LED lines need pull-down resistors
LED lines from FPGA are negative logic and are connected to front and
rear panel LEDs through inverting schmidt triggers.
These LEDs are supposed to light up immediately at board power up, hence why the negative logic is used. It was assumed that these lines will be Low during FPGA configuration.
However, user I/O pins of the FPGA are set to be floating during power up.
Moreover, the data sheet of the inverting schmidt triggers does not recommend using floating inputs.
Although the correct behaviour is observed on prototypes, the use of floating inputs on the schmidt triger may happen to be problematic in noisy environments,
If a new version of this board is to be produced, pull-down resistors are recommended at the input of the schmidt trigger inverters.