What are the differences between the -AURCT vs -AUR versions of Atmega32u4 MCU? I can't find any info on AURCT from the datasheet

  • \$\begingroup\$ I would say it is something related to the ordering and packaging. Really irrelevant if you plan to order insignificant amount or not going to use some special pick-and-place machines for production. \$\endgroup\$
    – Eugene Sh.
    Commented Oct 31, 2016 at 14:05
  • \$\begingroup\$ I am just worried that CT stands for crystal (as in the fuse bits are preconfigured to either run with an external crystal or not). \$\endgroup\$
    – Adam Lee
    Commented Oct 31, 2016 at 14:11
  • \$\begingroup\$ I support @EugeneSh. and think that this just specifies the default clock source: internal or external. If I understand correctly, if you want to program your chip for the first time, you will need to connect a crystal oscillator if the part has default external oscillator setting, whereas for the internal, you can program it directly see page 422. \$\endgroup\$
    – Nazar
    Commented Oct 31, 2016 at 14:13
  • \$\begingroup\$ Thanks. So which one requires external oscillator? It's really hard to tell. Searching for "AUR" does not result in anything! \$\endgroup\$
    – Adam Lee
    Commented Oct 31, 2016 at 14:16
  • 1
    \$\begingroup\$ Are you ordering from Digikey? Because if so, the "CT" means "Cut Tape". \$\endgroup\$ Commented Oct 31, 2016 at 15:12

1 Answer 1


I am not sure where you have got the part numbers that are in your question title, but it looks like for a ATmega32U4 you have only four options (see page 422): two package and two oscillators.

The -AU/-MU select the 44ML/44PW package respectively, whereas the 32U4/32U4RC select the oscillator option. They both should be the same internally except the default oscillator. They both need an external oscillator if you want to run them at > 8MHz. See page 29 section 6.2.1 and 6.2.2 - the difference is how the fuses are programmed when shipped. So, the 32U4-AU is set by default to work with the external oscillator, whereas the 32U4RC-AU is set to work with the internal oscillator and both are 44ML TFQP package. So, if you get the 32U4-AU, you will need to connect it to an oscillator to be able to reprogram it.

As for the additional -R letter I would guess that it signifies that the parts are packaged and shipped in a reel.

  • 1
    \$\begingroup\$ Per your last assertion re: the -R suffix, you are correct \$\endgroup\$ Commented Oct 31, 2016 at 15:16

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