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Speedgoat IO3xx Ring Buffer Example

Speedgoat IO3xx Ring Buffer Example — Example Showcasing a ring buffer on the FPGA as well as the DMA data path to the CPU domain

Model Name

The model is called IO3xx_ring_buffer_hdlc.slx.

Supported Modules

  • IO332-200k

  • IO333-325k, IO333-410k, IO333-325k-SFP, IO333-410k-SFP

  • IO334-325k, IO335-325k

  • IO342-1450k, IO342-1080k

Required Toolboxes

The list of basic software requirements are provided in the prerequisites section of the Getting Started page.

In addition, you must also install the Fixed-Point Designer.


The example uses the following interfaces:

User Blocks

The example uses the following user blocks:

DMA Overview

Direct Memory Access (DMA) is used to reduce the latency of data transferred between the FPGA and the target CPU domain, especially if larger amounts of data need to be transferred. There are different use cases depending on whether the data transfer is in the direction FPGA to CPU (data logging), or in the direction CPU to FPGA (playback) or bidirectional (coprocessor mode). A simplified setup including the basic blocks required to discuss the DMA use cases in a system comprising an FPGA-based I/O module and a Real-Time Target x86 CPU is illustrated below.

The FPGA I/O Module consists of the I/O channels (digital, analog, multi-gigabit transceiver, etc.), the FPGA itself, an external RAM (typically DDR3/DDR4 SDRAM), the design under test (DUT), the DMA engine and the PCIe Endpoint (used to communicate with the Target x86 CPU).

The Motherboard (x86) consists of the Target CPU (x86), the System Memory and the Solid-State Drive (SSD) for persistent data storage.


To test the HDL interface functionality, dedicated examples are included in the downloaded archive file. To open the examples, navigate to the corresponding folder. Note that the examples only test I/O channels for which the loopback test method is possible. The terminal board provided must be wired as described. Examples do not test I/O channels that require external hardware (for some examples a function generator or an oscilloscope is required), but running this example will still provide sufficient confirmation of the correct setup of this implementation. The examples only test interface channels which are provided by the base functionality of the I/O module. Please note that the examples provided have been color coded. The green colored subsystem (FPGA domain) is the part of the model which is actually compiled using HDL Coder and ultimately runs on the FPGA. The FPGA domain usually has a sample frequency in the range of 100 MHz and is set in the HDL Workflow Advisor (FPGA Synthesis Software Settings). The blue blocks (CPU domain) which surround the green subsystem are interfaces to the processor section of the model. The CPU domain usually has a sample frequency in the range of 1 kHz. The interrupt subsystem has been given another color (magenta), as its functionality is asynchronous to both the processor and FPGA. The interrupt source can be selected in the generated model in the Interrupt Setup block once the model has been run through the HDL Coder Workflow Advisor.

In this use case the data is buffered into the on-board RAM of the programmable FPGA module and is only transferred using DMA when triggered by a defined event (such as an error) on the FPGA. The event or trigger for transferring into the x86 CPU domain is flexible and can be modeled by the user within the design under test (DUT) algorithm. The example implemented here is an error event debug. The user signal is monitored, and based on signal anomalies (for example, spikes), stops logging and initiating a DMA transfer of the data into the x86 domain. The setup for the ring buffer use case can be illustrated as follows:

The data is logged from the DUT in the external RAM (1) in a ring buffer fashion (2) until an event (error) occurs, whereupon the RAM readout DMA is triggered and the data is transferred to the x86 System Memory (3).

Open the example model by navigating to the folder containing the "*.slx" model file and double clicking the file. If the example is provided as a Simulink Project, navigate to the corresponding example folder and extract the Simulink project zip file. Then double-click the "*.prj" icon to open the project. After opening the project, open the model by double clicking the "*.slx" file. The model is shown as follows:

The design under test (DUT) subsystem is shown as follows:

Data emulation models three sine waves with configurable amplitude and frequency (number of samples of the sine wave look-up table). The output of the sine generation subsystem is three sine wave signals that will be logged. The third sine wave passes through the fault_injection subsystem, which emulates a failure case on this user signal (it adds a significant value to the user sine wave at a configurable time). The configuration parameters of the sine waves are declared in the data dictionary file provided.

