33mhz into a 25mhz 486?

[technoid]

Hi all,

Would like to replace the 25mhz main oscillator in my 486 to 33mhz (or is that 33.33mhz?). Any thoughts? The cpu is originally a 25mhz 486sx embedded, but I upgraded it to an AMD 5x86 100mhz (capable to 133mhz of course) in the overdrive/upgrade socket. What sort of odd timing dependencies will I run into? The 128k L2 motherboard cache is rated at 25mhz. The SIMMs are 80. If you need further specs, let me know. I have done this before with 286 motherboards (up to 25mhz I think, and with a 286-to-486 cpu adapter), but have never tried it with a 486.

Thanks.

 

[smeezekitty]

Well its possible for other components such as that cache and the chipset to be rated at 25 MHz. Also your ISA divider will probably be somewhat wrong

Some things will be overclocked which may or may not be stable

 

[Chuck(G)]

Um--what's your motherboard?

 

[orion24]

I'm thinking of doing the same to an IBM board of mine. Problem is I still try to identify which chip is the oscillator on that board. I'm pretty sure there wouldn't be any instability problem.

 

[krebizfan]

I'm thinking of doing the same to an IBM board of mine. Problem is I still try to identify which chip is the oscillator on that board. I'm pretty sure there wouldn't be any instability problem.

For most 25MHz systems, the ISA slots will be running at 1/3 the clock of the main bus. That gives about a nice normal 8 MHz ISA slot. Go up to 33 MHz bus and the slots are now running at 11 MHz which sometimes isn't stable. If the motherboard has lots of jumpers and the right one can be found, the ISA divisor can be changed to 4 getting back to the proper 8 MHz.

 

[orion24]

For most 25MHz systems, the ISA slots will be running at 1/3 the clock of the main bus. That gives about a nice normal 8 MHz ISA slot. Go up to 33 MHz bus and the slots are now running at 11 MHz which sometimes isn't stable. If the motherboard has lots of jumpers and the right one can be found, the ISA divisor can be changed to 4 getting back to the proper 8 MHz.

Playing with the ISA clock on a 386-486 board, all my ISA VGA and I/O cards were stable even at 12 MHz. The only thing I didn't test is the sound cards. The extra clock speed gave measurable performance gains in VGA benchmarks.

 

[deathshadow]

Which mobo is indeed a good question, since anything made after about 1990 that's not from IBM is already capable of supporting 25 or 33 base clock. Hell, if the motherboard wasn't capable of selecting the speed (or even the voltage) I'd be looking for another motherboard (you can typically pick up something decent off e-fence for chump change)... well, unless it was something very system specific like a PS/2...

For example the Biostar I have my AMD 5x86/133 running at 150mhz in; since that board will support 25mhz, 33mhz, 40mhz and 50mhz clocks in a whole slew of voltages, letting you run pretty much any 486 class processor ever made in it... I got that WITH the processor for a song and a dance just a year ago.

Sounds like you've even got the same processor, you get the FSB up to 50mhz it will go to 3x just fine; if your board is worth a flying purple fish it should have jumpers for setting that. (though there were a LOT of really crappy boards that didn't)

 

[smeezekitty]

If you have a higher end 486 like that (didn't notice it before)
You might as well just invest in a decent board. If it doesn't eve have 33mhz it must be awfully limited

 

[technoid]

Thanks for the replies thus far...

Um--what's your motherboard?

It is a VTech Laser 486SX/3. It's actually the same PC in an earlier thread I had here from a couple years ago, the one where I was trying to, and successfully, installed the external 128k cache. As I mentioned in that thread, this PC had no ext. cache when it was bought new.

If you have a higher end 486 like that (didn't notice it before)
You might as well just invest in a decent board. If it doesn't eve have 33mhz it must be awfully limited

Well, that's not the point of this whole game. I am trying to get this old PC as close to modern day (without killing it of course) and one of those little ways is to increase the system clock. Isn't that what upgrading, modding, and overclocking is about? So, a 25MHz to 33MHz increase is hopefully about 30% faster (if not only 3% faster, which would be ok too, lol) if I'm correct, just as long as it will work. No need to go higher, just baby steps are ok. This PC also has emotional attachment, had good memories playing DOOM for the first time, as well as installing Geoworks PC/GEOS 2.0 (which I still have) as a Windows 3.x replacement for awhile.

And no, there are no clock divider jumpers anywhere, nor are there any sort of settings in the BIOS. The board/system was bought at Incredible Universe (anyone remember that superstore?) at around 1992. Since the embedded 486 was an SX-25MHz, I'm guessing that's why the entire system is only set at that frequency. As I said earlier, I already upgraded the system to the AMD 5x86 486 100MHz (via an Evergreen CPU upgrade) back in the late 1990's and this was a nice improvement, but increasing the oscillator would be another plus, if possible.

Here are some specs (off the top of my head). All of these are "improvements", i.e. stuff I added to the motherboard years later...

- External tag cache speed: 20ns
- External data cache speed: 25ns (128mb max)
- Videocard: ATI Graphics Pro Turbo 2MB ISA
- Soundcard: Can't recall, it's some generic one w/MIDI
- NIC: Currently a Trendnet 10mbps. Will install my Intel Pro 10/100 ISA soon.
- CPU: Evergreen upgrade w/AMD 5x86 @ 100MHz
- RAM: 48MB 80ns Simms (max capacity)
- HDD: Currently two IDE 428MB drives
- O/S: Win 98SE
- Also has a 1.44 & 1.2 FDD's, and a 120MB tape backup.

There are other additions I'd like to make, depending on free time, but right now a speed increase in the oscillator department would be nice. But if it's unstable, as some of you are saying, then it's no biggie. I can still try, I believe (but not totally sure) that I have a 33MHz clock osc. laying around in my electronics junkbox already. Actually I'm using this PC at the front desk at work, heh, attached to an LCD monitor. It's really only being used for Office 97 and an old medical network database. Still gets the job done, even at a slightly slower pace.