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
Best Answer
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.