Failure detection checks the sin3_fault user signal for any anomalies and if there is a failure (for example, a spike), it asserts its fault_flag output signal. This flag indicates that the failure case occurred and that the logging to the ring buffer can be stopped. The Post Logging subsystem delays the fault_flag signal and introduces a configurable amount of post-event logging.

The DDR RAM write controller handles the AXI4 Master signal interface to the External RAM. The DDR write controller block, implemented as a MATLAB function state machine, includes the management of addressing and signaling to the External RAM. The blocks have two mask parameters: Burst Size, which specifies the size in 64-bit words of a AXI4 Master transaction to the External RAM, and FIFO_size, which defines the supported buffer size to pre-store the data in the FIFO and initiate the next burst once enough data is pre-stored. The FIFO_sys subsystem is used to deal with backpressure introduced on the AXI4 Master interface, which requires a buffer to pre-store the data before a send transaction.

The DDR RAM Monitoring subsystem is used to monitor the various status flags from the DDR Write Controller, which can help identify any issues writing to the external RAM.


If the model is started by clicking the green run button in the models' toolbar, the following data may be observed in the Simulation Data Inspector (SDI) as illustrated bellow. The first subplot illustrates the three sine waves sine1, sine2 and sine3_fault, with the injected spike on sine3_fault. The second subplot shows the timing counter value. The third subplot shows the address signal of the AXI4 Master interface and illustrates the ring buffer’s behavior. The fourth subplot shows the fault_flag and fault_detected signals that finally initiate the DMA data transfer.

Target CPU Driver

On the target CPU, the extraction of the DMA data is done with the DMA read external RAM block which is located in the read RAM-enabled subsystem. The enabled subsystem is used in such a way that the DMA is only enabled when the logging on the FPGA is actually finished (pci_RAM_write_done signal). The input port of the DMA block offset is the actual address of the RAM being read at each sample step. The RAM is read out starting from address zero up to the specified logging size (RAM_depth parameter). Owing to the ring buffer setup, the realignment of the data is then done in the post-analysis script. The value for the DMA transfer size is pre-set to a variable value frame_size, which is defined in the Simulink data dictionary.

Test Wiring

This example does not require any test wiring.

Running HDL Workflow Advisor

Before the example can be deployed and run on the real-time target machine, you will need to run through the HDL Coder Workflow Advisor steps to actually generate HDL code and a FPGA bitstream using HDL Coder (FPGA Synthesis Software Settings).

New: Reference design parameters, set at step 1.2 now control which interfaces will be available to target in step 1.3 of the workflow. This has reduced the total number of reference designs, and the list of interfaces available. Please remember to select the front plug-in and rear plug-in setting that is appropriate for your module, as well as the Aurora settings that should be used for your model (if applicable). These additional reference design parameter settings are further described in the interface sections for which they are relevant.

New: Prior to running the workflow advisor, be sure to double click the Select Module block in the demo model. If one or more of your modules support the model (due to available interface compatibility), a pop-up will display prompting you to select the module you would like to target. If only a single module is installed, and providing it is compatible, it will be automatically selected when the box is double clicked.

Upon completion, a newly generated model containing the Simulink Real-Time interface subsystem appears. At first sight, this subsystem resembles the FPGA subsystem. However, inside, the Simulink algorithm has been removed and replaced with blocks that the real-time application will use to communicate with the FPGA during simulation execution. The newly generated model is now ready to be deployed to a real-time target machine. To download the FPGA bitstream and the Simulink model to the target, click the Build Model button on the Simulink Editor toolbar. The real-time application loads on the Speedgoat target machine and the FPGA algorithm bitstream loads on the FPGA. If you are using I/O lines, check that you have connected the lines to the external hardware under test. Please note that some example models do have Global Delay Balancing intentionally disabled. If an error is displayed about delay balancing in step 2.3 of the HDL Coder Workflow Advisor, it can be safely ignored by checking the Ignore warnings checkbox.

Running the Generated Model

Once the model has been successfully built using the HDL Workflow Advisor, it can be tested by running the IO3xx_ring_buffer_hdlc_run.m script provided. The script configures, builds and runs the model and retrieves the logged data after the run. Once the model has been downloaded and the target application has been started, the script will read out the logged data directly with SDI and illustrate them in a plot as shown below.