FPGA devices are often driven by clock signals with frequencies ranging from 10 to 100 MHz, while human eyes are much slower. Conventionally, resistors and capacitors on printed circuit boards are employed to create the necessary time constant for monostable circuits to drive LEDs. In the FPGA, however, it is more convenient to use multibit counters to produce signals in the frequency range of a few Hz, as shown in Figure 4.1a.