The Dallas Phantom Real Time Clocks are a family of
devices that offer the combination of a transparent
CMOS timekeeper and a nonvolatile static RAM meeting
the standard JEDEC bytewide pinouts. Some varieties
of the Dallas Phantom Real Time Clocks also provide
a transparent CMOS timekeeper for use with ROM.
The timekeeper is transparent to the RAM/ROM
memory map because it does not occupy any of the
existing RAM/ROM locations. These devices are
termed “Phantom” because the timekeeper is accessed
only when a predetermined 64–bit pattern has been
received by the device. When the timekeeper is not
being accessed, the RAM/ROM can be accessed normally.
The timekeeper keeps track of hundredths of
seconds, seconds, minutes, hours, day, date, month,
and year information. In the absence of power, a lithium
energy source maintains the timekeeping operation and
retains data in the CMOS static RAM.