Reversing the LED? i.e. sinking it to MCU?

Another important GPIO use is to control LEDs as indicators. The first way to perform this is to

simply connect the led to the GPIO and drive it using the pin. A resistor of an appropriate value

should be added in series with the LED to limit the current because the microcontroller will source

(provide) the current and its capabilities in this respect are limited. Typical LEDs require on the

order of tens of milliamperes. For low power microcontrollers this figure reaches the upper limit

of the current sourcing capability of the microcontroller. Because this current is shared among

the GPIO, adding LEDs can limit the drive capability of I/O. This might require buffering and

complicate the circuity. A solution commonly used is to connect the LED in reverse, as shown

in figure ??. Here, the microcontroller is used to sink the current, which it is usually able to

do more readily. Unfortunately TI does not provide much information in this respect and some

experimentation is in order.