UART Controlled Stopwatch Using an FPGA


Being able to interface an FPGA project with a device that has a serial port allows for a basic means of sending and receiving data between the FPGA and a PC. For this we need to implement a Universal Asynchronous Receiver Transmitter (UART) circuit within the FPGA system. You can then use a PC with an RS232 Serial port (shown above) to interface the FPGA with a PC, assuming your development board has the port and voltage conversion circuitry.

Because RS232 is a dated protocol that sometimes isn’t available on modern PC’s, another option is to use a USB-Serial converter chip, which connects to a USB port.

In this post I will detail a UART controlled stopwatch, designed for the Basys 2, using the same stopwatch implementation as in my previous post. To communicate with the PC through serial  we will implement a UART receiver and transmitter circuit, as well as a master control circuit that routes all the pieces together. We will then be able to control the stopwatch’s start, stop, and clear functions by sending ascii characters from the PC to the FPGA’s UART receiver circuit, and also send the current stopwatch time from the FPGA’s Transmitter circuit to the PC for viewing.

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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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BCD to Binary Conversion on an FPGA

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).

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