It is fairly common that microcontroller output pins can sink more current in the low state than they can source in the high state. As a result, designers got used to putting LEDs, or anything else that needs a high (for a microcontroller pin) current between power and the pin instead of between ground and the pin. When the micro has symmetric source/sink capability, this is not necessary, but does no harm either.

For example, here is a snippet from the PIC 16F1459 (a reasonably recent and certainly mainstream production part) datasheet:

Note how the currents for the Output Low Voltage case are higher at the same supply voltage than for the Output High Voltage case. And, the sink currents are specified for a 600 mV rise, while the source currents for a 700 mV drop. All in all, this micro has substantially stronger low side drivers on its regular I/O pins.

When the LED requires more current than a digital output can handle, or at least more than you want to let it handle, you need to use a external transistor. A low side switch is the natural and simple choice. The LED is then connected between power and this transistor.

It is fairly common (although not as common as it used to be) that microcontroller output pins can sink more current in the low state than they can source in the high state. As a result, designers got used to putting LEDs, or anything else that needs a high (for a microcontroller pin) current between power and the pin instead of between ground and the pin. When the micro has symmetric source/sink capability, this is not necessary, but does no harm either.

For example, here is a snippet from the PIC 16F1459 (a reasonably recent and certainly mainstream production part) datasheet:

Note how the currents for the Output Low Voltage case are higher at the same supply voltage than for the Output High Voltage case. And, the sink currents are specified for a 600 mV rise, while the source currents for a 700 mV drop. All in all, this micro has substantially stronger low side drivers on its regular I/O pins.

Many newer micros are symmetric, apparently particularly those that don't have much source/sink capability in the first place.

When the LED requires more current than a digital output can handle, or at least more than you want to let it handle, you need to use a external transistor. A low side switch is the natural and simple choice. The LED is then connected between power and this transistor.

It is fairly common that microcontroller output pins can sink more current in the low state than they can source in the high state. As a result, designers got used to putting LEDs, or anything else that needs a high (for a microcontroller pin) current between power and the pin instead of between ground and the pin. When the micro has symmetric source/sink capability, this is not necessary, but does no harm either.

When the LED requires more current than a digital output can handle, or at least more than you want to let it handle, you need to use a external transistor. A low side switch is the natural and simple choice. The LED is then connected between power and this transistor.

It is fairly common that microcontroller output pins can sink more current in the low state than they can source in the high state. As a result, designers got used to putting LEDs, or anything else that needs a high (for a microcontroller pin) current between power and the pin instead of between ground and the pin. When the micro has symmetric source/sink capability, this is not necessary, but does no harm either.

For example, here is a snippet from the PIC 16F1459 (a reasonably recent and certainly mainstream production part) datasheet:

Note how the currents for the Output Low Voltage case are higher at the same supply voltage than for the Output High Voltage case. And, the sink currents are specified for a 600 mV rise, while the source currents for a 700 mV drop. All in all, this micro has substantially stronger low side drivers on its regular I/O pins.

When the LED requires more current than a digital output can handle, or at least more than you want to let it handle, you need to use a external transistor. A low side switch is the natural and simple choice. The LED is then connected between power and this transistor.