hundt@wind.bellcore.com (tom hundt) (08/04/87)
THEORY
In a every XT there is an 8284 clock controller chip. This chip has
a couple of unused inputs: F/~C and EFI. Unused, that is, unless you
have a Turbo clone. The F/~C (Frequency/not Crystal select) chooses
the clock input from one of two places: a crystal on X1-X2 inputs, or
an External Frequency Input on EFI. A one-speed motherboard will
always select F/~C low (selecting the crystal) and get its clock from
the X1-X2 inputs. A two-speed board will have the F/~C tied to the output
of one of the 8255s on board, and will have maybe a 24 MHz oscillator
going to the EFI. (Note: The chip has a built-in divide-by-three
on these inputs, so 24 MHz in gives 8 MHz processor clock.)
The BIOS controls the clock speed by toggling the appropriate 8255 output.
PRACTICE
In my, and many other, XT clones the following does the job:
OUT 61H, 40H for slow speed,
OUT 61H, 4CH for fast speed.
COMMENT
The clone companies give you really lousy schematics (like, you get
a non-turbo motherboard drawing with your turbo machine...) and
the way I found this info out was by using Debug, doing I (INPs) from
all the 8255 registers, and seeing what changed when I hit CrtlAlt-
to toggle speed.
QUESTIONS
Does the above work on AT clones also?
Does anyone have a clone XT that works differently?
You could presumably replace your 24 MHz oscillator with a 30 MHz one,
so as to get 10 MHz operation. Anyone tried this? (I've been too lazy
to take apart the *whole* machine to get the m-board out...)
Caution: Don't try this with a 4-layer motherboard!
Did anyone get the *real* schematics with their system?
Happy trials.
/-^-\ Thomas M. Hundt / BELLCORE Morristown NJ / hundt@bellcore.bellcore.com
| | {seismo|ihnp4|ucbvzx|decvax|ulysses|allegra|clyde}!bellcore!hundt
/--_--\ john@hpcvlo.HP.COM (John Eaton) (08/06/87)
<<<<< < < The BIOS controls the clock speed by toggling the appropriate 8255 output. < Clock switching with the F/C~ line is easy if you don't care about runt pulses during the switch. The problem is that most processors do care. Do any turbo machines actually sync the clock during the switch to prevent bad clock problems? John Eaton !hplabs!hp-pcd!john
hundt@wind.bellcore.com (tom hundt) (08/07/87)
>Clock switching... is easy if you don't care about runt pulses >Do any turbo machines actually sync the clock during the switch Yeah, I thought about that too... Does anyone out there have *real* schematics for a Turbo PC? Ie. ones with the turbo-switching included? Do they do special sync'ing? /-^-\ Thomas M. Hundt / BELLCORE Morristown NJ / hundt@bellcore.bellcore.com | | {seismo|ihnp4|ucbvax|decvax|ulysses|allegra|clyde}!bellcore!hundt /--_--\