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.
Binary Coded Decimal format is a binary encoding of decimal numbers that represents each decimal digit by a fixed binary number. For example, 42 is represented in BCD format by the binary representations of 4 and 2, as shown above. The BCD format is common in electronic systems where numeric digits are displayed, as well as in systems where the rounding and conversion errors introduced by binary floating point representation and arithmetic are undesirable.
We will focus on designing a conversion circuit that converts a BCD formatted number to to a binary formatted number. I chose to detail this direction of conversion as binary to BCD conversion circuits are easily be found by a quick web search.
We will consider two algorithms to perform the conversion, the first being a direct arithmetic approach, and the second an iterative algorithm using a finite state machine with data path (FSMD).
We will be designing for the Basys 2 FPGA board which has 8 input switches. We can use the 8 input switches to encode 2 BCD numbers of 4 bits each. We will therefore concern ourselves with designing a circuit to convert a 2 digit BCD number to a 7 bit binary representation (27 = 128 > 99, the largest 2 digit BCD number we can input).
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.