DS2438 Dallas Semiconducotr, DS2438 Datasheet - Page 10

• Manufacturer Part Number: DS2438
• Description: Smart Battery Monitor
• Manufacturer: Dallas Semiconducotr

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
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
MSb
8-BIT CRC CODE
LSb
MSb
MSb
MSb
2
2
2
2
23
15
31
7
2
2
2
2
MSb
22
30
14
6
48-BIT SERIAL NUMBER
2
2
2
2
21
29
13
5
2
2
2
2
20
12
28
4
(unit = 1s)
(unit = 1s)
(unit = 1s)
2
2
10 of 29
2
2
19
27
11
3
2
2
2
2
18
26
10
2
LSb
2
2
2
2
17
25
1
9
LSb
LSb
LSb
2
2
2
2
24
16
0
8
MSb
8-BIT FAMILY CODE (26h)
MSB
LSB
DS2438
LSb