RichardE Posted December 15, 2012 Report Share Posted December 15, 2012 I wonder if anyone would care to comment on this:I'm just starting out on VHDL and following the IntroToSpartanFPGABook. One of the early examples configures C0 to C7 as digital intputs which are connected to switches and configures C8 to C15 as digital outputs which are used to drive LEDs. The states of the 8 switch inputs are written to the 8 digital outputs.However, not having a LogicStart MegaWing I only have the one switch and I move that around the digital inputs. What I was finding was that when only a single digital input was pulled high 2 of the LEDs would turn on - the one I expected and also an adjacent one.I was convinced I must have a short somewhere in the wiring to the LEDs or a solder bridge between pins on the header but I could not find it. Then it struck me that since I only had the one switch, all the remaining inputs were left floating so I changed the constraints so that all the digital inputs had pulldowns and now it works fine - only one LED turns on if only a single input is taken high.I realise it is not really a good idea to leave inputs floating but I have to admit that when working with PIC micros and the like I have on occasions left inputs floating with no ill effects. I have seen many designs where unused IO pins have been left configured as inputs.So my questions are:1. Is the behaviour I describe above something others have experienced and would this be expected?2. Are the Spartan 3E FPGAs particularly susceptible to this because of the construction of the inputs?3. Aren't all unused IO pins configured as inputs? Does this mean I should really connect all unused inputs to 0V (or with a pullup or pulldown resistor)? Link to comment Share on other sites More sharing options...
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