DS1925 datasheet(17/46 Pages) MAXIM

17 / 46 page

To enable password checking, set EPW to (AAh). The

default pattern of EPW, and any value other than AAh

allows the 64-bit read and 64-bit full access passwords to

be set. Once password checking is enabled, changing the

passwords and disabling password checking requires the

knowledge of the current full-access password.

Before enabling password checking, passwords for read-

only access and for full access (read/write/control) need

to be written to the password registers. Setting up a

password or enabling/disabling the password checking

is done in the same way as writing data to a memory

location, only the address is different. Because they are

located in the same memory page, both passwords can

be redefined at the same time.

The Read Access Password needs to be transmitted

exactly in the sequence RP0, RP1 . . . RP62, RP63. This

password only applies to XPC Read Memory function.

The DS1925 delivers the requested data only if the pass-

word transmitted by the master was correct or if pass-

word checking is not enabled. See Table 24 for the Read

Access Password Register bitmap. There is only write

access to this register. Attempting to read the password

reports all zeros. The password cannot be changed while

a mission is in progress.

The Full Access Password needs to be transmitted exact-

ly in the sequence FP0, FP1 . . . FP62, FP63. It affects

the functions XPC Read Memory, XPC Copy Scratchpad,

XPC Clear Memory, XPC Start Mission, and XPC Stop

Mission. The DS1925 executes the command only if the

password transmitted by the master was correct, or if

password checking is not enabled. See Table 25 for the

Full Access Password Register bitmap. There is only write

access to this register. Attempting to read the password

reports all zeros. The password cannot be changed while

a mission is in progress.

Due to the special behavior of the write access logic, the

Password Control register and both passwords must be

written at the same time. When setting up new passwords,

always verify (read back) the scratchpad before sending the

XPC Copy Scratchpad command. After a new password is

successfully copied from the scratchpad to its memory loca-

tion, erase the scratchpad by filling it with new data (write

scratchpad command). Otherwise, a copy of the passwords

remain in the scratchpad for public read access.

Table 23. Password Control Register Bitmap

Table 24. Read Access Password Register Bitmap

Table 25. Full Access Password Register Bitmap

DS1925

iButton High-Capacity Temperature Logger

with 122KB Data-Log Memory

www.maximintegrated.com

Maxim Integrated │ 17

ADDR

0227h

EPW

ADDR

0228h

RP7

RP6

RP5

RP4

RP3

RP2

RP1

RP0

0229h

RP15

RP14

RP13

RP12

RP11

RP10

RP9

RP8

. . .

022Eh

RP55

RP54

RP53

RP52

RP51

RP50

RP49

RP48

022Fh

RP63

RP62

RP61

RP60

RP59

RP58

RP57

RP56

ADDR

0230h

FP7

FP6

FP5

FP4

FP3

FP2

FP1

FP0

0231h

FP15

FP14

FP13

FP12

FP11

FP10

FP9

FP8

. . .

0236h

FP55

FP54

FP53

FP52

FP51

FP50

FP49

FP48

0237h

FP63

FP62

FP61

FP60

FP59

FP58

FP57

FP56

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