DS2438

10 of 29

Two other time-related functions are available. The first is the Disconnect Timestamp, which is written to

by the DS2438 whenever it senses that the DQ line has been low for approximately 2 seconds. This

condition would signal that the battery pack has been removed from the system; the time when that

occurs is written into the Disconnect Timestamp register, so that upon replacement into the system, the

system can determine how long the device has been in storage, to facilitate self-discharge corrections to

the remaining battery capacity. After the disconnect has been detected, the DS2438 reverts to a sleep

mode, during which nothing is active except the real time clock.

The other timestamp is the End-of-Charge timestamp, which is written to by the DS2438 whenever it

senses that charging is finished (when current changes direction). This timestamp allows the user to

calculate the amount of time the battery has been in a discharge or storage state, again to facilitate self-

discharge calculations.

The format of the ETM, Disconnect, and End-of-Charge registers are as shown in Table 10. Refer to the

“Memory Map” section for the address location of the time-related registers.

TIME REGISTER FORMAT Table 10

2

7

2

6

2

5

2

4

2

3

2

2

2

1

2

0

LSB

MSb

(unit = 1s)

LSb

2

15

2

14

2

13

2

12

2

11

2

10

2

9

2

8

MSb

(unit = 1s)

LSb

2

23

2

22

2

21

2

20

2

19

2

18

2

17

2

16

MSb

(unit = 1s)

LSb

2

31

2

30

2

29

2

28

2

27

2

26

2

25

2

24

MSB

64-BIT LASERED ROM

Each DS2438 contains a unique ROM code that is 64 bits long. The first 8 bits are a 1-Wire family code

(DS2438 code is 26h). The next 48 bits are a unique serial number. The last 8 bits are a CRC of the first

56 bits. (See Figure 3.) The 64-bit ROM and ROM Function Control section allow the DS2438 to operate

as a 1-Wire device and follow the 1-Wire protocol detailed in the section “1-Wire Bus System.” The

functions required to control sections of the DS2438 are not accessible until the ROM function protocol

has been satisfied. This protocol is described in the ROM function protocol flow chart (Figure 5). The

1-Wire bus master must first provide one of four ROM function commands: 1) Read ROM, 2) Match

ROM, 3) Search ROM, or 4) Skip ROM. After a ROM function sequence has been successfully executed,

the functions specific to the DS2438 are accessible and the bus master may then provide any one of the

six memory and control function commands.

64-BIT LASERED ROM FORMAT Figure 3

8-BIT CRC CODE

48-BIT SERIAL NUMBER

8-BIT FAMILY CODE (26h)

MSb

LSb

MSb

LSb

MSb

LSb