What Do Computers Use As A Clock?

Some PCs have RAM modules that have onboard didisplays. Those would be microntoler-based, but not all PCs have them.

Many servers have a light-out remote management feature that is clearly microntoler-based, but again not all PCs have them.

For something that is on all PCs, I am thinking that the RTC might qualify.

Below is part of a typical CMOS RAM memory map, which contains the the time (controlled by the RTC) as well as BIOS configuration data (read by the main CPU).

00h 00 1 byte RTC seconds. 01h 01 1 byte RTC seconds alarm. 02h 02 1 byte RTC minutes. 03h 03 1 byte RTC minutes alarm. 04h 04 1 byte RTC hours. 05h 05 1 byte RTC hours alarm. 06h 06 1 byte RTC day of week. 07h 07 1 byte RTC date day. 08h 08 1 byte RTC date month. 09h 09 1 byte RTC date year. 0Ah 10 1 byte Status Register A Bit 7 = Update in progress 0 = Date and time can be read 1 = Time update in progress) Bits 6-4 = Time frequency divider 010 = 32.768KHz Bits 3-0 = Rate selection frequency 0110 = 1.024KHz 0Bh 11 1 byte Status Register B Bit 7 = Clock update cycle 0 = Update normally 1 = Abort update in progress) Bit 6 = Periodic interrupt 0 = Disable interrupt (default) 1 = Enable interrupt Bit 5 = Alarm interrupt 0 = Disable interrupt (default) 1 = Enable interrupt Bit 4 = Update ended interrupt 0 = Disable interrupt (default) 1 = Enable interrupt Bit 3 = Status register A square wave frequency 0 = Disable square wave (default) 1 = Enable square wave Bit 2 = 24 hour clock 0 = 24 hour mode (default) 1 = 12 hour mode) Bit 1 = Daylight savings time 0 = Disable daylight savings (default) 1 = Enable daylight savings

I don't know the internal details of the RTC, but I know it runs when the main CPU is powered down, and the above sure looks like memory used by a microcontroller of some sort. i just don't see the 24/12 hour function or the DTS function as being likely to be done in hardware. What do you think? I am not saying the RTC qualifies, just wondering if it does.

It's hardware like the early RTC chips (just a handful of gates are required), and is now integrated as part of a MUCH larger chip. Eg. this Southbridge chip:

"The M1573 integrates a High Definition (HD) Audio/AC?97 Host Controller, 2-channel dedicated Ultra-66/100/133 IDE Master controller, SATA Host Controller, USB 2.0/1.1 Host controllers, IO APIC controller, as well as 1/10/100 Mb/s Fast Ethernet MAC layer and PCI Express interface. Additional features include Host Signal Processing (HSP) software modem solution, ACPI support, green function, and Real Time Clock."

Maybe some of the high end motherboards have some kind of custom feature implemented with a micro. It's possible.

Best regards, Spehro Pefhany

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