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When perusing the Digikey and Mouser listings for ATmega328 chips, there are some subtle differences that aren't apparent from the listing page. It is easy to choose between a 168 and a 328 chip, but what about the letters after those numbers. For the Atmega328 chip with either 28 or 32 pins, there are some suffixes that defy understanding from the standard listings. You can't discern any more information from the spec sheet either.

The part number format is ATMEGA328x-yy. The X identifier can be:  not there, 1 letter, or 2 letters. Those with 2 letters such as U2, M1, or C1 are a completely different chip than the ones used in standard Arduinos. The only suffixes of concern are the letter 'P' or nothing at all. The 'P' stands for picoPower which is ATmel's line of very lower power chips. There is usually very little price difference between the regular Atmega and the picoPower version, so for most projects, the picoPower version would be the best choice, as that gives the option for extremely low-power battery-powered applications.

For the last two digit suffix, the first digit stands for the package type. The choices are:
  • A - TQFP
  • M - VQFN
  • P - PDIP
The second digit gives the temperature range. The choices are:
  • N - industrial B, -40 to 105 degrees C
  • U - industrial A, -40 to 85 degrees C
There are some chips with additional letters in the suffix, but they are not a concern for most hobby applications. For the PDIP packaging, it boils down to just three choices. Two with picoPower and one without. In the picoPower selection, you have a choice of the two temperature ranges. For most applications, the lower industrial A range will suffice, so the choice is narrowed to this part number:

ATMEGA328P-PU

Now you know what chip to buy.

For more information on these distinctions, see the Atmel page for this series chip.

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