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IBM PC and AT Real Time Clock RTC Implementation

smplfyi

Experienced Member
Joined
Jan 31, 2007
Messages
259
Location
Warwickshire
Everybody,

Take a seat before you start to laugh. I'll eventually blog it (when with your forum help you have helped me uncover the technical detail). But I've found that the reason I've been waking up at exactly 03.00 numerous times in the last month is that an oldish PC in my study keeps powering on, downloading updates, then shutting down.

I caught it doing so yesterday since I was up late working on some code etc.

SO, I'm trying to remember how the IBM PC AT implemented the RTC circuitry to enable it to set a timer and power on the machine.

The original RTC for the AT was an MC 146818 but looking at the specs and some old 1980's hardware guides I don't see a way of it setting a timer to execute a poweron at any time.

Is this the case, i.e. was the AT design of the mid 1980's capable of receiving an instruction to wake the PC up at some later time? And if so EXACTLY HOW.

i RECALL that the RTC was home to multiple status bytes which were used for Disk hardware and time etc, but I don't recall any scheduling power abliity.

Comments please? And detailed answers would be welcome so when I know I'll publish at majzel.blogspot.com as usual.

thanks

marcus bennett
 
Looking at the MC146818 datasheet we can see that for a specific hour, minute and second we can set an alarm interrupt. This output could be used (with some additional logic) to activate the main power supply. So we can start every day - no longer off-phases.

I doubt the AT had such a mechanism because it has a mechanical power switch. ATX introduced this option as it provides a standby power supply to waste electricity.
 
I'm pretty sure wake timers are an ACPI feature. It uses your RTC to set an alarm, then the system power management responds to the alarm by waking up the system.
 
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