First post, by mkarcher
Dallas RTC chips (like the DS1287, DS12887 and DS12887A) are widely used on vintage motherboards. They have a specified life of 10 years and generally are useless after 20 years of operation. But this only applies if the clock is actually running. The Dallas data sheet at https://pdfserv.maximintegrated.com/en/ds/DS1 … 5-DS12C887A.pdf indicates a significantly lower power consumption if the "clock is not running". It lists 500nA maximum for running clock at 25°C and 100nA maximum for stopped clock over the whole operating temperature range which is up to 70°C. At 25°C, the actual power consumption is very likely considerable lower. The datasheet also tells you how to stop the clock by writing the "divider select" bits in the clock control register A.
This actually works in practice. I ordered 4 Dallas DS12887A chips from China, and they arrived as New Old Stock with a date code from 97 or 98. I did not yet expect the battery to contain any significant voltage and immediately ground the plastic case to get access to the power wires. I was surprised to measure a Lithium cell voltage of 3.2 volts, which indicates a nearly unused cell. The chips work perfectly, despite of their age.
This means that one can significantly extend the lifetime of a Dallas RTC by powering the oscillator down, just as we parked hard drives in the 80s before moving a computer.
To create a program that is able to stop the clock, run debug at the DOS prompt and input the following lines. It will create a file called stoprtc.com, which can be run from the DOS prompt to stop the clock (you will not lose CMOS settings by that). You should turn off AC power after running STOPRTC, without running a shutdown procedure. Otherwise, the operating system or the BIOS might possibly turn the clock back on (e.g. when making sure the RTC alarm is correctly programmed for the next wake-up)
a100mov al,8aout 70,almov al,06out 71,alint 20rcxanstoprtc.comwq
