What the difference between p/v and pa and a

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I have been searching every nook and cranny of the Atmel site and I can't figure out what the "a" means.

For instance:
First there was the atmega644
Then they changed to atmega644p and atmega644v
Now they have the atmega644pa and atmega644a

I think I read somewhere that the "p" stands for picopower, but I have know idea what the "a" stands for.

Does anyone know what the "a" stands for and also where you can find this info?

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A means "die shrink". Apart from one exception I think all A parts have retained the 3 signature bytes of their non-A bretheren and are, therefore, completely code identical. What changes because of the change in manufacturing process (mainly because Atmel closed all their own fabs and contracted out fabrication to 3rd parties using smaller geometries) is the power consumpion (and possibly the noise susceptibility). For every part that has gone A there's a migration noted between the two that explains the power differences.

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Thanks for the explaination! It is quite a releif to know I wont have to change my code.

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dpaulsen wrote:
Thanks for the explaination! It is quite a releif to know I wont have to change my code.
But that you could have figured out from the migration application notes Atmel publishes.

Stealing Proteus doesn't make you an engineer.

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I searched for migration and atmega644pa and it came right up. Now I know what to search for. Thanks.