ATMEGA328P-U vs ATMEGA328-PU

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:

328

328P

328PB

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:

http://ww1.microchip.com/downloads/en/DeviceDoc/ATmega48A-PA-88A-PA-168A-PA-328-P-DS-DS40002061A.pdf

ATmega328-AU

ATmega328-AUR

ATmega328-MMH

ATmega328-MMHR

ATmega328-MU

ATmega328-MUR

ATmega328-PU

And 328P options:

ATmega328-AU

ATmega328-AUR

ATmega328-MMH

ATmega328-MMHR

ATmega328-MU

ATmega328-MUR

ATmega328-PU

With these rated to 105C

ATmega328P-AN

ATmega328P-ANR

ATmega328P-MN

ATmega328P-MNR

ATmega328P-PN

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.

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 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.

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.