Spartan-3E Starter Kit Board User Guide www.xilinx.com 17UG230 (v1.0) March 9, 2006Rotary Push-Button SwitchRIn some applications, the BTN_SOUTH push-button switch is also a soft reset thatselectively resets functions within the FPGA.UCF Location ConstraintsFigure 2-5 provides the UCF constraints for the four push-button switches, including theI/O pin assignment and the I/O standard used, and defines a pull-down resistor on eachinput.Rotary Push-Button SwitchLocations and LabelsThe rotary push-button switch is located in the center of the four individual push-buttonswitches, as shown in Figure 2-3. The switch produces three outputs. The two shaftencoder outputs are ROT_A and ROT_B. The center push-button switch is ROT_CENTER.OperationThe rotary push-button switch integrates two different functions. The switch shaft rotatesand outputs values whenever the shaft turns. The shaft can also be pressed, acting as apush-button switch.Push-Button SwitchPressing the knob on the rotary/push-button switch connects the associated FPGA pin to3.3V, as shown in Figure 2-6. Use an internal pull-down resistor within the FPGA pin togenerate a logic Low. Figure 2-9 shows how to specify a pull-down resistor within the UCF.There is no active debouncing circuitry on the push button.Figure 2-4: Push-Button Switches Require an Internal Pull-Down Resistor in FPGAInput PinUG230_c2_03_021206BTN_* SignalPush Button3.3V FPGA I/O PinFigure 2-5: UCF Constraints for Push-Button SwitchesNET "BTN_EAST" LOC = "H13" | IOSTANDARD = LVTTL | PULLDOWN ;NET "BTN_NORTH" LOC = "V4" | IOSTANDARD = LVTTL | PULLDOWN ;NET "BTN_SOUTH" LOC = "K17" | IOSTANDARD = LVTTL | PULLDOWN ;NET "BTN_WEST" LOC = "D18" | IOSTANDARD = LVTTL | PULLDOWN ;
