feipoa wrote on 2024-03-29, 09:50:

I also experimented with some parallel resistors. No judgements please! I didn't want to put a whole lot of time into this. I […]
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I also experimented with some parallel resistors. No judgements please! I didn't want to put a whole lot of time into this. I don't have a large enough stockpile of power resistors in the 0-15 ohm range. The largest resistors I had in this range were 2 W, so I placed them in parallel to share in heat dissipation:
SXL2-90_17.JPG

I ultimately landed with this. It is 5.1-ohm, 5.1-ohm, 10-ohm:
SXL2-90_18.JPG
The parallel resistance was 2.0-ohms. Vin was 11.6 V, and the combination initially dropped the voltage by 2.1 V, but as time went on, deltaV increased to 2.5 V and reached 63 C after about 8 minutes. I am guessing that the resistance of the resistors is increasing as they heat up. From these values, we can see that the incoming current is 2.1 V / 2ohm = 1.05 A. Without the resistor, I guess we have to take the word of the DMM, which measured 2 A, but when the DMM leads are connected, the system won't POST - just black screen.

EDIT: Well this is embarrassing. I forgot to move the DMM's lead to the current terminal. Now that I've corrected the leads, I am getting 0.95 A while sitting at the DOS prompt. This measurement is without the parallel resistor pack. So, roughly 6.1 Watts to dissipate as heat ((11.6-5.15)/0.95)?

In action:
SXL2-90_19.JPG

Running Win3.11:
SXL2-90_20.JPG

My other set of finger wound paraelle resisitors also had a drop of 2.5 V as shown on the DMM here:
SXL2-90_21.JPG

I decided that I didn't want a single point of that much heat in the case without active dissipation (a fan). it was easier for me to cool the VRM with a fan. Also, I would have needed to paste on some silicone to insulate those leads. It is too easy to short it out on some metal in the case. Alternately, I might order some 10-20 W resistors on my next digikey order. I was thinking of this 2.2 ohm beast: https://www.digikey.ca/en/products/detail/yag … AJB-2R2/9167212

I haven't had any crashes with the SXL2 at 90 MHz. I will be setting up w95 and NT4 on a seperate CF card, so this will provide more insight concerning stability.

No judgments, that’s how experiments are done! 😀

I think you should give a murata buck converter a try look in my sig, pcb projects, top of first post.

Paralel wrote on 2024-03-28, 00:10:

According to the "Designing For The TI486SXL2-G" Application Report: […]
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feipoa wrote on 2024-03-27, 22:15:

What my experience here reminds me of is the original Pentium 66. While the 60 MHz variant would run well at 5.0 V, the 66 MHz […]
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Paralel wrote on 2024-03-27, 17:41:

I would argue these are the questions of the hour. I know I am hooked now seeing where this will go. Its interesting to see this progress, as it is treading in a similar path to the 486SX4 that Intel created, but never released, and the IBM SLC3 (although that was hobbled by the 16 bit bus) that was never released as a 33/100 (although I have read that it might have been released in Japan as a 33/100 upgrade, but I have never seen any proof of this), but could very likely be easily pushed into that territory.

What my experience here reminds me of is the original Pentium 66. While the 60 MHz variant would run well at 5.0 V, the 66 MHz variant often required higher voltages. The Intel Batman's Revenge socket 4 manual states,

Voltage Control Jumper: set at the factory to regulate CPU voltage at 5.0V for 60 MHz Pentium processor or 5.27V for 66 MHz Pentium processor. Improper operation may result if the factory setting is changed.

Many OEM suppliers of this motherboard bother providing the optional VRM required for operation at 5.27 V, presumably because they shipped with 60 MHz Pentiums. My two Batman's Revenge do not contain the onboard regulator circuit. Does anyone have a socket 4 with the 5.27 V regulator? If so, what type of regulator did Intel use, and is it step-up (from 5 V) or step-down (from 12 V)?

