The UC3C Datasheet doesn't provide a clear explanation about "drive strength", how it behaves and the side effects in projects. After fabricating my board I realized I placed a LED on a x1 strength pin (AVR32UC3C2512C pin PB00) and I calculate a 23mA current ((5V[input] - 1.5V[LED drop])/150ohm).

In table 40-6 there is a tip that x1 handles 3.5mA, so what happens if I turn this LED on? Will the LED drain more current than the pin can handle and burn the MCU or will the MCU limit the current to 3.5mA? Or does it means something entire different?

The Circuit:


simulate this circuit – Schematic created using CircuitLab


1 Answer 1


Do not trust an MCU output pin to effectively limit the current. It will likely damage your part. Even if it does it correctly once, or the first few times, or even the first few hundred times, you are compromising your design. You need either a higher current limiting resistor (your LED may turn on with 2 or 3 mA) or you need a buffer.

  • \$\begingroup\$ This is a general answer about MCUs. I am assuming that you have correctly read the datasheet and 3.5 mA is the maximum output current. \$\endgroup\$
    – scld
    Commented Dec 11, 2013 at 13:57
  • \$\begingroup\$ I thought the same, but since it's so bad documented maybe it could mean something else...there isn't a table with current limits for pins, just 2 ambiguous tables for maximum load on Vcc pins and output leve voltages given some current thresholds (tables 40-1 and 40-6) \$\endgroup\$
    – mFeinstein
    Commented Dec 11, 2013 at 13:58
  • \$\begingroup\$ I also answered without looking at the datasheet because 3.5 mA is a very reasonable number. As you said, it can be vague but as a rule I don't like to depend on my MCU to drive much current. \$\endgroup\$
    – scld
    Commented Dec 11, 2013 at 14:39
  • \$\begingroup\$ I was used with the PICs that can handle 25mA per pin \$\endgroup\$
    – mFeinstein
    Commented Dec 11, 2013 at 14:59

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