Hi all,

When connecting an LED to a microcontroller IO pin, what is actually the difference between connecting it in current sinking mode or current sourcing mode (apart from the face that in current sinking you have to output logic 1 for the LED to turn on and vice versa)?

 

Some CMOS chips can sink about 3X what they can source. With TTL it is even worse, more like 15X...

Consult the data sheet for the source/sink capability of any chip you use to drive a LED

 

Arrgh! Terminology and semantics!
It is my habit to think of the microcontroller to be sourcing or sinking current, so your last statement sounds backwards to me. The only difference I see is that you must look up the capability of the chip in both modes because some can conduct more current in one direction than the other.

Sometimes you might be doing a waste power budget and choose for best saturation voltage inside the chip.

and...MikeML got there first.

 

e.g an input device connected to +ve sources, the input device will sink the device.
The pdf shows how it pertains to PLC but the principle is the same.
Max.

 

Thanks for the reference, @MaxHeadRoom . It resembles my model of the universe, but I've learned a hundred times that my model is only one of the acceptable models.

 

Thanks a lot for the replies.
When it comes to calculation, is there a different formula for connecting an LED in sinking or sourcing?

 

The only difference is in the saturation voltage of the sourcer/sinker. All else is EIR and PIE, as usual.

 

Thanks a lot for the replies.
When it comes to calculation, is there a different formula for connecting an LED in sinking or sourcing?

The data sheet should state the source and sink current capability - if neither is enough for the LED you'll need a driver transistor anyway.

It should also state how close the output can get to Vss or Vdd at the specified current load - this indicates what voltage you need to subtract from the value of Vdd to calculate the current limiting resistor - the LED of course has a Vf volt drop which you have to add to the balance of Vo when you calculate the resistor.