On my Arduino UNO the chip part number is ATMEGA328P-U, i recently bought a ATMEGA328-PU to make a standalone Arduino. realized the part number differences after receiving the package...

I noticed that some arduino UNOs have the chip with part number of ATMEGA328P-PU! I'm totally confused!

Those chip are the same? can i use the one that i bought for an standalone Arduino?

If no what are the differences?

  • 1
    The datasheets say what...?
    – Majenko
    Commented Jun 29, 2019 at 10:30
  • 328 / 328p: forum.arduino.cc/index.php?topic=60001.0
    – Juraj
    Commented Jun 29, 2019 at 10:37
  • 1
    Uno has ATMEGA328P-PU. ATMEGA328-PU will work in Uno. ATMEGA328P-U doesn't exist
    – Juraj
    Commented Jun 29, 2019 at 10:46

3 Answers 3


The main difference is the bit before the -. That is, the 328 vs the 328P. The "P" there denotes "Picopower" which allows the chip to run at very low power consumptions.

Basically the P version is a more modern version of the non-P chip. There are probably other internal differences too but you will have to check the datasheets thoroughly for those.

Everything after the - is to do with the packaging and environmental grading of the chip. For example PU is a -40C to +85C grade DIP chip. ANR is a -40C to +105C grade TQFP chip, etc.

If you see 328P-U then that's probably a fake chip. There's no such thing as single U suffix.


There are three die types:




all have different signature bytes which may or may not impact loading code into them. Generally tho the programming software needs to know which part type is being used, at a minimum for installing bootloader code. Serial downloading may end up ignoring the signature bytes of a bootloaded part.

They can be had in different packaging, this is what Digikey is showing as available options:

-PU - 28 pin DIP

-AU - 32 pin TQFP

-AN - 32 pin TQFP

-MM - 28 pin leadless package, smaller than 32 pin leadless

-MN - 32 pin leadless package

-MU - 32 pin leadless package

-15AZ - 32 pin TQFP, rated for a higher temperature range

-15MZ - 32 pin leadless package rated for a higher temperature range

Any letters to the right of those seen when ordering indicate the method they are supplied - loose parts, parts in a reel, parts in a tube, etc.

Anything else is either an older part that did not follow the convention, or is a counterfeit part.

Tables 38.7 and 38.8 lists these options for the 328 and 328P for example, with H indicating a type of lead finish, and R indicating Tape & Reel availability:









And 328P options:








With these rated to 105C






I think you need to pull up the Automotive data sheet to see the Z numbered parts.

And the PB is a different data sheet as well.

  • That help a lot, but in case other people are finding this looking for ATmegaXX-XXPI, then: PI are 40-pin, PDIP
    – rasmusm
    Commented Nov 24, 2019 at 19:19
  • 1
    '328s are never 40-pin. There are 40-pin DIPs in the Atmega family, and some, like the '644, '1284P, have been brought into the Arduino IDE. I like the '1284P myself; 32 IO arranged as 4 8-bit ports, dual hardware serial, 128K flash, 16K SRAM (twice what the '2560 has).
    – CrossRoads
    Commented Dec 9, 2019 at 3:17
  • Good point and my comment was more general for ATmega's, as this is the first google hit and your answer are a good reference, so i just wanted to add the answer to my own question there was about ATmega16-16pi, just in case it could help somebody else finding this looking for that.
    – rasmusm
    Commented Dec 9, 2019 at 11:31
  • The 328PB also won't work with the stock Arduino IDE and has a number of differences from the earlier parts. Some of these are physical; e.g. a pin that was VCC is now a GPIO. For support in the Ardiuno IDE (or Eclipse/Sloeber) you'll need to install MiniCore. Commented Apr 5, 2021 at 4:46

User line_code from forum.arduino.cc recently received an ATMEGA328P-U (or actually ATMEGA328P U) from Mouser (a company which you would not suspect to send out fakes or source from the gray market). He asked Mouser (who contacted the manufacturer Microchip) and got the following response:

As part of Part marking change due to the migration from Atmel to Microchip, the package identifier has been removed from the device marking as this can be observed visually for different packages. This will be updated in the upcoming revisions of the device datasheet. As per this new marking the ATMEGA328P U are valid devices without any process change. [...]

(Source: https://forum.arduino.cc/index.php?topic=561928.msg3890391#msg3890391)

So the P stands for pico power (the low power modern version) and if the package is a 28-pin DIP like in the photo I took then this specific ATMEGA328P U is an ATMEGA328P-PU.

ATMEGA328P U in a 28-pin DIP is an ATMEGA328P-PU

Note that the Microchip response does say that the packaging is omitted from the marking. Hence we can expect ATMEGA328P Us to pop up in other packages. What the U stands for is unclear.

  • The U signifies ROHS compliance. For specialized applications other options are available. The ATmega328-MMH, for example, has a NiPdAu lead finish. Commented Apr 5, 2021 at 4:58
  • @Julian I went through all the Product Change Notification pdfs in this mouser link mouser.in/ProductDetail/Microchip-Technology-Atmel/… There is no mention of ATMEGA328p U. Do you have some official docs that is saying ATMEGA328P U is a legit IC Code?
    – Robot
    Commented May 24, 2021 at 10:05

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