Why are my LEDs partially lit?

I had finished using the diagnostic LEDs on the 3005 and have routed my output signals to various pins using the BRK3005 breakout board. However I notice the LEDs are often still lit to some degree, suggesting a signal of some sort is making its way to them? My .ucf file no longer specifies the led outputs so I am wondering what is happening?

Hi Chris–

Partially lit LEDs can be caused by a couple things.

First, if the device (FPGA) is not powered up, but you have some signals that are driving the FPGA I/Os externally, there can be some leakage current that gets to your LED I/Os.

Second, you should check the output files from synthesis (pad file) to see what was configured on the I/Os corresponding to the LEDs. They could either be left floating or configured to some other function causing this.

Generally, we would recommend specifying every pin on the FPGA that is connected to a peripheral. If that peripheral is not used, simply tie the signal to a known state that powers down the peripheral. This is the best way to assure known signals at the board level.

For example, if you’re not using the LEDs, just tie them all high to turn off the LEDs.