According to the "Designing For The TI486SXL2-G" Application Report:

The 3.3-V V cc can be no more than 1 V greater than V cc5 during power up. In a pure 3.3-V only system (3.6V for the TI486SXL2-G66), V ccs should be connected to the V cc supply (3.3 or 3.6 V).

So, 6V seems to be something of an upper limit. However, page 18 of the Application Report says that 5.25V is the hard upper limit with a maximum case temp of 65C. From what I can tell from looking at the VRM diagram in the Application Report, it is step up from 5V since they explicitly show a 5V input.

The Application Report indicates that the various voltage regulators in the attachment to this post can be used with anything from a TI486SLX2-G50 to DX4-100:

LT1085 is super common to see on late 486/early pentium motherboards. It has a unique single resistor design (uses a current reference instead of a voltage reference) that works well for this application. This also makes it easy to PWM control for jumperless designs.

I like to use the LT1573. It's specifically designed for microprocessors of this era. It's meant to handle all the transient response etc required by CPUs. It's unique in that the power transistor is external. So you can easily parallel the power transistors. This further decreases dropout, increases current capability, and more importantly spreads the heat load out. So instead of needing to pull 5W out of a single tiny SMD IC you only need to pull 2.5W. Depending on your PCB design this can be a lifesaver.

kingcake wrote on 2024-03-29, 15:19:

Paralel wrote on 2024-03-28, 00:10:

According to the "Designing For The TI486SXL2-G" Application Report: […]
Show full quote

feipoa wrote on 2024-03-27, 22:15:

What my experience here reminds me of is the original Pentium 66. While the 60 MHz variant would run well at 5.0 V, the 66 MHz variant often required higher voltages. The Intel Batman's Revenge socket 4 manual states,

Voltage Control Jumper: set at the factory to regulate CPU voltage at 5.0V for 60 MHz Pentium processor or 5.27V for 66 MHz Pentium processor. Improper operation may result if the factory setting is changed.

Many OEM suppliers of this motherboard bother providing the optional VRM required for operation at 5.27 V, presumably because they shipped with 60 MHz Pentiums. My two Batman's Revenge do not contain the onboard regulator circuit. Does anyone have a socket 4 with the 5.27 V regulator? If so, what type of regulator did Intel use, and is it step-up (from 5 V) or step-down (from 12 V)?

According to the "Designing For The TI486SXL2-G" Application Report:

The 3.3-V V cc can be no more than 1 V greater than V cc5 during power up. In a pure 3.3-V only system (3.6V for the TI486SXL2-G66), V ccs should be connected to the V cc supply (3.3 or 3.6 V).

So, 6V seems to be something of an upper limit. However, page 18 of the Application Report says that 5.25V is the hard upper limit with a maximum case temp of 65C. From what I can tell from looking at the VRM diagram in the Application Report, it is step up from 5V since they explicitly show a 5V input.

The Application Report indicates that the various voltage regulators in the attachment to this post can be used with anything from a TI486SLX2-G50 to DX4-100:

LT1085 is super common to see on late 486/early pentium motherboards. It has a unique single resistor design (uses a current reference instead of a voltage reference) that works well for this application. This also makes it easy to PWM control for jumperless designs.

I like to use the LT1573. It's specifically designed for microprocessors of this era. It's meant to handle all the transient response etc required by CPUs. It's unique in that the power transistor is external. So you can easily parallel the power transistors. This further decreases dropout, increases current capability, and more importantly spreads the heat load out. So instead of needing to pull 5W out of a single tiny SMD IC you only need to pull 2.5W. Depending on your PCB design this can be a lifesaver.

Excellent insight. It is greatly appreciated, as my understanding of VRMs and their application is limited.

Sphere478 wrote on 2024-03-29, 13:58:

I think you should give a murata buck converter a try look in my sig, pcb projects, top of first post.

I already have some of these mini adjustable buck converters I was considering as a fall-back option. I kind of like the feel of using the existing linear reg without any external components. At present, the onboard VRM only gets up to around 37 C steady state and the heat isn't affecting the SXL's operation. However, things might change when the system is inside the case. Let's see what happens first...

