• Please review our updated Terms and Rules here

Cyrix Cx5x86 Underclocking, Downclocking

L

lxx5101

Member
Joined
Apr 25, 2010
Messages
24
Location
Merida, Venezuela
Cyrix Cx5x86 Underclocking, Downclocking

Correction " The Micro is not a Cyrix cX5X86, it is an Am5x86 "

will like your opinion on this matter:

I have a PC Compatible Cyrix Cx5x86 running at 133MHz

I will like to Underclock the 5x86 and run the system at 16 Mhz
(in order to make it compatible with old software)

the mainboard has a 50Mhz integrated oscillator

the jumpers are set to: 50 Mhz x (2/3) x 4 = 133.33 Mhz
changing the jumper setting I've been able to run at 75Mhz, everything ok


I will like: 6 Mhz x (2/3) x 4 = 16Mhz

one solution is to replace the integrated oscillator, but I will like the
Pc to run at 133 Mhz and only when needed run at 16Mhz, (using the turbo switch)

I found this chip on Digikey LTC6903 ($3.90)
(1kHz – 68MHz Serial Port Programmable Oscillator)

this is from the datasheet
http://cds.linear.com/docs/Datasheet/69034fc.pdf

Underclocking.jpg


I plan to:
- Remove the 50 Mhz oscillator
- solder a socket (machined pins type) and try a 6Mhz oscillator (just to check)
- build a small PCB with a LTC6903 + pic, program the pic
- connect the turbo switch to the board
- plug the board, I intent to run MSDOS on that machine

Do you see any problems ?
timing issues with bridges, memory access ?
 
Last edited:
Wait, so you want to take the fastest pre-Pentium CPU and underclock it to that of a 286?
OpoQQ.jpg
 
Its going to be a LOT easier to just disable the onchip cache, I've heard of people using a pentium-133 and with just disabling the cache, it would run as fast as a 486.

But honestly if you have to have EXACTLY 16mhz, i think you are better of getting a 286 up and running.
 
The datasheet doesn't say you can't run the clock down to something stupid slow, (OTOH, the Am5x86, no relation, does). I'm not optimistic, but you can give it a try without making too much magic smoke.
 
How is the (2/3) multiplier is working its way into the equation? Is it through the BIOS or a jumper? Either way, the 2/3 multiplier option usually drops the PCI bus rate down, not the front-side bus. In which case, your 4X Cyrix is seeing 50 MHz x 4 = 200 MHz. Did 486 motherboards have an option to drop the whole FSB by some multiplier as well? If so, I've never seen it. With the Cyrix register bits, by way of DIR0, you can control the Core/bus ratio to 1x, 2x, 3x, or 4x. So it would seem to be, you can run this CPU at 25 MHz. If you wanted to run it closer to 16 MHz, you can probably see if your PLL has an unused ratio division output and rewire. You can also put in a different PLL circuit, or make your own. You can also try putting in a slower crystal oscillator. If your crystal is at 14.3 KHz, you'd want to find one that is 9.15 KHz. This way you can just use your existing PLL chip.
 
What exactly are you trying to run? Your best bet is to disable the cache. PM me and I can give you a "Cache ON/OFF" utility. Also, you can pick-up a 286 mobo for next to nothing on eBay.
 
I'll trade you a 286/386sx 12 or 16mhz motherboard, ram and cpu for the 586 and motherboard if your interested...
 
To run a Cyrix 5x86 CPU at 133 MHz and at 16 MHz with the push of a button will take some custom design work, as noted above.

If you let me know more about your application, maybe we can find another way to satisfy your requirements. I am open to trading your Cyrix CPU with other, pehaps more suitable, CPUs. I have all sorts of 386/486 CPUs, some at 16 MHz. Please include an image of your Cyrix CPU.
 
