Speed grade doubt

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I have already ordered ATmega2561-16AU which has maximum speed of 16MHz with 4.5V - 5.5V power supply. But I want to operate it at 8MHz at 3.3V power supply. Since for 8MHz another part no. is there ATmega2561-8AU (1.8V to 5.5V) i am in doubt whether it is possible?

If use 3.3V as Vcc then will VOH be typically 3.3V? Is there any option to set VOH etc to different voltage than Vcc? eg. Vcc=5V and VOH=3.3V

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Have you tried to read the datasheet by chance?

Warning: Grumpy Old Chuff. Reading this post may severely damage your mental health.

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Yes sir I read the datasheet. On pages 369-370 the graphs of Max. frequency vs Vcc is there for both 8MHz and 16MHz. But it is not clear whether the 16MHz version can be used down to 8MHz with less than 4.5V supply voltage.
About my other qn. i haven't found anything.

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The datasheet tells us that the max speed is related to voltage. Therefore you can run any clock rate up to the maximum for the given voltage.
So, 8MHz at 3.3v is allowed.

VOH is a specification and it is related to the supply voltage. You will find it is less than the supply voltage. Simple answer is No. You supply 5V, VOH will be close to 5V. You want 3V i/o, either use level translators or run the AVR at 3V

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^Thank you.

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Kartman:

There are actually four different speed grade plots in the datasheet.

On page 369 we have speed grade plots for the "V" variants of these devices. Figure 31-1 is for all "V" variants during normal operation. Figure 31-2 is only applicable to the 256KB "V" variants in the specific scenario that the bootloader section of Flash is in use.

On page 370 we have speed grade plots for the "non-V" variants of these devices. Figure 31-3 is for the 64KB and 128 KB "non-V" variants of these devices. Figure 31-4 is for the 256KB versions.

Figure 31-4 clearly shows that the 256KB "non-V" variants of these devices are not certified to operate below 4.5V under any circumstances. It's possible that they may work, but Atmel does not guarantee it.

- Luke

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Kartman wrote:
The datasheet tells us that the max speed is related to voltage. Therefore you can run any clock rate up to the maximum for the given voltage.
So, 8MHz at 3.3v is allowed.

The ATMega256x are different from many (most? all?) other ATMegas in that the "high-speed" (here: 16MHz) version is NOT qualified outside the 4.5V-5.5V range.

The "low-speed" version has suffix V (i.e. ATMega2561V-8AU) is qualified for wide supply range up to 5.5V, but only up to 8MHz operation.

JW