Some photos of the 2A bucks I have on hand, they be tiny:

27C is absolutely nothing to worry about. Not even close to it. Why are you regulating 12V to CPU VCORE instead of 5V? Are you trying to overvolt or something.

Yes, he needs at least 5.15V for stable 2x45.

As pshipkov mentioned, I need 5.15 V going to the SXL2 for stable operation at 90 MHz. I only need 3.85 V for stable operation at 80 MHz.

That should read 37 C on VRM heatsink, steady-state, with only the linear reg. I've fixed it now.

While waiting for my kids to get ready to go out (they take forever), I ran some quick tests with my $1 buck module. The trim pot screw does not adjust well in the 3.5 - 10 V range. Others also report this. It was as if there is a gap in the trimmer for this range. After a few minutes, I was able to get it to output 6 V. Seems to work fine, but the issue with the trim pot in this range may cause faults in time. After a few minutes running, that 6 V output went to 5.6 V, still acceptable because I'm using a ultra low drop-out linear regulator, but may drop too low. To use this in-line buck, I'd have to swap the trim pot with a unit that can do more turns. I definitely would not trust this $1 buck without the existing linear reg in place.

With the buck, I'm reading 0.92 A going into the linear reg. The temperature of the linear reg's heatsink now is 25 C w/VRM fan. The $1 buck didn't get hot, just slightly above room temp - I didn't measure though.

I'll see if I have a proper trim pot to replace on my $1 buck.

At any rate, I'm not sure if it is worth the effort. With the existing linear regulator only, the CPU temp only gets up to 28.5 C. I'll have to run some in-chassis test to determine if I'll keep the buck. I need to determine how lower, if any, the CPU temp gets while using the buck at 6 V.

Huh, I guess the 6V from the Application Report was accurate. I honestly didn't think it would allow for it. I'm impressed at the operating voltage range of the 486 SXL2-G processors. I don't think I know of another processor in the 386/486 family that can operate anywhere close to that large of a range.

With the temp only getting to 28.5, as long as the case has decent air inflow and outflow to guarantee a certain volume of air change I don't see how it would ever be possible to hit or exceed 65C.

feipoa wrote on 2024-03-30, 01:21:

As pshipkov mentioned, I need 5.15 V going to the SXL2 for stable operation at 90 MHz. I only need 3.85 V for stable operation […]
Show full quote

As pshipkov mentioned, I need 5.15 V going to the SXL2 for stable operation at 90 MHz. I only need 3.85 V for stable operation at 80 MHz.

That should read 37 C on VRM heatsink, steady-state, with only the linear reg. I've fixed it now.

While waiting for my kids to get ready to go out (they take forever), I ran some quick tests with my $1 buck module. The trim pot screw does not adjust well in the 3.5 - 10 V range. Others also report this. It was as if there is a gap in the trimmer for this range. After a few minutes, I was able to get it to output 6 V. Seems to work fine, but the issue with the trim pot in this range may cause faults in time. After a few minutes running, that 6 V output went to 5.6 V, still acceptable because I'm using a ultra low drop-out linear regulator, but may drop too low. To use this in-line buck, I'd have to swap the trim pot with a unit that can do more turns. I definitely would not trust this $1 buck without the existing linear reg in place.

With the buck, I'm reading 0.92 A going into the linear reg. The temperature of the linear reg's heatsink now is 25 C w/VRM fan. The $1 buck didn't get hot, just slightly above room temp - I didn't measure though.

I'll see if I have a proper trim pot to replace on my $1 buck.

At any rate, I'm not sure if it is worth the effort. With the existing linear regulator only, the CPU temp only gets up to 28.5 C. I'll have to run some in-chassis test to determine if I'll keep the buck. I need to determine how lower, if any, the CPU temp gets while using the buck at 6 V.

Mini_switching_vrm_3.JPG

I almost wonder if maybe we should define maximum voltage with our own tests.

Is this a 3.6v chip or a 5v chip.

One might raise vcore slowly over the course of a day .25v a day with benchmarks running non stop until chip failure.

