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What are most important differences between ATMega32u2 and ATMega32u4?

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Compared to the u2, the u4 has:

  • More RAM (2.5k vs. 1k)
  • More pins (48 vs. 32)
  • Analog-to-digital converters instead of just analog comparators
  • Low frequency crystal option instead of full swing crystal option
  • An I2C interface.

To answer this, I did a DigiKey search for each part number and compared the result tables. This is easier than comparing the datasheets, although datasheets are the most accurate place to find information. For example, the oscillator information in Digikey suggests that the u4 requires an external oscillator, while the u2 requires an internal oscillator. This is not the case; the u4 ships with the external oscillator as the default and the u2 ships with the internal oscillator as a default, but both can use internal and external clock sources.

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  • \$\begingroup\$ The datasheets says (section 6.2.2 for u4 and section 8.3.1 for u2) that for both the default enabled clocksource is the internal RC oscillator. \$\endgroup\$ – PetPaulsen Oct 9 '11 at 8:22
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    \$\begingroup\$ It should be noted that for u2 parts the datasheet is inconsistent in several places. Atmel's USB chips actually come preloaded with a USB bootloader, which requires an external oscillator to run -- the chips therefore have the external oscillator selected as the default clock source. \$\endgroup\$ – avakar Oct 9 '11 at 9:12
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    \$\begingroup\$ @avakar the same with ATMega8u2 - it is pre-configured for external oscillator - USB would've not worked otherwise and I surely hadn't changed any of the fuses since I only programmed it through USB DFU \$\endgroup\$ – Mihailo Oct 9 '11 at 11:05
  • \$\begingroup\$ Ok, so I understand there are quite a lot diffs, and not that easy to find them... thanks. Also it was very interesting for me to learn that there are reportedly inconsistencies in the datasheets/manuals, and I must not necessarily believe them fully... \$\endgroup\$ – akavel Oct 27 '11 at 18:26
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    \$\begingroup\$ The datasheet *u2 section 25.2 Fuse Bits states "The default setting of CKSEL3..0 results in External crystal Oscillator 8MHz. See Table 8-1 on page 29 for details."(pg 248) BUT the cross ref says "The device is shipped with internal RC oscillator at 8.0 MHz " For some reason Atmel won't fix this. \$\endgroup\$ – Andrew Mar 1 '16 at 14:20
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The U2 and U4 indicate the USB module version of atmel's chip. That mean that U4 has more usb capabilities/flexibility than the U2.

You can find in the LUFA documentation the description for every USB class, the USB modules supported.

For example, the Joystick Device Demo say :

Demo Compatibility:

 The following list indicates what microcontrollers are compatible with this demo.

 - Series 7 USB AVRs (AT90USBxxx7)
 - Series 6 USB AVRs (AT90USBxxx6)
 - Series 4 USB AVRs (ATMEGAxxU4)
 - Series 2 USB AVRs (AT90USBxx2, ATMEGAxxU2)
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