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.