Model Name

The model is called IO3xx_playback_hdlc.slx.

Supported Modules

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.

Interfaces

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.

Example

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 directly streamed from the CPU through PCIe Endpoint and DMA engine to the design under test (DUT) of the FPGA. The setup for the Direct Stream use case can be illustrated as follows:

DMA Playback

In the DMA playback, the FPGA initiates the DMA transfer by triggering an interrupt to the x86 CPU. Since the DMA write driver is placed in the Interrupt subsystem, the DMA will be initiated as soon as the interrupt is received on the CPU side.

The example represents a small playback application, on which data originates from the CPU and is finally output on the analog output of the FPGA. The sampling of the analog output interface runs at a fixed but configurable rate and therefore must always have new data available to pass on to the AO interface. The data itself is passed on from the CPU through DMA, and a small FIFO in the design under test (DUT) is used to buffer enough data to bridge the gap when new data is requested by the CPU and passed to the FPGA through DMA.

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:

The DUT_IO3xx_DMA_Playback subsystem, which is the part intended for HDL code generation, consists of a AO_Triggering and a Fifo Buffer subsystem.

AO_Triggering generates trigger pulses in the sample frequency configured through the PCIe_CounterValue_DA_Trigger port from the CPU. The Enable port of the subsystem only starts generating trigger pulses when the Fifo Buffer has received enough data to get started.

Fifo Buffer implements a small FIFO that handles the incoming data from the DMA interface and transfers it to the AO interface. The Fifo is needed to bridge the gap until new data is received from the CPU. Everytime the DMA frame completes, new data is requested by initiating an interrupt. The tReady port of the AXI4-Stream interface is used to apply backpressure on the incoming DMA data stream to avoid overloading the FIFO.

Simulation

The simulation of the model is started by clicking the green run button in the model toolbar. The simulation illustrates in SDI (Simulation Data Inspector) how the FIFO is first filled and then how the first few samples are output on the AO interface (AO_Trigger, AO_Data).

Target CPU Driver

On the target CPU, the write of the DMA is done with the DMA write block which is located in the INTA Write subsystem. The block itself is placed in a Function-Call Subsystem, as the DMA transfer is initiated on the CPU side via interrupts. The size of the DMA transfer is configured in the frame_size variable, defined in the IO3xx_playback.sldd data dictionary.

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 Example

To run the generated model, simply run the IO3xx_playback_run.m example script. The script configures, builds and runs the model. Finally, it retrieves the logged data after the run is complete. Once the model has been downloaded and the target application has been started, the script will read out the logged data directly from the Simulation Data Inspector and illustrate them in a plot.

To see the actual signal output on the analog interface, connect an external oscilloscope. The signal illustrated in the image below, which is the logged data of the CPU, is undersampled, as the data is only retrieved with a CPU sample rate of 1 ms. The data on the analog output port, however, is sampled with a rate of 48 kHz.

Logged data of the top-level model