Real Time Clock

 

 

 

 

 

The real time clock , or RTC, is part of a chip on the computer motherboard which is responsible for knowing what the correct time and date is.

The RTC is usually part of the CMOS battery powered memory chip.

It can therefore keep on working when the computer is switched off.

The RTC is basically a counter which counts the pulses coming from a crystal.

The counter is hardware programmed to divide down the pulses into units of time, and a typical counter will produce a series of outputs which indicate :-


Seconds
Minutes
Hours
Day of the week
Day of the month
Month
Year

These outputs are in BCD format, and are continuously updated as time and date passes. They are stored in the CMOS RAM section of the same chip.

Up until quite recently, all RTC`s only provided two digits for the year output, and this ouput ranged from 00 to 99.

The RTC provides no information therefore about the century.

The century information is added at a higher software layer, normally within the BIOS, which is the software layer that actually reads the output of the RTC.

As the RTC is basically a counter, when the year count gets up to 99, at the end of that year, it rolls over to 00, and then starts again.

Because there is no century information, there is no information to tell the BIOS that the century has changed.

So unless it can work it out for itself, the BIOS can think that the year is 1900 instead of 2000.

And this is only the first reason that pc`s might have problems.

There are two possible hardware solutions to this defect in the RTC :-

  • The first is to fit a small expansion card in one of the expansion slots within the PC - the card uses the 8 bit section of the standard ISA or EISA expansion slot.

    No other conections to the card are required.

  • The second solution is to fit a replacement CMOS chip, one which contains an RTC which outputs century digits as well as year digits, ie, it outputs four digits.

    These replacement CMOS chips are a plug in replacement for the existing CMOS chips in some PC`s, and are increasingly being used in new PC`s.

    However, there is no certainty that they will work in every PC.

There are also possible software solutions to the RTC defects, shown in later web pages.

A second problem that can be caused by the RTC not knowing about centuries, is that the RTC mignt not know about leap years - different centuries have different leap year configurations.

The year 1900 was not a leap year, but the year 2000 is.

So if the RTC only knows it is the year 00, does it put in a leap year, ie, does the RTC use the date 29th. February ?

And that is the next reason why PC`s have year 2000 problems !

 

© 1999 Ron Turner

Return to the Year 2000 index page