the DS1925.

Note that both register pages are write protected dur-

ing a mission. Although the Write Scratchpad command

works normally at any time, the subsequent XPC Copy

Scratchpad to a register page fails during a mission.

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 are the target

address. The next byte is the ending offset/data status

byte (E/S) followed by the scratchpad data beginning at

the byte offset (T4:T0), as shown in Figure 8. The master

may continue reading 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 1s

from the DS1925 until a reset pulse is issued.

DS1925

iButton High-Capacity Temperature Logger

with 122KB Data-Log Memory

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