Understanding the NI Ettus USRP X410 Reference FPGA Project

This article describes how to use and configure the NI Ettus USRP X410 FPGA Reference Project.  

 

NI-USRP installs two flavors of the reference FPGA project to the path <LabVIEW>\instr.lib\niUSRP\ReferenceFPGA:

1. referenceFPGA (USRP-X410 4ch-1spc) 

2. referenceFPGA (USRP-X410 4ch-4spc) 

The first project is configured for using all 4 channels on the USRP X410 while using 1 Sample Per Cycle (SPC).  This yields a max rate of 122.88 Mhz (Daughterboard 0\DataClock). The second project is set up for 4 channels and 4 Samples Per Cycle (SPC), giving a rate of 4*122.88Mhz = 491.52 Mhz.  You can configure the project for other combinations such as 2ch-4spc through Conditional Disable Symbols.  See the section Conditional Disable Symbols for a description of common symbols.  

Reference FPGA X410.vi diagram 

The diagram above is divided into three main sections: 

1. Instruction Framework (Register Bus) 

2. RX 

3. TX 

Instruction Framework 

The Instruction Framework allows the NI-USRP driver to read and write registers used by the different subsystems (Rx Core, Tx Core, DSP, Streaming, and Synchronization).   

For the Rx and Tx core subsystems, the driver queries the downloaded bitfile running on the X410 to determine how many channels the bitfile supports and whether it is compiled for 1spc or 4spc. 

For the Rx and TX DSP subsystems, the driver queries which DSP blocks are enabled in the running bitfile. DSP blocks are enabled or disabled by setting the corresponding Conditional Disable Symbol. 

For the Rx and Tx streaming subsystems, the driver can set registers with information such as the number of channels configured, channels enabled, finite acquisition mode, etc.  As well as reading back the state of the active streaming session, samples processed, FIFO under or overflows, etc. 

For the Synchronization subsystems, the driver can read and write trigger type as well as specific trigger configuration such as trigger time, etc. 

For example, look at the Bus Interface.vi for the Rx Core class: 

Here you can see that reading the Rx Core class at hex offset 0x2 returns the number of channels that this project is configured for through the Conditional Disable Symbols. 

RX 

The Rx section consists of reading the ADC IQ data, passing it through a series of DSP VIs before streaming the data back up to the host through a Target to Host-DMA FIFO. Note that not all the DSP VIs are enabled by default. These are the DSP VIs available in the RX path: Digital Offset, Digital Gain, Equalization, IQ Impairments, Frequency Shift, Decimation. 

• By default, Equalization is not enabled 

• NI-USRP use the Frequency Shift and Decimation blocks when you set the Carrier frequency and the IQ Rate 

• NI-USRP initialized the IQ Impairment block to a gain of 1, but it is not currently used 

• Users can set the Digital Offset and Digital Gain.  See the section named Using niUSRP_SetAttribute.vi to directly control DSP for how to do this. 

TX 

The TX section reads data from a Host to Target-FIFO, passes this IQ data through a series of DSP VIs before writing it to the DAC.  These are the DSP VIs available in the TX path: Digital Pre Gain, Interpolation, IQ Impairments, Frequency Shift, Equalization, Digital Gain, Digital Offset. 

• By default, Equalization is not enabled 

• NI-USRP use the Frequency Shift and Interpolation blocks when you set the Carrier frequency and the IQ Rate 

• NI-USRP initialized the IQ Impairment block to a gain of 1, but it is not currently used 

• Users can set the Digital Offset, Digital Pre Gain, and Digital Gain.  See the section named Using niUSRP_SetAttribute.vi to directly control DSP for how to do this. 

Both RX and TX DSP VIs can be disabled through Conditional Disable Symbols.  

Note that changing the behavior of DSP blocks used by the NI-USRP can make the driver unusable. 

Configuring Component-Level IP (CLIP) 

Several CLIPs are installed along with the X410 Reference Project.  These CLIPs can be found here: 

..\LabVIEW xxxx\Targets\NI\FPGA\USRP\X410\CLIP 

For a 1 SPC configuration, use the RF2x2_100M_CLIP 

For a 4 SPC configuration, use the RF2x2_400M_CLIP 

To add a CLIP, right-click on the FPGA (USRP-X410) Target in the Reference project and choose Properties >> Component-Level IP 

Use the + icon to browse and add the needed CLIP. 

Once a new CLIP has been added to the project, right-click on the Daughterboard nodes in the project to select the CLIP declaration. 

Conditional Disable Symbols 

The Reference projects contain several Conditional Disable Symbols.

To configure these, right-click on the FPGA (USRP-X410) Target in the Reference project and choose Properties >> Conditional Disable Symbols 

Table of Conditional Disable Symbols used in the Reference FPGA project 

Loading a bitfile 

Use the following Open Session VIs to load a specific bitfile created by the Reference FPGA project. 

• niUSRP Open Rx Session with Custom Bitfile.vi 

• niUSRP Open Tx Session with Custom Bitfile.vi 

Below is an example of a desktop VI that loads a specific bitfile and starts generation and acquisition of the IQ data.
Note: this VI is not included in the example files. Users need to recreate it.

