In this third part of the series (as promised), we will show how to implement the timers block by using, not registers, but memory blocks.
Memory blocks are an often unused capability of modern FPGAs and can in many cases (as in this one) be a nice alternative to save on scarce resources like registers and LUTs. As we commented in the previous entry, implementing a block of 32 x 16 bit timers took about 7% of the LUTs of a Cyclone, and we wanted to see if we can reduce the quantity of resources taken.
On the first two chapters of this Tutorial we started with a simple LFSR module and added a test bench. Then, on chapters three and four we upgraded our module with some features and learned to export the test bench data to files.
Chapter 5 – Matlab Formal Verification
Our VHDL block implements an algorithm that generates pseudo-random numbers. If the register is large enough, the output of the block will be hundreds or thousands of different numbers. How can we be sure that our block is working OK?
On the first part of this tutorial, we started with a simple implementation of an LFSR block (Chapter 1) and it test bench (Chapter 2). Let’s make our code a bit more professional.
Chapter 3 – Upgrading the LFSR code
Good code doesn’t use hard-coded constants as were used on the first part (to define the LFSR width). The downside of using constants is that code updates and maintenance is cumbersome at best. If we want to change the width of the register… we must scan the code and change each and every instance of the constant. There is a great possibility of making mistakes while doing that. Forgetting to change one of the ‘3’s… or changing one that was not related to the register width. This is not clearly seen in a short piece of code, but as our code gets longer, maintaining hard-coded constants is a sure recipe for trouble.
In this tutorial we will see how to design a block. We will start with a very simple block and gradually add features to it. We will also simulate it and test its output with Matlab. Over the process we will see:
How to start with a simple block and gradually add features and improvements
How to add a test bench (simulation)
Adding parameters to the VHDL block
Saving the block data output to files (from simulation)
Author Mike Fields wrote this book as an introduction to FPGAs and VHDL. The book examples are mainly oriented to Xilinx Spartan 3E FPGA (but as in other books, the concepts are general. If you don’t have that FPGA, adapting the book examples to your own device can be an excellent way to learn).