DS2437S Maxim Integrated Products, DS2437S Datasheet - Page 9

• Manufacturer Part Number: DS2437S
• Description: IC MONITOR SMART BATTERY 16-SSOP
• Manufacturer: Maxim Integrated Products

Specifications of DS2437S

• Function: Battery Monitor
• Battery Type: Smart Batteries
• Voltage - Supply: 2.7 V ~ 10 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

DS2437
OPERATION – REAL TIME CLOCK
The 32.768 kHz crystal oscillator is used as the time base for the timekeeping functions. The timekeeping
functions are double-buffered, allowing the master to read time without the data changing while it is
being read. To accomplish this, a snapshot of the counter data is transferred to holding registers which the
user accesses. This occurs after the eighth bit of the Recall Memory command.
The real-time clock is a 4-byte binary counter with a 1-second resolution. The four bytes are a count of
seconds. The real-time clock can accumulate 136 years of seconds before rolling over. Time/date is
represented by the number of seconds since a reference point, which is determined by the user. For
example, 12:00 A.M., January 1, 1970 could be used as a reference point.
Two other time-related functions are available. The first is the DISCONNECT TIMESTAMP, which is
written to by the DS2437 whenever it senses that the DQ line has been low for more than 1 second. 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 DS2437 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 DS2437 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 RTC, Disconnect, and End of Charge registers are as shown in Table 6. Refer to the
“Memory Map” section for the address location of the time-related registers.
Time Data Relationships Table 6
Crystal Selection
A 32.768 kHz crystal, Daiwa Part No. DT26S, Seiko Part No. DS-VT-200 or equivalent can be directly
connected to the DS2437 via pins 11 and 13 (X1, X2). The crystal selected for use should have a
specified load capacitance (CL) of 6 pF. The crystal is connected directly to the X1 and X2 pins. There is
no need for external capacitors or resistors. Note: X1 and X2 are very high impedance nodes. It is
recommended that they and the crystal be guard ringed with ground and that high frequency signals be
kept away from the crystal area. For more information on crystal selection and crystal layout
considerations, please consult Application Note 58, “Crystal Considerations with Dallas Real Time
Clocks.”
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