It probably doesn’t matter if someone with a lot of chips used their lowest clocking chip for this. It should still define max voltage. But a little overclock would be good.

Sphere478 wrote on 2024-03-30, 03:20:

I almost wonder if maybe we should define maximum voltage with our own tests. […]
Show full quote

feipoa wrote on 2024-03-30, 01:21:

As pshipkov mentioned, I need 5.15 V going to the SXL2 for stable operation at 90 MHz. I only need 3.85 V for stable operation […]
Show full quote

As pshipkov mentioned, I need 5.15 V going to the SXL2 for stable operation at 90 MHz. I only need 3.85 V for stable operation at 80 MHz.

That should read 37 C on VRM heatsink, steady-state, with only the linear reg. I've fixed it now.

While waiting for my kids to get ready to go out (they take forever), I ran some quick tests with my $1 buck module. The trim pot screw does not adjust well in the 3.5 - 10 V range. Others also report this. It was as if there is a gap in the trimmer for this range. After a few minutes, I was able to get it to output 6 V. Seems to work fine, but the issue with the trim pot in this range may cause faults in time. After a few minutes running, that 6 V output went to 5.6 V, still acceptable because I'm using a ultra low drop-out linear regulator, but may drop too low. To use this in-line buck, I'd have to swap the trim pot with a unit that can do more turns. I definitely would not trust this $1 buck without the existing linear reg in place.

With the buck, I'm reading 0.92 A going into the linear reg. The temperature of the linear reg's heatsink now is 25 C w/VRM fan. The $1 buck didn't get hot, just slightly above room temp - I didn't measure though.

I'll see if I have a proper trim pot to replace on my $1 buck.

At any rate, I'm not sure if it is worth the effort. With the existing linear regulator only, the CPU temp only gets up to 28.5 C. I'll have to run some in-chassis test to determine if I'll keep the buck. I need to determine how lower, if any, the CPU temp gets while using the buck at 6 V.

Mini_switching_vrm_3.JPG

I almost wonder if maybe we should define maximum voltage with our own tests.

Is this a 3.6v chip or a 5v chip.

One might raise vcore slowly over the course of a day .25v a day with benchmarks running non stop until chip failure.

It probably doesn’t matter if someone with a lot of chips used their lowest clocking chip for this. It should still define max voltage. But a little overclock would be good.

It is a 3.6v chip.

I'm running a 3.6 V CPU at 5.15 V.

I might be willing to run my worst SXL2 chip at 6V to see if it can handle it. The worst CPUs can only do 75 MHz though.

Yeah, yeah, we know what the label says but the 2.45 mobile pentium is a 2.8v chip, the 1.6v k6-3+ is a 2.0v chip, what is the sxl2.🤔 is it a 3.6 or a 5v

At above reply, you should, for science! But also, it would be sad to hurt it. I’m conflicted 🤣

Depends on process node used, I think up to 0.6um is fine at 5V. First Pentium was on 800 nm.

Paralel wrote on 2024-03-30, 02:05:

Huh, I guess the 6V from the Application Report was accurate. I honestly didn't think it would allow for it. I'm impressed at the operating voltage range of the 486 SXL2-G processors. I don't think I know of another processor in the 386/486 family that can operate anywhere close to that large of a range.

With the temp only getting to 28.5, as long as the case has decent air inflow and outflow to guarantee a certain volume of air change I don't see how it would ever be possible to hit or exceed 65C.

From what I understand it's "only" running at 5.15V. The 6V from the buck regulator is to feed the actual VRM

In the "1994_TI486SXLC_and_TI486SXL_Microprocessors_Reference_Guide", pages 209-210,

For the TI486SXL-G(40/50/66):
Absolute max VCC is 5.5V
Absolute max temp is 110C (power applied), 150C storage
Recommended max VCC is 5.25V
Recommended max temp is 85C (power applied)

5.25V should be "guaranteed" if all the chips are systematically tested for that... but regular fluctuations above this is a question mark, and whether the voltage is also going to other chips on the mainboard via IO...

