Memory Design Using Verilog

By Arnav Bansal

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Presented here is a memory design project using Verilog hardware description language (HDL). This project is simulated using ModelSim software, and the design is tested through a simulation process. Simulation is done to check, verify and ensure that what is wanted is what is described.

Simulation is carried out through dedicated tools. With every simulation, the results are studied to identify errors in the design description. Errors are corrected and then another simulation is carried out. Simulation and changes to design description together form a cyclic iterative process, repeated until an error-free design is evolved.

Design description is an activity independent of the target technology or manufacturer. It results in a description of the digital circuit. To translate it into a tangible circuit, one goes through the physical design process.

The same constitutes a set of activities closely linked to the manufacturer and the target technology. For designing projects in HDLs, you have to follow the very large scale integration (VLSI) design flow and verification steps required in each step.

Single-port memory

The single-port memory is basically the design as per your defined specifications. Then, as per the specified width and depth, define the memory block that can also be verified using field programmable gate array (FPGA) boards. Design hierarchy also plays an important role in designing the basic building blocks required in each step of verification.

In this project we design a 64-bit x 8-bit, which is a single-port design with common read and write addresses in Verilog.

While designing this project in ModelSim and test benches, follow the guidelines mentioned below:
1. When modelling sequential logic, use non-blocking assignments
2. When modelling latches, use non-blocking assignments
3. When modelling combinational logic with an always block, use blocking assignments
4. When modelling both sequential and combinational logic within the same always block, use non-blocking assignments
5. Do not mix blocking and non-blocking assignments in the same always block
6. Do not make assignments to the same variable from more than one always block
7. Use $strobe to display values that have been assigned using non-blocking assignments
8. Do not make assignments using #0 delays

Memory design

The memory block diagram is shown in Fig. 1. It takes a few assumptions into consideration for easing the operations of the circuit. While data input pin and address pin may have any value depending on the specifications of memory used and your need, clock used in the circuit is active high.

Block diagram of 64x8 bit memory
Fig. 1: Block diagram of 64×8 bit memory

Enable pin triggers the circuit when it is active high, and read operation is performed when read/write pin is high, while write operation is performed when read/write pin is active low.

Software

Verilog has been used for register-transfer logic coding and verification. The bottom-up design has been followed in this project to facilitate easy debugging of various modules. ModelSim is an easy-to-use yet versatile simulator from Mentor Graphics. It supports behavioural, register-transfer- and gate-level modelling.

To start memory (64-bit) design simulation, install ModelSim V 10.4a on a Windows PC and follow the steps mentioned below:
1. Start ModelSim from the desktop; you will see ModelSim10.4a dialogue window

2. Create a project by clicking Jump Start on Welcome screen

3. Create Project window pops up (Fig. 2). Select a suitable name for your project. Set project location and leave the rest as default, and click OK

Create Project window for Memory design
Fig. 2: Create Project window

4. Add items to Project window pops up (Fig. 3). Select Create New File option

Add items to the Project window
Fig. 3: Add items to the Project window

5. Create Project File window pops up. Give an appropriate file name (say, Memory_SP.v) for the file you want to add, and choose Verilog as Add file as type and Top Level as Folder (Fig. 4)

Create Project File window
Fig. 4: Create Project File window

6. On the workspace section of the main window (Fig. 5), double-click on the file you have just created (Memory_SP.v in this case)

Workspace window
Fig. 5: Workspace window

7. Type your Verilog code (Memory_SP.v) in the new window. Since 64-bit x 8-bit memory is considered here, 64-bit address is considered, and the main goal is to input 8-bit data that should be written in this address location. Note that these inputs are arbitrary and you can enter any 8-bit data

8. Save your code from File menu

Note. Generally, memory internal architecture design may contain D-flip-flop, mux, basic cell, 8-bit word, core and so-on.

Compiling/debugging project files

1. Select Compile→Compile All options

2. Compilation result is shown on the main window. If you get ‘Compile of memory-SP.V was successful’ message, it means there are no errors in the project (Fig. 6)

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