ST AN923 Application note

AN923

APPLICATION NOTE

TIMEKEEPER Rolling into the Year 2000 (21st Century, Y2K)

The member s o f the TIMEKEEPER family, from STMicroelectronics, ar e listed in Ta b le 1 . Each contains,
in a single-package footprint, non-volatile RAM (NVRAM) and a real time clock (RTC), offering dedicated
time-keeping and alarm functions.

Table 1. Family of TIMEKEEPER Devices

M41T56
M48T86 RTC primarily utilized for its timekeeping functions with little additional memory

M48T559 RTC with 64 Kbit memory that supports Address, Data Multiplexed I/O pins

Low cost RTC that is compatible with Serial I

2

C bus applications

M48T02, M48T12, M48T08, M48T18,
M48T58, M48T59, M48T35, M48T37

M48T201 TIMEKEEPER SUPERVISOR

CENTURY ROLLOVER

The date and time information is distributed over several internal registers, with two BCD digits in each

register. Fabricated in ST’s SRAM technology, the individual registers can be accessed by the user like a
conventional bank of SRAM (read accesses to read the time, write accesses to set a new time).

The year is divided into two fields: the Century fiel d, and t he Y ears field (each i s 8 bits wide, hold ing two
BCD digits). TIMEKEEPER devices that were designed in 1997, and after, have a register for each of
these fields; those that were designed before 1997, though, do not have the Century register.

At the turn of the century, all TIMEKEEPERS will increment the Years register from 99 to 00 (from
1001,1001 to 0000,0000). Those that have a Century register, will, at the same moment, increment it from
19 to 20 (from 0001,1001 to 0010,0000).

The Century register, where present, is positioned 14 address locations before the most significant byte
address, using the device’s 16-byte register map definition. For a 32K x 8 device, for example, the Century
register is at address location 7FF1h.

For TIMEK EEPER d e vices that do no t h a v e a Ce nt u r y r e g i ste r , but which utilize the 16-byte register map
definition, it is recommended that the user sets up the same location (location 7FF1h for a 32K x 8 device,
for example) for storing the Century information. This byte, of course, is not updated automatically by the
clock, but has to be updated manually by the user’s software. It is recommended that the user stores the
BCD value 19 (0001,1001) in the location now, and on 1st January 2000, at 12:00:01 AM, updates it to
the BCD value 20 (0010,0000).

TIMEKEEPER devi ces that do not have a Centu ry register , and that do not u tilize the 16-byte reg ister map
definition, can still adopt the same methodology. Any address location that is convenient to the application
can be used for storing the Century information.

Note: All TIMEKEEPERS automatically allow for 31, 30, 29 and 28 day months (correctly adjusting for
leap-years until the year 2100).

RTCs with 16 Kbit to 256 Kbit of additional asynchronous SRAM non-volatile
memory

May 2000 1/2

AN923 - APPLICATION NOTE

If you have any questions or suggestions concerning the matters raised in this document, please send
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to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not
authorized for use as cri tical comp onents in life support dev i ces or systems wi thout express written approval of STMicroe l ectronics.

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