In this board, to keep the design simpler we do not use a transistor to drive the LEDs, differently to other mezzanines/carries.
- With the FPGA output at "0" the LED is ON and with the limiting
resistor of 120 Ohms, there are 11mA to be sinked by the FPGA pin
(LVCMOS25 with default driving/sinking strength 12mA).
From the LED specs, with If=10mA the intensity of the light is: red: 13 mcd, green: 9 mcd (see picture)
- With the FPGA output at "1" there is no current flowing as the LED Vf
= 2V, so the LED is OFF.
- With the FPGA output at "highZ" there is some current flowing (through the FPGA pin protection diode probably) that slightly lights the LED (see picture).