Project 6: UART

B441/E315, Fall 2018                                                        Version 2018.4

Autograder Due: 11:59pm Sunday, Oct 21, 2018

Demo Due: 2:15pm Friday October 26, 2018


In this lab you will practice using state machines by designing and implementing a UART (or RS232) interface.  



UART is short for “Universal Asynchronous Receiver / Transmitter”.  It allows serial communication between two devices.  In our case we’re using it to communicate between the Host machine and the FPGA.  There are many different flavors of UART, but we will be using the RS-232 standard.  

Side note:  We’re actually using RS-232 encoded USB.  Luckily, there is a chip on the Basys3 board that is translating the USB into RS-232 for us.  

UART (RS-232) has two wires, RX (receive) and TX (transmit).  The are connected as follows, were the TX of one device is the RX of another, and vice versa.

UART is composed of 1+8+1 bit “frames”.  The transmitter keeps the TX wire high when not transmitting.  To transmit, the transmitter first pulls the TX wire low for one “bit” (called the start bit), followed by  8 data bits, followed by one high “bit” (called the stop bit).  This is shown below.


You know that data is coming when the line goes low.  You discard the first start “bit”, then capture 8 bits, then discard the last stop “bit”.  The tricky part of UART is determining when the next “bit” is on the wire.  For our project, we’re using 9600 baud, or 9600 bits-per-second.  Bits here includes the start and stop bits.  Therefore, each bit should require roughly 9600Hz^-1 = 104.166 us. Trying to sample a bit at the beginning or end of it’s 104us time window is unreliable.  A better approach is to sample at the middle of the time window or midbit.  In our case that’s 52 us from the beginning of the bit.  This is illustrated below.  

You will need to create a timer that uses the 100MHz Basys3 clock to trigger both a 104us and 52us sampling times.   When your uart module detects the RX line going low, it should wait 52us before sampling the start bit.  It should then wait another 104us before sampling the first data bit (D0).  It should continue to wait 104us between sampling each bit.   After 1+8+1 bits, the UART packet is complete.  You can throw away the start and stop bits, and are left with an 8-bit value.  


You get to drive the data line here.  When not transmitting, it should be high.  To begin a transmission, simply drive the data line low.  Then wait 104us (representing the passing of the start bit), then drive it with D0.  Then wait 104us and drive it with D1, et cetera.  To finish, drive it high for 104us (representing the passing of the stop bit).  

Clock Domains

When crossing clock domains (ie going between the desktop computer clock and the FPGA’s clock) weird (bad) things can happen when the clocks don’t line up exactly.  Data values can become corrupted, entirely lost, or may reach some flip flops one clock cycle and others the next cycle.  This is illustrated below.  

As such, it is important to synchronize your input signals, ie UART’s RX line.  We recommend using the following Verilog module to synchronize your input buffers:

module synchronizer (

        input           clk, //your local clock

        input           async, //unsynchronized signal

        output          sync //synchronized signal


        // Create a signal buffer

        reg [1:0] buff;

        always @ (posedge clk) begin

                if (async)  buff <= 2'b11;

                else        buff <= {buff[0], 1'b0};


        assign sync = buff[1];


An example of using the synchronizer module is shown below:

// sync signals crossing clock domains

synchronizer syncUartRx(





Connecting over UART with Screen

You can use the Unix program screen to connect to your FPGA over UART.  You need to specify the USB device that is encoding the RS-232 (see side note above).  This is a little tricky.  On a Mac, it should look something like this:

 On Linux, it looks something like this:

Note:  9600 is optional. You can exit screen with the following key combinations:

Assignment Description

Your assignment is to create the following Verilog modules and testbenches as specified below.  


Create a Verilog file named uart_rx.v which defines a module as follows:

module uart_rx (

    input       clk,

    input       rst,

    input       uart_rx_in,

    output [7:0] data,

    output       ready //high for 1 clk


Clk and rst are the standard clock and reset signals.  uart_rx_in is RX line into your UART module (this is TX on the host machine).  ready is a signal that tells the rest of the system that uart_rx has received a valid UART frame.  When ready is high, the uart_rx module will drive the data signal with the received byte.  

The ready signal should reset to low (ie if rst is asserted).  After that, the ready signal should remain low until the first UART frame/packet is completely received.  It should then remain high until the beginning of the next UART frame.  Essentially, the ready signal indicates when the UART module has received data to work with.


Create a Verilog file named uart_tx.v which defines a module as follows:

module uart_tx (

        input       clk,

        input       rst,

        output      uart_tx_out,

        input [7:0] data,

        input       req, //request

        output      empty, //able to accept new data

        output      err //request error


Clk and rst are the standard clock and reset signals.  uart_tx_out is TX line out of your UART module (this is RX on the host machine).  The data signal is ignored until req (or request) is high.  On the first positive clock edge that req is high, uart_tx should latch the data into an internal register.  It should then begin transmitting the UART frame of the latched data.  The latched data should be transmitted least-significant bit (LSB) first, ie data[0] to data[7].  After one cycle, the req line should be lowered.  All subsequent changes to data should be ignored until the next request.  

The empty signal indicated that uart_tx is not currently transmitting a frame, ie “empty”, and is ready to accept a new transmit request.  This signal should go low when a transmit request is made, and stay low until the completion of the entire transmission (including the stop bit).  If a new request is made while the module is transmitting a previous request, the err (error) signal should be asserted to indicate the request will not be honored.  


Create a Verilog file named uart.v which defines a module as follows:

module uart(

    input        clk,

    input        rst,

    input        uart_rx_in,

    output       uart_tx_out,

    input [7:0]  tx_data,

    input        tx_req,

    output       tx_empty,

    output       tx_err,

    output [7:0] rx_data,

    output          rx_ready,


This module just connects the two uart submodules, uart_tx and uart_rx.  


Create a Verilog file named SevSegDriver.v which defines a module as follows:

module SevSegDriver (

    input clk,

    input rst,

    input [7:0] byte0,

    input [7:0] byte1,

    output  [6:0] seg,  

    output  [3:0] an



NOTE:  You do not need to write this module yourself.  The code for this module will be available on Piazza.  


Your module should receive UART packets over the RX link, and display the hex contents of each byte on the right-most two Seven-Segment Displays. It should also echo the byte back to the sender over the TX link. An diagram is shown below.  

To do this, create a Verilog file named top.v which defines a module as follows:

module top(

    input           clk,

    input           btnC, //aka rst

    input           RsRx, //aka uart_rx_in

    output          RsTx, //aka uart_tx_out

    output   [15:0] led,  //this is optional

    output   [6:0]  seg,

    output   [3:0]  an



wire RsRx_sync;

synchronizer syncUartRx(





//Your code here



For this project, you need to create two testbenches.  The first will test uart_tx and is named uart_tx_tb.v.  The second will test uart_rx and is named uart_rx_tb.v. We do not require a testbench for uart, SevSegDriver, or top. 

For testing, we recommend creating two UART modules, connecting the TX of one to the RX of another, and using one to test the other.  

Remember to select “System Verilog” from the “File Type” drop-down menu. 


You will also need to reconfigure your constraints file to align with the top-level module declaration.  The names should line up properly by default.  A reference file is available in the Google Drive folder.  


The evaluation will have two steps, first submission of your source code and testbench to the autograder.  Second, you will need to synthesize your design, download it to the FPGA and do a demonstration for the TA.

Autograder (70%)  

Log on to and submit your code as per Project 1.

Demonstration (30%)

Program your FPGA with your demultiplexer and demonstrate your working system to the TA.  You will not receive full points until the TA has approved your demonstration.