I have one of those JCS286 router motherboards that comes with a 16 MHz capable chipset but 12 MHz CPU and crystal.

When I overclocked to 16 MHz, I could get into CMOS setup but had bizarre issues. Like page up would work but page down wouldn't, or something to that effect. Certain keys didn't work. And oddly it was reliably reproducible.

If I either went back to 12, or then put in a 16 MHz CPU, all worked fine.

keenmaster486 wrote on 2025-03-24, 17:08:

I've been searching around and can't find much evidence of anyone overclocking Intel 286 12.5 MHz chips. Everyone seems to focus […]
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I've been searching around and can't find much evidence of anyone overclocking Intel 286 12.5 MHz chips. Everyone seems to focus on the faster third party chips that can go to 25 MHz or beyond.

In my IBM 5170, before I acquired it someone had already overclocked the 6 MHz CPU it came with to 8 MHz. It's perfectly stable.

Has anyone tried going to 16 MHz with a 12.5 MHz Intel chip? Or do they just not run stable at higher frequencies since that was already the highest bin for Intel production? There's something about it being the original Intel 286 rather than a third party licensed chip that is attractive.

Hi, be careful, the original AT BIOS has a speed-lock.
If it runs between 6-8 MHz, everything is fine but otherwise, the BIOS prevents the AT from booting.

Third-party BIOSes don't have this limit.

I'm trying to overclock a 12.5 MHz Intel CPU in a different 286. I was just using the 5170 as an example.

keenmaster486 wrote on 2025-03-24, 20:59:

I'm trying to overclock a 12.5 MHz Intel CPU in a different 286. I was just using the 5170 as an example.

If you get a good Intel 12.5, If I changed to 60ns ram I could never overclock to 16 or 16.66mhz but usually could overclock to 14.37 or 15mhz.

Because finding in between clock cans was tough 14.37mhz it was.

Maybe very late Intel chips would overclock better but on my various systems it seemed like Intel 12.5 chips had very little overhead and oddly 70/80ns ram made things unstable past 12.5 on the machines I used. (Back in the day using 60ns ram was a bit of an aggravation)

Worth noting One of the 12.5mhz systems I had running at an even 15mhz actually burned a trace in the keyboard controller area.

Intel 12.5 chips are 1u process so they “should” overclock more easily but I could never get them to do anymore than the 286-10 I had. Maybe the cpu or maybe it was just the boards and ram I had.

Although I ended up with several Intel 12.5 systems from government/ tech college salvage auctions, I am told they are very rare to be found in an actual AT/Baby AT PC, with most going into some sort of machine/device/equipment.

I have a couple computers based on the Motorola 68010-12.5mhz cpu as well which is also somewhat rare in original space heater spec.