DS2415
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TIMEKEEPING
A 32.768kHz crystal oscillator is used as the time base for the real-time clock counter. The oscillator can
be turned on or off under software control. The oscillator must be on for the real-time clock to function.
The real-time clock counter is double-buffered. This allows 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 a
read/write buffer, which the user accesses.
DEVICE CONTROL BYTE
The on/off control of the 32.768kHz crystal oscillator is done through the device control byte. This byte
can be read and written through the Clock Function commands.
Device Control Byte
76543210
U4 U3 U2 U1
OSC
OSC 0 0
Bit 0 - 1 0 No function
Bits 0 and 1 are hard-wired to read all 0s.
Bit 2 - 3 OSC Oscillator Enable/Disable
These bits control/report whether the 32.768kHz crystal oscillator is running. If the oscillator is running,
both OSC bits will read 1. If the oscillator is turned off these bits will read 0. When writing the device
control byte both occurrences of the OSC bit should have identical data. Otherwise, the value in bit
address 3 (bold) takes precedence.
Bit 4 - 7 Un General-purpose user flags
These bits have no particular function within the chip. They can be read and written under the control of
the application software and remain non-volatile as long as there is sufficient voltage at the V
DD
pin. If
the DS2415 is located inside a battery pack, for example, these bits could convey data on the charging
status from the charging station to the equipment that uses the battery.
Real-Time Clock
The real-time clock is a 32-bit binary counter. It is incremented once per second. 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 a reference point.
CLOCK FUNCTION COMMANDS
The “Clock Function Flow Chart” (Figure 5) describes the protocols necessary for accessing the real-time
clock. With only four bytes of real-time clock and one control byte the DS2415 does not provide random
access. Reading and writing always starts with the device control byte followed by the least significant
byte (LSB) of the time data.
