Overclocking a Freescale 9S12DP256C MCU

It appears that modern HCS12 can be overclocked by as much 3 x rated clock. The clock is configured by the PLL register settings.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

although I'm also rather sure that they can run somewhat faster than

24MHz, do you have any sources, measurements, test suites proving your "x 3" claim?

Oliver

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Oliver Betz, Munich
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Why can't you just try and see for yourself? I have a device with S12D64. I overclocked it just for curiosity. It appeared to be operable up to ~70MHz. Probably the silicon can go even higher; 70MHz is where the internal PLL got stuck. Of course, I didn't check every fine detail of the operation however it seemed to work normally.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

I _did_ overclock S12 devices when I checked the PLL, but made no thorough tests whether I can _safely use_ the higher frequency. Just wanted to know whether you have better information.

This "test" doesn't IMNSHO justify your statement in your previous posting. You shouldn't state "It appears that modern HCS12 can be overclocked by as much 3 x" if you didn't reasonable tests.

Oliver

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Oliver Betz, Munich
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Why do you want to do this ?

Does your software consume so much resources that the CPU is not capable of handling it at the nominal maximum clock frequency ?

While overclocking might work in a one off (such as a student) project, you are not going to be able to use this approach in real world professional design jobs. In those situations, you either have to improve your software or use a more efficient processor.

Chip manufacturers have to be conservative when specifying the maximum clock frequency so that the chip will work with any operating voltage within the specification and within the whole temperature range for any chip produced at various fabs with slightly different processing.

Typically the limiting factor will be the minimum supply voltage and the maximum ambient temperature. Running the chip at the maximum specified operational voltage will typically allow running at a higher clock frequency. However, this will typically increase the power dissipation proportional to the clock frequency and the supply voltage squared, so the junction temperature will also increase.

Apart for the direct meltdown, the increase of the junction temperature will decrease the MTBF (Mean Time Before Failure) quite rapidly. To avoid this, you have to limit the maximum allowed ambient temperature or increase the cooling by a large heatsink or forced air cooling.

If these are acceptable limitations and especially if you are able to select the best chips from different production lots, you certainly would be able to overclock quite a lot, but you would typically not be able to design a commercially successful product with these limitations.

Paul

The maximum clock frequency specified is at highest temperature and lowest supply voltage. Freescale are in that busyness not since yesterday so I would be very surprised if there is a lot of reserve to be had over temperature, voltage and of course batch variations. But if you can sacrifice some range of the above - say, this is just a prototype you want to have running faster and not put in production - just try to raise the frequency until it stops working, then settle for say 90 or 95% that and there you are.

Didi

------------------------------------------------------ Dimiter Popoff Transgalactic Instruments

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There are also non-technical reasons for the vendors to under rate their chips: marketing, legacy compatibility, export regulations.

Motorola is one of the oldest players and this is why modern HCS12s have that much of potential reserve (3 times of speed). The techology improved many times since HCS12 was introduced; there is no advantage in the manufacturing of the initial slow variant.

Fie, how boring. First of all, overclocking is sport. It is fun :-)

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Yes it can be. But one thing to watch would the A/D Convertors. Once you start overclocking them, they lose linearity.

And as others have asked, why? Do you need more speed? Do you have tasks not completing within their allowed time? If not, there is no real reason to crank up the clock.

Probably not an issue, but in real world EMI/EMC is an issue, and higher clock speeds cause more problems.

JW

tell me more - what exactly did they change?

Oliver

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Oliver Betz, Munich
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