Thanks to all of you for your quick response

feipoa

How is the (2/3) multiplier is working its way into the equation? Is it through the BIOS or a jumper? Either way, the 2/3 multiplier option usually drops the PCI bus rate down, not the front-side bus. In which case, your 4X Cyrix is seeing 50 MHz x 4 = 200 MHz. Did 486 motherboards have an option to drop the whole FSB by some multiplier as well? If so, I've never seen it. With the Cyrix register bits, by way of DIR0, you can control the Core/bus ratio to 1x, 2x, 3x, or 4x. So it would seem to be, you can run this CPU at 25 MHz. If you wanted to run it closer to 16 MHz, you can probably see if your PLL has an unused ratio division output and rewire. You can also put in a different PLL circuit, or make your own. You can also try putting in a slower crystal oscillator. If your crystal is at 14.3 KHz, you'd want to find one that is 9.15 KHz. This way you can just use your existing PLL chip.
Click to expand...

after your comment I checked the board and turns out that I was totally wrong

The clock is not a 50 Mhz integrated oscillator (some another board has that kind of oscillator)

IT has a UM9515 + 14.318 Mhz crystal (sorry my mistake)
and the chip is not a cyrix It's a Am5x86 (again my mistake)

see the attached image
Um9515.jpg

Am5x86.jpg

The board has 3 jumpers for:
25 MHz
33 Mhz (that's where I got the 2/3, thinking in the 50 Mhz oscillator)
40 Mhz
50 Mhz

there is another jumper to select 4x or 3x (Amd and Cyrix share the same config,
the manual was checked as a cyrix chip )


I like your Idea to check for PLL unused ratio division, but haven't found the data sheet yet


I will like to have the normal 133 Mhz operation and a stupid slow mode if possible


------------------------------------------------------
Chuck(G)
(OTOH, the Am5x86, no relation, does)
Click to expand...

turns out this is the case (8 Mhz minimum) thanks for the tip

------------------------------------------------------

Agent Orange
What exactly are you trying to run? Your best bet is to disable the cache. PM me and I can give you a "Cache ON/OFF" utility. Also, you can pick-up a 286 mobo for next to nothing on eBay.
Click to expand...

if there is a software solution, perfect I will PM you, thanks

I intent to run

copywrite (max freq 33 Mhz, as far as I know)

copyIIpc (max freq 20 Mhz, as far as I know)

Disk explorer (max freq 33 Mhz, as far as I know)

all of them work fine at 16 Mhz in a 286 machine

--------------------------------------------------
luckybob
you have to have EXACTLY 16mhz
Click to expand...

no, may be anything below 33Mhz will work

--------------------------------------------------
Why ?? :D ( for almost everybody), NeXT

I have a 286 @ 16 Mhz but I don't have HDD for that machine
my LPT Zip died last year, I also live overseas, hard to get almost anything here
(my company has a PoBox in the US, but this is personal use, the shipping is expensive for me, I only import small parts)

my FDDs and/or Floppy Disks are giving me hard time, I have to copy everything twice

I prefer the 5x86 It has LBA HDD support and a PCI USB card
that works fine with MSdos

----------------------------------------------------------
twolazy

I'll trade you a 286/386sx 12 or 16mhz motherboard, ram and cpu for the 586 and motherboard if your interested..
Click to expand...

thanks I already have a 286 board, thanks for your offer
 

Attachments

  • Am5x86.jpg
    Am5x86.jpg
    29.5 KB · Views: 1
I think the moment you mentioned it was an AMD X5-133 instead of a Cyrix 5x86-133, you lost half your audience! To the best of my knowledge, I have the only currently in-use Cyrix 5x86-133. It was working fine with this forum up until about 1 month ago. There may have been some host-side software change that made it run too slow with NT4-IE6. Oh well, vogons still works fine.

I may have mixed up my KHz and MHz units for the oscillator. I'd have to hook it up to the scope to see. The concept is the same though. They often leave off the units on the package. I think you'll have a very difficult time finding the spec. sheet for the PLL circuit. Please let me know if you find it.
 
Thanks Agent Orange for the files !!!

