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Maxim > Design Support > Technical Documents > Application Notes > Real-Time Clocks > APP 52
Keywords: phantom clock, real time clocks, RTCs, DS1216, DS1315, DS1244, DS1248, DS1251, DS1254
APPLICATION NOTE 52
Using the Dallas Phantom Real-Time Clocks
(RTCs)
Sep 25, 2002
Abstract:
This application
note describes how to interface a Phantom Real-Time Clock (RTC) in a
microcontroller memory system. It also describes some of the programming considerations and common
problems.
Description
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.
Family Overview
DS1315
The heart of the Dallas Phantom Real-Time Clock family is the DS1315 Phantom Time Chip. This
integrated circuit is a combination of a CMOS timekeeper and a nonvolatile memory controller. In the
absence of power, an external battery maintains the timekeeping operation and retains data in the CMOS
static RAM. The watch keeps track of hundredths of seconds, seconds, minutes, hours, day, date, month,
and year information. The last day of the month is automatically adjusted for months with less than 31
days, including correction for leap year every four years. The real-time clock operates in one of two
formats: 12-hour mode with an AM/PM indicator or a 24-hour mode. The nonvolatile controller supplies
all the necessary support circuitry to convert a CMOS RAM to a nonvolatile memory. The DS1315 can
also be used to provide timekeeping functions with ROM.
DS1216
Stemming from the DS1315 are the DS1216 SmartWatch Intelligent Sockets. The SmartWatch is a 600-
mil wide DIP socket with a built-in DS1315 (providing timekeeping functions and a nonvolatile RAM
controller), an embedded lithium energy source, and a 32.768kHz crystal. When the socket is mated with
a bytewide CMOS static RAM, it provides a complete solution to problems associated with memory
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