Very simple question regarding LEDs and FPGA

[Sink0]

On a lot of FPGA designs i see LEDs connected with the Anode at the VCC and Cathode at the pin. What is the advantage of this configuration considering a Push-Pull output (open-drain with internal pull-up would be obvious)?

Thank you!

 

[tsvetelin_velkov]

possibly the fpga pin is not symmetrical, can sink more current than source or vice versa. you have to refer to the datasheet for that. in general it is not a good practice to power leds directly from the fpga, better use some kind of driver.

 

[Sink0]

I am talking about signaling leds... i can't see a reason use an external driver for that, would be a huge waste of space and money with no meaningful gain. Anyway i will take a look at this scpecification ata datasheet but i can't see why that's the reason. Most of FPGAs (at least low end ones) can drive up to 24mA on any pin.

Thank you!

 

[FvM]

Some FPGA power up with a weak pull-up resistor at all I/O pins. If you won't signals turned on before configuration, you should define I/O signals active low. It's also usual, to have higher drive strength for low side transistors, but the ration isn't very high for usual CMOS I/O standards.

I don't see a problem to drive moderate LED currents (e.g. 5 or 10 mA) from FPFA outputs directly.