VOGONS


Reply 40 of 53, by Anonymous Coward

User metadata
Rank l33t
Rank
l33t

I would confirm your suspicions with another person who has performed the upgrade before desoldering.

"Will the highways on the internets become more few?" -Gee Dubya
V'Ger XT|Upgraded AT|Ultimate 386|Super VL/EISA 486|SMP VL/EISA Pentium

Reply 41 of 53, by scorp

User metadata
Rank Newbie
Rank
Newbie

Hi guys, sorry for reviving this thread, but I have a question regarding 386sx upgrade. It is not related to the Cyrix deiscussion, but to 386sx upgrade topic in general. I have here a nice mainboard, the same one as this Looking for 386 motherboard ID (3SIUD), but with i386sx-16. Just for fun I was experimenting with overclocking and had apparently Am386sx-33 around, so I replaced the CPU. The old 16MHz CPU worked absolutely stable at 20MHz, but refused to work at 25MHz completely. As I can see, the same mainboard was used with Am386sx-25, so I thought, I'll give it a try. Well, I ended up with working, but unstable system at 25MHz and it doesn't work at 33MHz now. At 16MHz however it works just fine. Do you have any suggestions, if there is something I can do? Is it possible, that the chipset doesn't support anything above 25MHz? Why do I get then instability issues with 25MHz in my case? There are obviously 25MHz versions of the same mainboard in the wild?

My Youtube channel (Necroware):
https://www.youtube.com/channel/UCKU5nwSWYXa3xfXGBlKyvdw

Reply 42 of 53, by Anonymous Coward

User metadata
Rank l33t
Rank
l33t

If the CPU is a really old one, then maybe there just isn't enough overclocking headroom.
Do you know the production date of the CPU? Does it have "C-Step" printed on it?

"Will the highways on the internets become more few?" -Gee Dubya
V'Ger XT|Upgraded AT|Ultimate 386|Super VL/EISA 486|SMP VL/EISA Pentium

Reply 43 of 53, by scorp

User metadata
Rank Newbie
Rank
Newbie

The old CPU marking is NG80386SX-16 (L1344284), however, the old CPU is not the problem, I can even overclock it to 20MHz. The thing is, that the "new" CPU is actually Am386SX/SXL-33 with markings NG80386SXL-33, NG80386SX-33 (C 224BA8E), however I can't get it booting with 33MHz and even 25MHz seem to be instable. The question is, if that's motherboard issue, or not? I mean, can all 386sx chipsets handle 25/33 MHz CPUs?

My Youtube channel (Necroware):
https://www.youtube.com/channel/UCKU5nwSWYXa3xfXGBlKyvdw

Reply 44 of 53, by candle_86

User metadata
Rank l33t
Rank
l33t
scorp wrote on 2020-07-16, 14:00:

The old CPU marking is NG80386SX-16 (L1344284), however, the old CPU is not the problem, I can even overclock it to 20MHz. The thing is, that the "new" CPU is actually Am386SX/SXL-33 with markings NG80386SXL-33, NG80386SX-33 (C 224BA8E), however I can't get it booting with 33MHz and even 25MHz seem to be instable. The question is, if that's motherboard issue, or not? I mean, can all 386sx chipsets handle 25/33 MHz CPUs?

What's the crystal for the Isa bus or it's divider.

Phenom II X4 840T @ 4ghz - ASUS M3N72D-SLI - GTX 560 Ti- 4GB DDR2 1066 - 1TB HDD - Windows XP
Pentium 4 3.4C - MSI 865PE NEO2 - x850 XT PE - 2GB DDR 400 - 500GB HDD - Windows XP
Duron 1600 - ASUS A7N8X - 512MB DDR 266 - Radeon 8500 LE

Reply 45 of 53, by scorp

User metadata
Rank Newbie
Rank
Newbie

I thought about that and, unfortunately, I'm not sure. For the CPU I tried four crystals 32, 40, 50 and 66 MHz. For the old 16MHz CPU I could boot with 32 and 40MHz crystals. With the 33MHz CPU I can boot with 32,40 and 50 MHz. For 33MHz CPU I'd actually need 66MHz crystal. That wouldn't even be overclocking yet. Anyway, regarding the divider, there are no jumpers for it and in BIOS I can't set anything either. However, I measured the frequency on ISA B20 pin and got 7,16MHz, which should be ok, I guess. The chipset obviously tries to keep the ISA Bus at that frequency, doesn't matter which crystal I'm using. Or do I understand it wrong and B20 is completely unrelated?

