System PLL--actual or logical clock?

[mkaresh]

Someone just asked this in another thread, but it's gettig buried in there without a response.

Is the System PLL clock a real or logical clock. If I set it to 450, is anything in the G400 actually running 450mhz? Or is that just a logical number the actual clocks are based on through the divisors?

I ask because I like to run 150/180, which requires the use of 3/2.5/3, and a 450 clock.

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PIII 500e @667
Soyo SY6BA+IV
Iwill Slotket II
G400 32MB DH @144/180
Nokia 445Xi 21"
Aureal Vortex Superquad
128MB PC133
IBM DPTA 20.5GB 7200rpm
Toshiba 8/40 DVD

[Greebe]

It's logical

[Ashley]

I'm not sure what you mean by "logical", but it is a very real frequency you can measure off a pin on the chip itself, before it gets divided down and fed to the clocks you are really interested in (e.g., the memory clock).

[mkaresh]

I replied to this earlier, but don't see my reply.

The reason I asked this question is to know whether a higher syspll in itself risks frying anything. I like to do 150/180 which requires a syspll of 450. Is anything in there really doing 450mhz? Or is it just a number that the real, slower clocks are based on?

[Ashley]

The answer would have to be: Yes, there is risk. At 450MHz you are exceeding frequency limitations by a considerable margin.

Moreover, chances are your attempt at 450MHz is so far beyond the PLL lock range limits that either that clock speed can't be synthesized to begin with, or you have introduced so much jitter that the clock is now wildly erratic.

[mkaresh]

Actually, with the PLL at 450 and dividers of 3/2.5/3 there is no degradation in quality. That's why I was wondering if anything is actually running 450mhz. The gclock is a rather normal 150 and the mclock is 180. Can't get these settings without going to 450.

I ask this question because I too find it hard to believe that anything in the card is actually capable of 450mhz.

Another way to put my question is:

Is there anything different about having a PLL of 300 with 2/1.5/2 dividers and a PLL of 450 with 3/2.25/3 dividers. Both give the same gclock and mclock. (No, my card isn't capable of these Max settings. It's just an example to make my question clearer.)

[This message has been edited by mkaresh (edited 23 January 2000).]

[Ashley]

That may very well be. But I think you wanted to know if anything is actually running at 450MHz and the answer is Yes (assuming it actually locked at this frequency).

So: yes, it is better to use a system PLL that does not overly exceed the max. speed the RAMDAC is rated for.

[mkaresh]

Thanks for the replies, Ashley.

So the syspll is the speed of the RAMDAC? That doesn't seem right, since stock syspll for a vanilla is 250 (or maybe the 252 mgatweak reports for my card) and about 300 for the Max. Both of these are below the RAMDAC mhz.

The card runs well at this frequency. Benchmark scores are right where you'd expect them for 150/180, which is to say a touch above those for the 144/180 possible with lower divisors and even more above those for the 135/180 possible with the standard divisors.

450 is so far out of the spec range, I wonder why MGATWeak includes a divisor of 3, since it necessarily requires a very high syspll.

[Ashley]

The system PLL is *not* the speed of the RAMDAC - I'm just suggesting that you shouldn't set any of the PLL frequencies higher than what the RAMDAC is rated for.