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Getting Started with RAM and ROM in Simulink

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This example shows how to utilize RAM resources in your FPGA design using HDL Coder™.

Introduction

Dedicated RAM blocks in FPGA are valuable resources for digital designs. It is easy to design with RAM and ROM in Simulink®, and utilize the dedicated RAM blocks available in your FPGA using HDL Coder.

RAM Blocks in the HDL Example Library

HDL Coder provides following types of RAM blocks in the HDL RAMs block library. Use hdllib to display blocks that are compatible with HDL Coder™, and then select the HDL RAMs library under HDL Coder:

  • Single Port RAM

  • Single Port RAM System

  • Simple Dual Port RAM

  • Simple Dual Port RAM System

  • Dual Port RAM

  • Dual Port RAM System

  • Dual Rate Dual Port RAM

Run this command navigate to RAM blocks in HDL library:

hdllib

The RAM blocks are masked subsystems built using Simulink blocks for behavioral simulation. For code generation, HDL Coder generates predefined templates that describe RAM structures in HDL. Most synthesis tools recognize the RAM structures in the templates, and map them to RAM resources on the FPGA.

Using Generic RAM Coding Style for RAM Blocks

By default, HDL Coder provides RAM template that uses clock enable for the RAM structures. As an alternative, HDL Coder also provides a style of generic template that does not use clock enable. The generic RAM style template implements clock enable with logic in a wrapper around the RAM. You can control this using RAM Architecture option in the HDL Coder global settings panel.

You may want to use the generic RAM style if your synthesis tool does not support RAM structures with a clock enable, and cannot map the HDL to FPGA RAM resources as a result.

The example hdlcoderfirram is an example of how to use the generic RAM style for your design.

open_system('hdlcoderfirram');
open_system('hdlcoderfirram/FIR_RAM');

Selecting RAM Coding Style

The RAM Coding Style is selected by choosing the desired RAM Architecture on the Coding Style tab of the HDL Code Generation Global Settings configuration page.

Building a ROM Using Simulink Blocks

HDL Coder does not provide a ROM block, but you can easily build one using a Lookup Table block and a Unit Delay block from Simulink, as shown in the following example.

open_system('hdlcoderrom');
open_system('hdlcoderrom/ROM');

Follow these modeling guidelines when building a ROM from Simulink:

  • For an n-bit address, specify all $2^n$ entries of the Lookup Table data. Otherwise, your synthesis tool may not map the generated code to RAM, and the code may not match your Simulink model.

  • Place the Lookup Table and Unit Delay blocks in the same model hierarchy.

  • Support of RAM reset logic varies among FPGA devices and synthesis tools. For best synthesis result, suppress the generation of reset logic for the Unit Delay block by setting its ResetType property to none, in the HDL Block Properties dialog box. Also set the IgnoreDataChecking property to 1 in the HDL Test Bench configuration parameters to ignore the initial simulation mismatch caused by suppressing the reset logic.

If you follow the preceding guidelines, most synthesis tools will implement the ROM using dedicated RAM blocks in an FPGA.

Minimum RAM Size Requirement

If the size of the RAM or ROM in your design is small, your synthesis tools may map the generated code to registers instead of dedicated RAM blocks for better speed performance. Check your synthesis tool for any minimum RAM size requirement, and if desired, how you may override that requirement.