First post, by feipoa
- Rank
- l33t++
I recently obtained the 3.6 V version of the Texas Instruments 486SXL2-66 on a PGA132 inteposer board. I was able to run this chip at 2x40 MHz, instead of the traditional 2x33 MHz, on various motherboards, including those based on the VIA 481/495, UMC 481/482, SiS 310/320/330, CHIPS 351/355/356, and SiS 460 chipset. I would type cyrix.exe -cd and this would set the CPU into clock doubled mode. Confirmation was checked with Landmark speedtest and CHKCPU. Both confirmed it was in clock-doubled mode.
Of course, my main motherboard of interest to run this CPU in, an AMI Mark V Baby Screamer based on the VLSI 320/331/332, is the one which causes trouble. I first tested the SXL2-66 at 66.66 MHz using a 66.666000 MHz crystal. It clock doubles just fine as indicated by Larnmark Speed.exe and CHKCPU.exe. I'm attaching a photo of the motherboard below, not because it is needed, but it seems like posts get better attention with more photos. This photo is from user elianda. My motherboard is in the case and I didn't want to take everything apart to snap a photograph.
My trouble starts when I try to run the CPU at 80 MHz. Before acquiring the 486SXL2-66, I would run a SXL-40 in this system, so it is accustomed to using a 40 MHz FSB. Everything works fine with 1540 SCSI DMA and L1 cache in Windows 3.11. Now when I try to clock-double the SXL2-66 CPU to run at 80 MHz using the cyrix.exe -cd command, CHKCPU.exe indicates that the freq. did not change. I ran Landmark's speed.exe and it confirmed that the ALU results are the same as if the CPU is in single-clock mode. How can this be? I tried using Evergreen TI486SXL program to enable the clock-doubling, e.g. 486CACHE.exe /c2, which says it ran just fine. But when I check with CHKCPU.exe and SPEED.exe, the CPU appears to still be in single-clock mode. So why does the 486SXL2-66 clock double with a 33 MHz FSB, but not with a 40 MHz FSB? And why only when placed in the AMI Mark V Baby Screamer motherboard?
I have other oscillators between 66.66 and 80 MHz which I tried. I ran the Baby Screamer with a 70 MHz OSC, so the FSB would be 35 MHz. The CPU clock doubled just fine. CHKCPU indicates that the internal CPU freq. is 79.7 MHz though, not the 70 MHz I would expect. CHKCPU is not always right with these older oddball configurations though. I then ran the system with a 72 MHz OSC, thus an FSB of 36 MHz. The CPU was able to clock-double and CHKCPU indicated that the system is running at 80.1 MHz. Moving up the ladder, I then tried a 74.5 MHz OSC, and the SXL2 would not clock double. Thus, for 74.5 and 80 MHz oscillators, the SXL2-66 would not clock doubled when used in my AMI Mark V Baby Screamer motherboard. Why? What is preventing it from clock doubling and only in this motherboard and at >36 MHz FSB?
Is it just a coincidence that right around where CHKCPU is indicating the freq. to be 80 MHz is where the CPU stops clock doubling? Is there some kind of function built into the CPU which prevents clock doubling if it thinks the FSB is above 40 MHz? Is there a way to tricking the CPU to thinking the FSB is less than 40 MHz? And why is CHKCPU over reporting the FSB? Even with a SXL-40, CHKCPU thinks the FSB is 45 MHz.
This is not the first time I have witnessed odd behaviour like this. If you use a 486SXL-40 with a 25 MHz FSB (or was it 33 MHz, I forget) and try to clock-double it, it just won't clock double. But if you use a FSB less than a certain amount, e.g. 25 MHz, it will clock double. I don't recall which motherboard I witnessed this on. It may have been the Baby Screamer or another one. The point is that the issue is not specific to the 3.6 V of the 486SXL2-66.
The last thing I can think to try is to use the Cyrix DRx2 software as opposed to the two utilities I have already tried. On this forum, http://www.uncreativelabs.net/phpBB2/viewtopic.php?t=922 , I read that the SXL2 can have issues clock-doubling (no specifics given), but it the following was suggested:
I now have TI486SXL2 working properly in clock doubled mode with cache enabled and coprocessor installed. I had to do some goofy things to get this working properly. I used the Cyrix 486DRx2 control software, but instead of using the software frontend I manually programmed the cx486.cfg to configure CCR0 (control register 0) to hex value of 55.
Anyone have any novel (and practical) ideas to try to get this Baby Screamer working at 2x40 with the 486SXL2?
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