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I am looking into AVR chips (big fan) for a personal project. I need capabilities similar to Arduino Uno, so I recently compared Atmega 328p and Atmega 324p.

To my surprise the 324p has significantly more features and on-board hardware than the 328p. To me, higher model number == more powerful/more features product, so discovering this slightly confusing me.

Could anyone explain to me why Atmel chose such weird convention for naming their chips? I know this is not a big deal, but for the future reference I would like to know how to choose AVR MCU's for my needs properly (their buyers guide is not very helpful).

edit: the 328p consumes twice less amount of power, so is it all down to smaller, less power hungry product?

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    \$\begingroup\$ How to choose ICs? Read the data sheet, compare the features, don't compare the part numbers. VTC as my opinion is just as good as the next person's about why they chose particular part numbers. \$\endgroup\$
    – Neil_UK
    Commented Aug 11, 2018 at 13:34
  • \$\begingroup\$ @Neil_UK thanks for that, I am used to commercial CPU and GPU marketing where higher number is always better :) \$\endgroup\$
    – Shibalicious
    Commented Aug 11, 2018 at 13:43
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    \$\begingroup\$ There is always a tradeoff... I'd pay special attention to tolerances and accuracy specifications, especially surrounding clocks and voltages. I was looking at the differences between the PIC10F200 and the ATTiny9. The ATTiny9 seemed better in every respect AND was cheaper, until i found out that the ATTiny9 clock calibration is +/- 10% while the PIC10F200 is +/- 2%. \$\endgroup\$
    – gregb212
    Commented Aug 11, 2018 at 13:56
  • \$\begingroup\$ @gregb212 thank you, I will double check the tolerances! \$\endgroup\$
    – Shibalicious
    Commented Aug 11, 2018 at 14:06
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    \$\begingroup\$ Atmel/microchip have a parametric search tool for selecting parts. That is the 'proper' way to select parts rather than inferring anything from the part numbers. The SAMs and competing products like the STM32s have incomprehensible part numbering. \$\endgroup\$
    – whatsisname
    Commented Aug 11, 2018 at 16:44

1 Answer 1

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The numbers of Atmega devices follow a quite simple basic scheme. Let's take the Atmega644PA-AU as an example.

  • "64" The first digits always mark the size of the Flash in Kibibyte
    • This is always a power of 2, between 2 and 256. The 4808 is the only exception with 48kB of memory
    • The RAM is in most cases a factor of 8 or 16 smaller than the Flash
  • "4" The last digit marks the series of chips. Within a series they are often pin-compatible and share a similar set of features. Some prominent series are:
    • 'no digit' - these are the first generation chips with 8 to 128 kiB Flash
    • '8' - a series from 4 to 32 kiB Flash, all in the same housing. More or less an improved version of the original chips
    • '4' similar to '8', but in larger packages with more pins (~40 instead of ~30) and up to 128 kiB Flash
    • '5' similar to '4', but with more timers and PWM channels
    • '0','1' rather old family with large packages (60 - 100 pins) and up to 256 kiB Flash.
    • '9' with integrated LCD controller
    • 'U2', 'U4' are the two sizes of USB-enabled controllers
    • '08', '09' newest family with additional configurable logic and more CPU-bypassing features
    • '50', '90' the largest chips with 100 pins, but few peripherals
  • 'PA':
    • 'A' and 'B' are newer revisions, usually without major changes.
    • 'P' is 'pico-power' - chips with very deep sleep modes and quite low power consumption, ideal for battery-powered applications
    • 'L' and 'V' are sampled for lower voltage operation at slower clock speeds (old series only, newer have this 'built-in').
  • '-AU' The letters after the dash mark
    • the kind of packaging (LGA, DIP, QFN...)
    • the temperature range (industrial, consumer)
    • the shipping packaging (tube, reel)
    • default fuse settings (e.g. internal oscillator on 32U4)
    • lead content (obsolete)

The even smaller Attiny controllers follow a similar scheme, but have more and smaller families with more specialized sets of features. Notable is the Attiny[2,4,8,16,32][0,1][4,6,7] (e.g. Attiny204 up to Attiny3217) series that extends the Atmega[8-48]0[8,9] towards smaller memory and fewer pins.

As of 2020, there are a two new Atmega-related series: AVR-DA and AVR-DB which seem to be very similar to Atmega, but with some improved and more modern features.

In summary: You can learn quite a lot about the device from its number, but for details and precise numbers you always have to look into the datasheet. Microchip offers a handy Quick Reference Guide where families of chips are grouped - but be careful, some entries are wrong (e.g. 324PB, 32U4).

Basic buying guide for Atmega: Use a ..4 or ..8, depending on the number of I/O and peripherals you need. 'P' and 'B' are always favorable. For communication with a PC, use a U2 or U4. Battery powered devices that have to react on external signals might profit from the new ..[0-1][4-9] series.

For special needs, check the rest of the species.

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  • \$\begingroup\$ How did you figure out 324PB has less features than 324PA? I am comparing the datasheets and PB seems to win in every aspect, even though PA is 3x more expensive, weird! \$\endgroup\$
    – Shibalicious
    Commented Aug 13, 2018 at 8:54
  • \$\begingroup\$ @Hypomania Thanks for pointing this out, The entry in the Quick Reference Guide is wrong. \$\endgroup\$
    – asdfex
    Commented Aug 13, 2018 at 9:46

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