ATMega128L Maximum Crystal Speed

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I'm using an ATMega128L chip and have experienced intermittend strange behavoirs in the chip's operation. In another post I was informed about the "M103" fuse. I fixed that and things got much better but not completely better.

I'm just realizing that I'm running the chip with a 20MHz crystal. Based on what is in the data sheet it looks like the maximum clock speed for this chip is 8MHz. (oops!)

Quote:

• Speed Grades
– 0 - 8 MHz ATmega128L

1) Am I reading the data sheet correctly? Is the maximum clock speed for this chip 8MHz?
2) If so, have I done permanent damage to the chip by running it at 20Mhz for several days? I'm thinking that running it at this higher rate could cause heat damage to the chip?

I don't have any extra ATMega128L parts on hand to experiment with.

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If you run the L version at 5V then it should run at 16MHz AFAIK. Does the chip still work? ie no smoke has escaped?

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Hi,

You will not have caused any permanent damage, in fact some people run their processors at higher speeds (overclocking)... you can search out many threads on the subject.

The only result is the chip won't run, the AVR line won't generate any noticable extra heat from running it that much faster.

But the 'intermittend strange behaviours' could be from a higher-than-specified clock speed. Especially if the problems were with EEPROM / FLASH programming... but 20 MHz isn't really that much higher.

And yes, for the L device it is 8 MHz max.

-Colin

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Thanks for you input.

I did not let out any of the "magic smoke" from the chip yet. :-)

The chip still runs and I can still communicate with it via the JTAG. I'll try running it from an 8MHz crystal now and see if the strange behaviours go away.

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Let's face it, Atmel almost certainly don't produce separate silicon dies for the 128 and 128L versions. They are all going to be exactly the same chip. It's just the ones they package as "L" are presumably those that failed some part of the the QC procedure for the non-L variant. But for single, home, one-off use you would probably get away with driving it up to 16MHz on 5V

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Once Atmel app engineer told me that Mega88V is essentially same as Mega88 but "binned" for operating at lower voltages. That is why Mega88V is relatively expensive than M88.

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clawson wrote:
Let's face it, Atmel almost certainly don't produce separate silicon dies for the 128 and 128L versions. They are all going to be exactly the same chip. It's just the ones they package as "L" are presumably those that failed some part of the the QC procedure for the non-L variant. But for single, home, one-off use you would probably get away with driving it up to 16MHz on 5V

My guess is that it only depends on if the lower BOD voltage level can reset the core before core stops working because of too low supply voltage at rated speed.

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There is probalby no difference in the geometry (masks) for the low voltage chips. But its possible to have a thinner gate oxide and thus slightly different electrical properties. This could be on purpose or just from normal variations.

Very likely many of the chips would qualify for both specifications. Its just not garantied.

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I tried this experiment on an ATmega128L. On the particular example I had, the chip stopped functioning at about 14 MHz.

As for all users who ask about "overclocking", my advice is: If you can get it to work once, fine. If this is a hobby application and you don't care about long-term functionality or edge conditions, rock on. But don't think about shipping your solution in a commercial product or you will incur the Wrath Of Field Support. :evil:

Stu

Engineering seems to boil down to: Cheap. Fast. Good. Choose two. Sometimes choose only one.

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