My Youtube channel (Necroware):
https://www.youtube.com/channel/UCKU5nwSWYXa3xfXGBlKyvdw

Reply 46 of 53, by scorp

User metadata
Rank Newbie
Rank
Newbie

Hi, this mainboard has the chipset UM82C366F by UMC. Does anybody know, where I can get the datasheet for it? Unfortunately google gives me nothing, but may be there are some "secret" places whatsoever 😀

My Youtube channel (Necroware):
https://www.youtube.com/channel/UCKU5nwSWYXa3xfXGBlKyvdw

Reply 47 of 53, by Anonymous Coward

User metadata
Rank l33t
Rank
l33t

If ISA is 7.16MHz it sounds like it's probably derived from the OSC signal. The other two methods being derivative of system clock, and asynchronous mode. So it seems ISA clock is not your problem. Is there no way to adjust memory timings? What kind of RAM is currently installed? If any of the RAM is less than 70ns it might be a problem. Especially check the DIP RAM if installed.
I'm not familiar with the UMC 286 and 386SX Chipsets, but you wouldn't think 25MHz would be much of a challenge for those.

"Will the highways on the internets become more few?" -Gee Dubya
V'Ger XT|Upgraded AT|Ultimate 386|Super VL/EISA 486|SMP VL/EISA Pentium

Reply 48 of 53, by scorp

User metadata
Rank Newbie
Rank
Newbie
Anonymous Coward wrote on 2020-07-17, 12:24:

If ISA is 7.16MHz it sounds like it's probably derived from the OSC signal. The other two methods being derivative of system clock, and asynchronous mode. So it seems ISA clock is not your problem. Is there no way to adjust memory timings? What kind of RAM is currently installed? If any of the RAM is less than 70ns it might be a problem. Especially check the DIP RAM if installed.
I'm not familiar with the UMC 286 and 386SX Chipsets, but you wouldn't think 25MHz would be much of a challenge for those.

A good point, I'll try to install 60ns memory and remove DIP RAM chips. Currently I have 80ns SIMMs and 70ns DIPs in there.

My Youtube channel (Necroware):
https://www.youtube.com/channel/UCKU5nwSWYXa3xfXGBlKyvdw

Reply 49 of 53, by ph4nt0m

User metadata
Rank Member
Rank
Member

80ns RAM is generally good up to 33MHz and 70ns up to 50MHz, but it depends. 386SX based mainboards were low end back in the day, so don't expect many good capacitors there. Even if there are no electrolytic ones, I'd suggest to replace some on the +5V rail near the CPU, chipset and RAM slots with 100uF or 47uF tantalum capacitors 10V+ rated. It helped me to bring many mainboards back in the business.

Last edited by ph4nt0m on 2020-07-18, 12:28. Edited 2 times in total.

My Active Sales on CPU-World

Reply 50 of 53, by scorp

User metadata
Rank Newbie
Rank
Newbie
ph4nt0m wrote on 2020-07-18, 12:24:

80ns RAM is generally good up to 33MHz and 70ns up to 50MHz, but it depends. 386SX based mainboards were low end back in the day, so don't expect many good capacitors there. Even if there are no electrolytic ones, I'd suggest to replace some on the +5V rail near the CPU, chipset and RAM slots with 100uF or 47uF tantalum capacitors 10V+ rated. It helped me to bring many mainboards back in the business.

The suggestion with the capacitors sounds plausible. I'll try this out too, I guess 😀

My Youtube channel (Necroware):
https://www.youtube.com/channel/UCKU5nwSWYXa3xfXGBlKyvdw

Reply 51 of 53, by ph4nt0m

User metadata
Rank Member
Rank
Member

It is also worth to mention that chipsets also have their overclocking limits. Many 386SX chipsets were 20MHz or 25MHz rated. There is no guarantee they will do 33MHz or 40MHz even if everything else is in a great shape.

My Active Sales on CPU-World

Reply 52 of 53, by scorp

User metadata
Rank Newbie
Rank
Newbie
ph4nt0m wrote on 2020-07-18, 12:29:

It is also worth to mention that chipsets also have their overclocking limits. Many 386SX chipsets were 20MHz or 25MHz rated. There is no guarantee they will do 33MHz or 40MHz even if everything else is in a great shape.

Yes, this is what I assume. That's why I was asking for a datasheet, but nobody seems to have an answer. I guess this chipset is ok for up to 25MHz. Probably, it is near the limit even at that frequency, because it is not even stable at 25MHz. However, may be this instability is related to bad capacitors, just as you mentioned....

My Youtube channel (Necroware):
https://www.youtube.com/channel/UCKU5nwSWYXa3xfXGBlKyvdw

Reply 53 of 53, by scorp

User metadata
Rank Newbie
Rank
Newbie

I tried everything, another RAM down to 60ns, SIMM or DIP only, checked and replaced caps. No difference. Interesting enough, here is written, that the mainboard supports 16,20 and 24 MHz. https://stason.org/TULARC/pc/motherboards/U/U … 6-UM-386SX.html

The last frequency is kind of weird, there were no 386SX CPUs with 24MHz 😀 And at 25MHz this mainboards gets instable. I guess, the chipset is the bottleneck and UM82C366A is not made for anything faster. Still, it was a nice experiment 😀

My Youtube channel (Necroware):
https://www.youtube.com/channel/UCKU5nwSWYXa3xfXGBlKyvdw