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PLTW S7 Reference Manual

Basic I/O

The PLTW S7 includes one RGB LED, 4 individual LEDs, and 2 push buttons, as shown in Figure 6.1. The push

buttons are connected to the FPGA via series resistors to prevent damage from inadvertent short circuits (a short

circuit could occur if an FPGA pin assigned to a button was inadvertently defined as an output). The buttons are

"momentary" switches that normally generate a low output when they are at rest, and a high output only when

they are pressed.

The four individual high-efficiency LEDs are anode-connected to the FPGA via 330 Ohm resistors, so that they will

turn on when a logic-high voltage is applied to their respective I/O pins. Additional LEDs that are not user-

accessible indicate power-on, FPGA programming status, and USB-UART traffic.

The single RGB LED has three input signals that drive the cathodes of three smaller internal LEDs: one red, one

blue, and one green. Driving the signal corresponding to one of these colors low will illuminate the internal LED.

The RGB LED will emit a color dependent on the combination of internal LEDs that are currently being illuminated.

For example, if the red and blue signals are driven low and green is driven high, the RGB LED will emit a purple

color.

Note: Use of pulse-width modulation (PWM) is strongly recommended when driving the RGB LED. Driving any of

the signals to a steady logic '0' will result in the LED being illuminated at an uncomfortably bright level. This can be

avoided by ensuring that none of the RGB signals are driven with more than a 50% duty cycle. Using PWM also

greatly expands the potential color palette of the RGB LED. Individually adjusting the duty cycle of each each color

between 50% and 0% causes the different colors to be illuminated at different intensities, allowing virtually any

color to be displayed.

Figure 6.1 shows how each of the LEDs and push buttons are connected to the Spartan-7.

Other product and company names mentioned may be trademarks of their respective owners.

Figure 5.1. USB-UART bridge.

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