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I'm not sure what this symbol represents:

enter image description here

It comes from figure 5-2 on page 19 of the data sheet for the ATSAM3U2CA-AU

Heres a picture of that page: enter image description here

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  • \$\begingroup\$ There may be some subtlety to the choice of the symbol. Normally one would expect a ceramic capacitor for decoupling the microcontroller core supply. The datasheet specifies on page 1089 that it must have ESR > 0.5 ohm, but at the same time claims that ceramic capacitors are ok, which is a bit conflicting. \$\endgroup\$
    – jpa
    Jun 22 at 12:28
  • \$\begingroup\$ @jpa MLCCs can have surprisingly high ESR for ceramics. You can also add a series resistor to compensate for too low ESR. \$\endgroup\$
    – Hearth
    Jun 22 at 15:23
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This is one of the ways of representing a capacitor: enter image description here

The extra line below is the ground connection.

The image is taken from the link below: https://iastate.pressbooks.pub/electriccircuits/back-matter/appendix-c/

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Regarding the datasheet specifically, it is unfortunately another case of "Atmel-quality" documentation. As an example they inconsistently use different symbols for polarized caps across the manual.

Normally, we avoid polarized caps for low values since ceramics are available - far cheaper and far more reliable than tantalum or aluminium electrolyte. So when one draws a polarized cap, it usually means that ESR of that cap is important.

Looking at the unnamed caps in your picture, they are secretly named on p.1089 and specified as:

  • The cap on VDDIN CDin is specified to be typically 10uF.
  • The cap on VDDOUT CDout (Note 2) is specified to be typically 4.7uF. It needs to have ESR between 0.5 to 10 Ω

As suspected by the symbol, ESR is important. There's the following foot note 2 regarding ESR:

  1. To ensure stability, an external 4.7μF output capacitor, CDOUT must be connected between the VDDOUT and the closest GND pin of the device. The ESR (Equivalent Series Resistance) of the capacitor must be in the range 0.5 to 10 Ω. Solid tantalum, and multilayer ceramic capacitors are all suitable as output capacitor.

Peeking at typical ESR ranges for different cap chemistries at Wikipedia https://en.wikipedia.org/wiki/Equivalent_series_resistance then we see that ceramics likely have much lower ESR than what's specified. Which is normally just a good thing.

Oddly, other SAM parts don't have that requirement. SAMC for example says max 0.5Ω ESR. It might be a newer part though.

So this is where we could probably just use common sense, roll our eyes at the "Atmel-quality" manual and just pick some standard X5R cap. Voltage rating isn't important here, so any 0402 or larger should come with 4.7uF.

If you would rather not gamble but follow the recommendations, then before picking some expensive "low ESR" tantalum (or series resistance), I would contact Microchip support and ask them why their Atmel documentation is being strange yet again. Likely they'll respond that it's fine to use plain old ceramics.

Make sure to also check silicon errata, as always.

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