286 modification

[TidalWave]

Hi,


For fun I am "upgrading" a 286 from 12 to 16 MHz. I already exchanged the processor with a 16 MHz type, but now I have to speed up the bus clock since the 286 has an 1x multiplier.

I thought it was as simple as changing the crystal (oscillator)which sets the bus speed, but I see a lot of them and I am not shure which one is responsible for the 12 MHz bus clock.

Does anybody know which oscillator to change, and what rating the new oscillator should have?

Unfortunately it is not as easy as "looking for a 12 MHz one and zip it with a 16 MHz type".

Thanks,

Mike.

[Mstoumba]

The last time I did one "many life's ago" it was the one that was pluged into a socket that was mounted on the system board.

[TidalWave]

Unfortunately they are all non-socketed and thus means soldered.

Well, in addition I list here the following available oscillators on the mobo:

- osc1 36.0000 MHz
- osc2 25.1750 MHz
- osc3 42.000 MHz
- osc4 28.322 MHz
- osc5 9.600 MHz
- osc6 1.8432 MHz
- osc7 14.31818 MHz
- osc8 25.000 MHz
- osc9 16.000 MHz
- osc10 30.000 MHz

One of them is responsible for the 12 MHz clock on which the processor depends, where osc 7 to 10 are located somewhere around the processor. I guess osc8 is the one since the processor runs actually at 12.5 MHz, so it could be divided by 2.

Any ideas guys?

[Bogg]

The 14.31818 osc still exists on todays computers, in one way or another (don't argue, it might not be so anymore, but on most of the Pentiums and Pentium MMX's i have seen it is so).
So it should have something to do with the whole deal,...but i have no clue really..

[darkclaw]

Pretty sure it is the 25.000 osc. Not positive but I know on all my macs they're divided by 2 to get the processor speed. I think Apple lifted that idea from IBM.

[TidalWave]

Bogg,

That frequency is somewhat a standard from where a lot of other frequencies are derived from, still on pentiums, thats right.

darkclaw,

I know about the divide by 2 or 3 trick to get a more accurate clock signal. So I think I will try to change the 25 MHz osc.

First with a 32 MHz one, and maybe a 36 MHz one to overclock this board ;)


I let you know what the result is.


Cheers,

Mike.

[arthur888]

It is the 25mhz. I've done this trick with some old 286 and 386 computers and for the cpu frequency it is divided by two. Maybe you have to change the bios setting for isa bus frequency, because many old isa cards can't handle higher frequencies than the 8mhz they were designed for. Good luck

Arthur

BTW: Eindhoven de gekste!!!!!!!!!!!!!!!

[TidalWave]

Cool!

Thanks,

I 'm up to getting this on!

btw Almere sucks :D