The CLK2 (clock input) also has a VCC minimum of VCC -0.3V. If VCC is 5.25V, but CLK2 only reaches <4.95V then CLKS may not be read.

MikeSG wrote on 2024-03-30, 10:41:

The CLK2 (clock input) also has a VCC minimum of VCC -0.3V. If VCC is 5.25V, but CLK2 only reaches <4.95V then CLKS may not be read.

This is interesting.

The 5V rail on my motherboard, at a point near the cyrstal osc, is 5.08 V. From the values you mentioned, I should be able to take the CPU up to 5.38 V without missing clocks. 5.15 V is well within this range.

I did some inspection of the 1 buck and determined that the trim pot was bad. The other buck I have doesn't have a blank out in the 4-10 V range, however the fine adjustment still isn't great. I wouldn't rely on it solely for 0-300 mV adjustments. I replaced the pot with a more standard 25-turn unit and now it works beautifully. I may or may not use it in the final system.

feipoa wrote on 2024-03-30, 01:21:

As pshipkov mentioned, I need 5.15 V going to the SXL2 for stable operation at 90 MHz. I only need 3.85 V for stable operation at 80 MHz.

That should read 37 C on VRM heatsink, steady-state, with only the linear reg. I've fixed it now.

Wire your regulator to +12V and +5V to get 7V Vin. You'll need an amp of load on your +5V power supply rail to ensure it doesn't try to sink current. A 5R power resistor would work fine hooked up to a molex connector. This will drastically cut your linear regulator's heat load.

kingcake wrote on 2024-03-30, 15:24:

feipoa wrote on 2024-03-30, 01:21:

As pshipkov mentioned, I need 5.15 V going to the SXL2 for stable operation at 90 MHz. I only need 3.85 V for stable operation at 80 MHz.

That should read 37 C on VRM heatsink, steady-state, with only the linear reg. I've fixed it now.

Wire your regulator to +12V and +5V to get 7V Vin. You'll need an amp of load on your +5V power supply rail to ensure it doesn't try to sink current. A 5R power resistor would work fine hooked up to a molex connector. This will drastically cut your linear regulator's heat load.

Pretty sure it will mess some things up, as the regulator probably need to be on the same ground potential as the cpu and rest of the system for that sake.

H3nrik V! wrote on 2024-03-30, 15:46:

kingcake wrote on 2024-03-30, 15:24:

feipoa wrote on 2024-03-30, 01:21:

As pshipkov mentioned, I need 5.15 V going to the SXL2 for stable operation at 90 MHz. I only need 3.85 V for stable operation at 80 MHz.

That should read 37 C on VRM heatsink, steady-state, with only the linear reg. I've fixed it now.

Wire your regulator to +12V and +5V to get 7V Vin. You'll need an amp of load on your +5V power supply rail to ensure it doesn't try to sink current. A 5R power resistor would work fine hooked up to a molex connector. This will drastically cut your linear regulator's heat load.

Pretty sure it will mess some things up, as the regulator probably need to be on the same ground potential as the cpu and rest of the system for that sake.

Hmm yeah that's a good point.

I had assumed that wouldn't be an issue for CMOS, compared to TTL open collector stuff. But I could be wrong.

kingcake wrote on 2024-03-30, 15:24:

feipoa wrote on 2024-03-30, 01:21:

As pshipkov mentioned, I need 5.15 V going to the SXL2 for stable operation at 90 MHz. I only need 3.85 V for stable operation at 80 MHz.

That should read 37 C on VRM heatsink, steady-state, with only the linear reg. I've fixed it now.

Wire your regulator to +12V and +5V to get 7V Vin. You'll need an amp of load on your +5V power supply rail to ensure it doesn't try to sink current. A 5R power resistor would work fine hooked up to a molex connector. This will drastically cut your linear regulator's heat load.

You would need a floating power supply and to tie one of the legs to negative of the chip. Probably the +5 of the new floating psu

It can be done. I thought of it earlier but it’s kinda silly when you can just buck down from
+12v into some capacitors