• In this example, note that both Rx config and Tx config are set to none.  The niUSRP Open Rx Session with Custom Bitfile.vi allows you to specify that in this case, we are not using the driver for Rx and Tx front-end configuration.  This test VI runs a bitfile set up for internal loopback between Tx and Rx on the FPGA: 

•  

..instr.lib\niUSRP\Reference FPGA\Reference FPGA X410 Debug.vi 

This allows you to use the NI-USRP driver to set up and configure parts of the system while you are responsible in this case for the Tx and Rx portion.  Similarly, you can disable streaming and implement that locally on the FPGA.

Using the RIO interface 

It is possible to open an FPGA VI Reference in addition to using the niUSRP Open.  This allows you to use the Read/Write Control to read or write to Front Panel controls on your main FPGA VI.  See the example below. 

This is a very useful technique for adding debug information to your FPGA VI. Note that the Open FPGA VI Reference must use the same bitfile that is used by the niUSRP Open and that you can not use the download method on the RIO session while a niUSRP session is open. 

Using niUSRP_SetAttribute.vi to directly control DSP 

This utility VI allows you to directly set attributes in the ni-usrp driver: 

C:\Program Files\National Instruments\LabVIEW <Version>\instr.lib\niUSRP\niUSRP Set Attribute (DBL).vi



These are the DSP attributes that can be changed:
1. DSP Attribute: DCOffsetReal   Type: double  Attribute ID: 0x118E34
2. DSP Attribute: DCOffsetImaginary   Type: double  Attribute ID: 0x118E35
3. DSP Attribute: DigitalGainReal   Type: double  Attribute ID: 0x118E3D
4. DSP Attribute: DigitalGainImaginary   Type: double  Attribute ID: 0x118E3E
5. DSP Attribute: DigitalPreGainReal   Type: double  Attribute ID: 0x118E3F
6. DSP Attribute: DigitalPreGainImaginary  Type: double  Attribute ID: 0x118E40
 

Here is an example of where this is used to add a DC offset using the DCOffsetReal attribute in the RX DSP path.  You can see that the real component is now shifted up by 0.2V compared to the imaginary component. 

Streaming Engine and optimization 

With FPGA compiles there are tradeoffs between a number of features and speed versus resource usage.  Thus, you will have a greater bitfile compilation success rate if you compile a 1spc bitfile v.s. a 4spc as well as using smaller FIFOs and disabling DSP features you do not need.  The two flavors installed (4ch-1spc and 4ch-4spc) are just examples.  If your solution only requires 1 channel at 4spc, it is more efficient to set up a project that compiles a 1ch-4spc bitfile instead of enabling 1 channel in a 4ch-4spc bitfile.  Here are some parameters you can adjust. 

FIFO Size 

When transferring acquired data through the Target to Host DMA FIFO, there are several limiting factors.  There is the speed of the PCIe bus which will limit the number of samples you can fetch using finite acquisition at high rates since the IQ rate is greater than the PCIe speed.  At an IQ rate of 491.52M, the speed of the PCIe bus will result in you retrieving a finite number of samples less than the size of the Rx Stream FIFO.  At slower rates, the PCIe bus speed is not the limiting factor, and the host side buffer becomes the limiting factor resulting in more finite samples being transferred. 

Continuous streaming to the host will be limited by the PCIe speed.  The max PCIe speed is slower than the full 491.52M IQ rate thus you can’t keep up.  Streaming data from the FPGA over Aurora would be an alternative. 

Streaming Benchmarks 

Below are some benchmarks on streaming performance.  These benchmarks were done on the following equipment: 

• PXIe-8880 controller 

• PXIe-1095 chassis 

DSP 

In the shipping Reference Project, these DSP VIs are enabled. 

Reduced resource usage can be obtained by disabling other DSP functions, especially when compiling a 4 spc bitfile. 

Bit packing/FIFO optimization 

The Reference FPGA project uses bitpacking to optimize transfers between the host and the target. 

The Reference FPGA X410.vi (FPGA) can, as mentioned earlier, through Conditional Disable symbols be configured to only use 1 or 2 channels in a 4 channel configuration, or 1 channel in a 2 channel configuration. To optimize the Tx and Rx FIFO which is configured for 4 elements in a 1 spc configuration and 16 elements in a 4 spc configuration the stream engine will fill up all elements in the FIFO regardless of how many channels are enabled.  

For the Tx stream controller, this FIFO logic is found in this sub VI: 

.. \niUSRP\Reference FPGA\Stream Control\Protected\Tx Disable channel logic 2ch 4SPC.vi 

The equivalent code for the Rx stream controller is found in this sub VI: 

..\niUSRP\Reference FPGA\Stream Control\Protected\RX Disabled channel logic 2ch 4SPC.vi 

General Notes

1. If you are using UHD configuration and Rio streaming in niUSRP Open Rx Session with Custom Bitfile.vi or niUSRP Open Tx Session with Custom Bitfile.vi, then this optimization cannot be turned off.
2. Be aware that support for the NI Ettus USRP X410 device was introduced in version 21.0 of the NI-USRP driver. You must install this version or a newer one to access the example explained here. Also, be aware this device is only supported in LabVIEW 2020 or newer.