It worked !!!!!!!


check out the results:

Jumper setting: 75 Mhz (25Mhz x 3)
Amibios detects: Am486Dx4-SE
Speed tool: quaplus 4.2

Cache Enabled

Turbo on: 72.50 Mhz
Turbo off: 72.32 Mhz

Cache Disabled:

Turbo on: 71.80 Mhz
Turbo off: 28.02 Mhz <-- Disk explorer, copywrite, copy II pC Work OK


Jumper setting: 133 Mhz (33Mhz x 4)
Amibios detects: Am5x86
Speed tool: quaplus 4.2

Cache Enabled

Turbo on: 128.82 Mhz
Turbo off: 128.27 Mhz

Cache Disabled:

Turbo on: 126.64 Mhz
Turbo off: 37.38 Mhz <-- Disk explorer, copywrite, copy II pC Work OK


also thanks to luckybob (same idea)
 
There's a DOS program called 'slowdown that can be used to slow the speed down purely with software. You can use it to get any speed you want.

It can be downloaded here: http://bretjohnson.us/
 
The problem with DOS "slowdown" programs is that they do so with rather large granularity; somewhat like taking your Maserati and driving 120 MPH between dead stops to get a longer average travel time.

Some systems that still use DMA channel 0 for memory refresh can be slowed by bumping up the refresh rate.

Another method would be to design a little ISA board that added wait states to the CPU.

One could also enable single-step interrupts (x86 interrupt 1) and kill some time between instructions. A few programs are smart enough to disable this, but most aren't.
 
FishFinger
There's a DOS program called 'slowdown
Click to expand...

I tried that before start this post, Didn't work, thanks

Chuck
Another method would be to design a little ISA board that added wait states to the CPU.
Click to expand...

Chuck do you mean like holding the /READY signal high for some cycles, until the end of a T2 cycle, release and wait for some more cycles, and then start all over again ?
 
I know my dell xps 486 has a feature in bios for XT/AT compatible mode and a pin header that can use a switch. Is getting a new motherboard an option? I can take pics if necessary. Its supports 486s 586s and Pent OD's. (mines not for sale btw LOL)

As for the isa card idea, why not use a Hercules monochrome/cga card or maybe an isa ems card? Should slow it down and won't have to build anything... The EMS has the ability to be turned on and off with software. So you could I guess just do a custom config.sys etc to choose which way you want it to run , regular or slow as a crawl ems mode. :)
 
Last edited:
twolazy

thanks for your reply

Is getting a new motherboard an option?
Click to expand...
I'm happy with the Agent orange/luckybob solution, It worked !!!


As for the isa card idea, why not use a Hercules monochrome/cga card
Click to expand...

I can try the Hercules idea, as soon as a I have a chance, I'll post the results

or maybe an isa ems card?
Click to expand...

sorry I don't have one


Chuck
Yeah, pretty much. That should work.
Click to expand...

I like your idea I will give a try

sounds like a CPU's mind control experiment :D
 
Last edited:
lxx5101 said:
The board has 3 jumpers for:
25 MHz
33 Mhz (that's where I got the 2/3, thinking in the 50 Mhz oscillator)
40 Mhz
50 Mhz
Click to expand...
Sorry for the late reply, but I did some testing on 486 boards to find their undocumented jumper settings.
Having 3 jumpers means theres 2x2x2 options. 4 are official, meaning theres 4 more you could try ;)
I've found the newer boards would often have a jumper for 60Mhz and one even had 66Mhz!
The older ones tend to have slower choices which are not documented. Perhaps this could help you in your efford to underclock some more?
Good luck ;)
 
hi Tetrium2

Having 3 jumpers means theres 2x2x2 options. 4 are official, meaning theres 4 more you could try
Click to expand...

that is a great idea

here the results

JP3A-C

A B C
on on on 33 Mhz
on on off No boot
on off on No boot
on off off 50 Mhz
off on on 40 Mhz
off on off No boot
off off on 25 Mhz
off off off 20 Mhz <-- new

thanks for your help
 
You must log in or register to reply here.
Back
Top