Dual Rate Dual Port RAM

Dual Port RAM that supports two rates

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Libraries:
HDL Coder / HDL RAMs

Description

The Dual Rate Dual Port RAM block models a RAM that supports simultaneous read and write operations to different addresses at two clock rates. Port A of the RAM can run at one rate, and port B can run at a different rate.

In high-performance hardware applications, you can use this block to access the RAM twice per clock cycle. If you generate HDL code, this block maps to a dual-clock dual-port RAM in most FPGAs.

Simultaneous Access

You can access different addresses from ports A and B simultaneously. You can also read the same address from ports A and B simultaneously.

However, do not access an address from one RAM port while it is being written from the other RAM port. During simulation, if you access an address from one RAM port at the same time as you write that address from the other RAM port, the software reports an error.

Read-During-Write Behavior

The RAM has write-first behavior. When you write to the RAM, the new write data is immediately available at the output port.

Ports

Input

din_A — Port A write data input

Scalar (default)

Data that you write into the RAM memory location when we_A is true. The data inherits the width and data type from the input signal. din_A can be a double, single, integer, or a fixed-point (fi) object, and can be real or complex.

Data type: scalar fixed point, integer, or complex

Data Types: int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | fixed point

addr_A — Port A write address

Scalar (default)

Address that you write the data into when we_A is true. This value can be either fixed-point(fi) or integer, must be unsigned, and have a fraction length of 0.

Data Types: uint8 | uint16 | uint32 | uint64 | fixed point

we_A — Port A write enable

Scalar (default)

Write enable for RAM port A. Set we_A to true for a write operation, or false for a read operation.

Data Types: Boolean

din_B — Port B write data

Scalar (default)

Data that you write into the RAM memory location when we_B is true. The data inherits the width and data type from the input signal. din_B can be a double, single, integer, or a fixed-point (fi) object, and can be real or complex.

Data type: scalar fixed point, integer, or complex

Data Types: int8 | int16 | int32 | int64 | fixed point

addr_B — Port B write address

Scalar (default)

Address that you write the data into when we_B is true. This value can be either fixed-point(fi) or integer, must be unsigned, and have a fraction length of 0.

Data type: scalar fixed point, integer, or complex

Data Types: int8 | int16 | int32 | int64 | fixed point

we_B — Port B write enable

Scalar (default)

Write enable for RAM port B. Set we_B to true for a write operation, or false for a read operation.

Data Types: Boolean

Output

dout_A — Port A output data

Scalar (default)

Output data from RAM port A address, addr_A.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | Boolean | fixed point | enumerated | bus

dout_B — Port B output data

Scalar (default)

Output data from RAM port B address, addr_B.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | Boolean | fixed point | enumerated | bus

Parameters

Address port width — Address bit width

8 (default)

Minimum bit width is 2, and maximum bit width is 28.

Programmatic Use

Block parameter: ram_size

Type: string scalar | character vector

Value: A minimum value of 2 and maximum value of 28

Default: '8'

Algorithms

HDL code generated for RAM blocks has:

• A latency of one clock cycle for read data output.

• No reset signal, because some synthesis tools do not infer a RAM from HDL code if it includes a reset.

Code generation for a RAM block creates a separate file, blockname.ext. blockname is derived from the name of the RAM block. ext is the target language file name extension.

RAM Initialization

Code generated to initialize a RAM is intended for simulation only. Synthesis tools can ignore this code.

Implement RAM With or Without Clock Enable

The HDL block property, RAMArchitecture, enables or suppresses generation of clock enable logic for all RAM blocks in a subsystem. You can set RAMArchitecture to the following values:

• WithClockEnable (default): Generates RAM using HDL templates that include a clock enable signal, and an empty RAM wrapper.

• WithoutClockEnable: Generates RAM without clock enables, and a RAM wrapper that implements the clock enable logic.

Some synthesis tools do not infer RAM with a clock enable. If your synthesis tool does not support RAM structures with a clock enable, and cannot map your generated HDL code to FPGA RAM resources, set RAMArchitecture to WithoutClockEnable.

RAM Inference Limitations

If you use RAM blocks to perform concurrent read and write operations, verify the read-during-write behavior in hardware. The read-during-write behavior of the RAM blocks in Simulink^® matches that of the generated behavioral HDL code. However, if a synthesis tool does not follow the same behavior during RAM inference, it causes the read-during-write behavior in hardware to differ from the behavior of the Simulink model or generated HDL code.

Your synthesis tool might not map the generated code to RAM for the following reasons:

• Small RAM size: your synthesis tool uses registers to implement a small RAM for better performance.

• A clock enable signal is present. You can suppress generation of a clock enable signal in RAM blocks, as described in Implement RAM With or Without Clock Enable.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

HDL Code Generation

Generate VHDL, Verilog and SystemVerilog code for FPGA and ASIC designs using HDL Coder™.

HDL Coder™ provides additional configuration options that affect HDL implementation and synthesized logic.

HDL Architecture

Note

For simulation results that match the generated HDL code, in the Solver pane of the Configuration Parameters dialog box, clear the checkbox for Treat each discrete rate as a separate task. When the checkbox is cleared, single-tasking mode is enabled. If you simulate the block with this check box selected, the output data can update in the same cycle but in the generated HDL code, the output data is updated one cycle later.

This block has one default HDL architecture.

HDL Block Properties

General
ConstrainedOutputPipeline	Number of registers to place at the outputs by moving existing delays within your design. Distributed pipelining does not redistribute these registers. The default is 0. For more details, see ConstrainedOutputPipeline.
InputPipeline	Number of input pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see InputPipeline.
OutputPipeline	Number of output pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see OutputPipeline.

Note

The RAMDirective property is not available for use with the Dual Rate Dual Port RAM because the block does not have a single clock interface.

Complex Data Support

This block supports code generation for complex signals. You cannot have mixed real-value and complex inputs.

Version History

Introduced in R2014a