FPGA Reaction Timer

Reaction time is the duration of time it takes for the brain to interpret a stimulus and do something in reaction to it. The stimulus may be something visual such as a light turning on, something auditory such as a beep, or a touch cue such as a poke. The time it takes for the brain to interpret a stimulus and respond to it can be used as a basic benchmark to measure and compare mental acuity.

We will be implementing a reaction timer on an FPGA that turns on an LED after a psuedorandom period of time, and uses a pushbutton as a reaction input. There will be 3 input buttons: clear, start, and stop. The system will begin in an idle state waiting for the user to press the start button. When the start button is pressed, a random time interval will elapse before the LED turns on. When the LED turns on a reaction timer will begin counting the number of milliseconds until the user presses the stop button. When the stop button is pressed, the reaction time will be shown on a 4 digit 7-segment display in the format “0.000” seconds, up to a value of 9.999 seconds. The user can then press the clear button to reset the time display and go back to the idle state.

For this project we will be using the Basys 2 FPGA development board to implement the design, as it has the 4 digit display, pushbuttons, and LED that we need onboard.

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Stopwatch with the Basys 2 FPGA

A stopwatch is a good FPGA project that covers many basic, yet interesting areas of FPGA design. We will need display multiplexing for the multi-digit display, synchronous cascaded counter circuits to increment time registers for seconds and minutes, and a finite state machine to give us start, stop, and reset functionality.

I recently acquired a Basys 2 FPGA development board, which has an on-board 4 digit seven-segment display that lends itself nicely to keeping track of time in MM.SS format. Let’s build a stopwatch.
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