Learn Verilog by Example

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Serial Receiver and Transmitter (UART) in Verilog | FPGA

Note: This was a post I wrote back in 2014 as I was teaching myself Verilog, but never got around to finishing the code for it and thus this post remained as a draft. I apologize that this is not in line with my other previous blog posts as it does not have a working code and is not demonstrated working in a video. But I am publishing this in hopes that someone researching this topic may find my introductory text on the topic useful. UART stands for Universal Asynchronous Receiver and Transmitter. It is a system that is a capable of sending parallel data using a serial line. As the name indicates, the UART consists of a transmitter and a receiver. The transmitter accepts data in parallel and then by using a shift register send the data out bit by bit using predetermined parameters. Similarly the receiver accepts the data serially and then compiles the data using a shift register. Data is recognized by following a fixed format while transmitting and receiving, data sent consists of a s...

Reaction Timer | Reflex Tester in Verilog and FPGA

This post will be about programming a reaction timer in verilog and make it ready for FPGA implementation which in our case will be on the BASYS2 (Spartan 3E) board but can be programmed for any other board by simply changing the .ucf file. The reaction timer or reflex tester will check and time how fast you can respond after seeing a visual stimulus or in other words it will test your hand eye co-ordination. The code for this is a bit more busy than any of my other projects, but it has been heavily commented so just by reading the code you can easily understand what is going on. The code will be followed by a video demonstration in Isim along with it working on the FPGA board. Here we will be using three inputs; reset , start , stop and one output led along with displaying the time on the seven segment display . When the reset button is pressed it will obviously reset all registers and counters and will make the system ready for the next reflex test. Also when in this state it ...

Random Number Generator in Verilog | FPGA

In verilog a random number can be generated by using the $random keyword but that works only for simulation purposes and cannot be implemented. So when we need a random number for implementation i.e. in hardware it can be achieved using an LFSR (Liner Feedback Shift Register). An LFSR is simply a shift register with some of its bits (known as taps) XOR'd with themselves to create a feedback term. When implementing an LFSR it's width and it's repeatability must be kept under consideration .An N-bit LFSR will be able to generate (2**N) - 1 random bits before it starts repeating. For example a 30 bit LFSR will have 1073741823 random states before repeating, so for most practical purposes this can be considered true random. In an LFSR the MSB will always be the feedback point also the main thing to take care of while coding an LFSR is to know which bits are the taps (to be selected for XOR ). This is confusing as the taps are different for different size registers. For e...

Universal Shift Left - Right Register/Circuit in Verilog

A shift register will "shift" itself to the left or right by one position every clock cycle. A shift left - shift left circuit, as simple as it sounds is a very important part of many circuits and will be a part of many of my future projects and that is why it needs to be addressed. The code for this circuit in verilog is written below. It is very simple and does not need any detailed explanation. It is controlled by 2 control signals which determines whether the circuit will shift right or left. If control[0] is high the circuit will shift right and if control[1] is high the circuit will shift left and if both are low stay with the default value. The data shifted is stored in an internal register and is output as a bus. I have coded it using if - else statements. This can also be coded using a case block if further control is required. module shift( input clock, input reset, input [1:0] control, input in, output [7:0] out ); reg [7:0] r_reg, r_nex...

Scrolling or Moving Text Using 7 Segment LED's in Verilog | FPGA

This one is going to be a quick easy project. The objective here is to get use the 7-segment LED display on the board to display a scrolling text. it should be slow enough so that it can be easily read. I have made the delay between each word shift to be 1 second. This is long enough to read the text comfortably without missing anything. I have explained in my previous post how to create an accurate delay using verilog, you can read about it there: Create accurate delay in Verilog . Now since I need a 1 second delay here the counter monitoring this count must count to 50,000,000. And the register needed to hold this count must be 29 bits wide. Using this method you can scroll and display anything. Here I have chosen to display the text "Hello There" to scroll across the LED's and appear to be continuous without any stops. The concept is pretty simple. I create a separate counter that increments every time a count of 50,000,000 is reached. This counter will count till ...

FIFO(First In First Out) Buffer in Verilog

A FIFO(First in First Out) buffer is an elastic storage usually used between two subsystems. As the name indicates the memory that is first written into the FIFO is the first to be read or processed. A FIFO has two control signals i.e. write and read. When write is enabled data is written into the buffer and when read is enabled data is "removed" from the buffer to make room for more data. This concept of write and read (remove) can be best understood from the conceptual diagram of a FIFO below: As can be seen, once the data is read, it can be considered as removed and thus allowing more data to be written into the buffer. Implementation of FIFO in Verilog To implement FIFO in verilog imagine the memory components to be arranged in a circular queue fashion with two pointers; write and read . The write pointer points to the start of the circle whereas the read pointer points to the end of the circle. Both these pointers increment themselves by one after each read or wr...

00 to 99 Two Digit Decimal Counter via 7 Segment Display Using Verilog

Since I have already made a detailed post regarding 7 segment LED multiplexing , this post is going to be a short one, in which I will only explain the code via comments in code. Here I am going to make a 2 digit counter that counts from 00 to 99 and then rolls over back to 00. The counter will increment every 0.1 second. The 0.1 second interval is produced by another counter that will produce an enable tick every 0.1 second to increment our main counter. How to make a 0.1 second accurate delay in Verilog We know that the board being used has a 50 MHz clock. So to produce a 0.1 second delay simply multiply the two.. 50Mhz * 0.1 sec = 5000000. So every 5M ticks is equal to 0.1 second. So using the simple log formula ( (x)log(2) = log(5000000) ) we can calculate that a 23 bit wide register will be able to hold a count of 5000000. The code for this counter is given below: module twodigit_onefile( input clock, input reset, output a, output b, output c, outp...

Seven Segment LED Multiplexing Circuit in Verilog

The seven segment LED circuit uses seven different and individual LED's to display a hexadecimal symbol. It has 7 wires to control the individual LED's one wire to control the decimal point and one enable wire. The demo board I am using here consists of four such 7-segment LED's(As do any other demo board). To reduce the number of wires a multiplexing circuit is used to control the display. Using the multiplexing circuit the number of wires required to light up all 4 displays are reduced from 32 to 12 (8 data bits and 4 enable bits). All bits here are active low, such that to enable them a '0' is required. For example the figure below shows how to display a 3 on the seven segment. The multiplexing circuit can take 4 inputs and have only one output. But the inputs should be displayed on the output fast enough to fool the viewer into thinking all outputs are enabled individually and simultaneously. This is achieved by having an enable signal that changes so fast t...

Simple Button Debouncing Code in Verilog

So you have made a counter and after programming it onto your board you realize that every button press increments the counter by 30 or 40 units. This problem is knows as bouncing and to overcome this a debouncing circuit is needed to compensate for the mechanical button bounces. The push buttons and switches on the FPGA boards are mechanical devices and tend to bounce multiple times when pressed. And since the code related to the button is usually placed in the always @ (posedge clock) block, every bounce of the button is picked up and processed. It has been found that the bounces last around 20 ms after which it stabilizes. The debouner circuit should be able to filter out these bounces and only pick up the stabilized state of the button. Since it is known that the bounces last around 20 ms the first thing the code should have is a timer, a timer that will outlast the 20 ms of instability. I will use a 10 ms counter and after every 10 ms will check the state of the button, if it...