Having used the Arduino prototyping platform (a loose combination of specific pieces of somewhat open/free hardware and a more open/free software stack) for some time for educational and tinkering purposes in my local hackerspace (geeklabs.dk) I have seen and studied the Arduino UNO hardware and lots of its “clones/compatibles/knockoffs” and their common MCU (MicroController Unit);

Atmel ATmega328P

I had begun wondering what the P in the microcontroller model name actually meant. So here is an attempt to decode the Atmel megaAVR chip numbering system. The other existing AVR based series UC3, tinyAVR, XMEGA, Battery & Automotive, will probably employ similar naming schemes.

The remainder of the product name following “ATmega” expresses the available flash memory and the approximate pin count of the package in an integer and optionally other features as either integer or letter (like the initial wondering of P in 328P above).

Starting with the integer, it is a concatenation of two separate integers encoding the flash size and pin count as defined below. The division of the two is non-ambiguous leaving some interpretation to be done.

1st integer: onboard flash size
8 = 4 KiB
8 = 8 KiB
16 = 16 KiB
32 = 32 KiB
64 = 64 KiB
128 = 128 KiB
256 = 256 KiB

2nd integer: total pin number
(none) = standard pin count (differs)
8=28/32-pin
4= 40/44/49-pin
5= 64-pin
0= 100-pin

Suffix (char or integer), multiple possible
P = picoPower (max. consumption 9mA@8MHz,5v vs. 12mA@8Mhz,5v for non-P)
9=LCD controller
U2 = USB controller
U4 = USB controller
A  = ?

Exceptions
Note that some of the product names are completely void of these rules. Others employ different numbering but still with a familiarity to the above.

An example:
ATmega6490A: 64KB flash, 100-pin, LCD Controller

Sources

• http://www.atmel.com/products/microcontrollers/avr/megaavr.aspx
• http://www.atmel.com/products/microcontrollers/avr/

“Every good work of software starts by scratching a developer’s personal itch.”

Eric S. Raymond, “The Cathedral and the Bazaar” (@Goodreads)

The above quotes the first lesson from Eric S. Raymond‘s (ESR) essay/book “The Cathedral and the Bazaar“ (link to full book, summary at Wikipedia), which has become a kind of bible within the FOSS ecosystem (also nicknamed CatB). In his text Eric investigates motivations and social organisation of free and open source software projects. Itches are known initiators of many both large projects and minor changes to FOSS software. Itches, and the scratching of those by developers in the FOSS community, highlights a FOSS software user’s right to access, modify and redistribute the source codes behind FOSS software. With access to the underlying source code of FOSS software, a developer is able to scratch an itch, and is usually very motivated by this, because it often is a very personal itch.

You can listen to an audio recording of Eric elaborating about the central topics of CatB in a recording from a talk at Linux Kongress all the way back to May 22th 1997 17:15 CEST (48k MP3, 96k MP3):

My Itches

I’ve long been trying to keep a list of itches I want to scratch in free software projects/products. Realizing that most of these were lost in transit in the chaotic neuron mess of my brain, my intention now is to, also,  keep track of them textually using the mechanisms of this site.

This effort will be an ongoing, and probably ever expanding, mix of my private personal itches and itches related to and spun-off from my software development work done as a professional embedded developer, but still personal itches.

You can head over to the static page at mikini.dk/what/itches and take a look at my past and present itches.

EDIT 2021-08-24: add prominent quote source, add GR quote link, add CatB main page link, add para. with audio recording, minor copyediting