View Full Version : IBM AT clone clock speed issue

October 29th, 2017, 10:13 AM
IBM AT clone. Works well, except system timer is slow. Only about 11 seconds pass on the system timer during an actual 60 seconds.

Swapped the BIOS to a similar board and vice versa with the same BIOS revision. No issues there. Seems to be an issue with the board itself.

Never seen this problem myself before, any ideas?


Unrelated, but this one did not have a keyboard BIOS installed - do these boards actually need one?

October 29th, 2017, 10:48 AM
Most do, yes--the keyboard controller manages a number of tasks, such as switching into real mode from protected mode.

I assume that you're not talking about the "CMOS" RTC chip's timekeeping, but rather the time seen by DOS. If it's the former, check the 32.767 KHz crystal on the chip. If the latter, the very-late timer can be caused by either a hardware fault (8254) or by the IRQ 0 (interrupt 8 ) routines not getting service when needed.

October 29th, 2017, 01:41 PM
So far I have only been testing with the keyboard and video card installed. I may not have the equipment to test the accuracy of the crystal on the existing chip, but I do have a couple dead AT boards I could salvage another RTC chip from.