Digital to Analog Converters (DACs) are an essential part of modern communication systems requiring high-speed data conversion, such as cellular base stations and Direct Digital Synthesis (DDS) instruments, where signal accuracy is paramount. An eye diagram is a valuable tool for evaluating signal integrity, allowing engineers to assess the quality of the signal at a glance, just by looking at the shape of the “eye”, but also providing detailed information that can be useful for troubleshooting or optimizing the system.

In a recent project, Antmicro developed a Python-based tool for verifying signal integrity of DAC3xJ8x devices, allowing users to trigger an eyescan, i.e. the process of collecting data and generating eye diagrams. The DAC eyescan tool can be used from the command line interface, which makes it easy to integrate with automatic testing tools, such as our open source Protoplaster framework.

In this article we will describe the eyescan testing tool and its integration with Protoplaster, and show you how to configure tests and extract and visualize the results, using a DAC38j8x device as an example.

Automated DAC SI verification with eyescans and Protoplaster

Eyescan testing

The eyescan tool conducts a SerDes (Serializer/Deserializer) test of the DAC device, using a JTAG interface to configure the test and extract the results from the DAC. The test is run for voltage offsets in the -320 to 310 mV range and phase offset in the -0.5 to 0.5 UI range with 1/32 increments. The output is aggregated in a TSV file (Tab Separated Values), containing information about lane number, bit number, voltage offset, phase offset and the amplitude readout:

0       7       31      16      4095
1       7       31      16      4095
2       7       31      16      4095
3       7       31      16      4095

The testing setup uses JTAG (IEEE 1149.1) to conduct an eyescan test. Our solution allows us to test multiple DACs connected in a daisy-chain configuration. The script is able to test a single DAC device in a daisy-chain at a time by indicating how many devices are in the chain and which one should be tested.

The output is then parsed and visualized in a static HTML file. The HTML file contains an eye diagram and two dropdown lists for switching between diagrams generated for each lane and bit number. The HTML file also contains eye height and width values that are calculated as the maximal distance between minimal values on both axes. This diagram can be used by a designer to decide whether the input parameters of the DAC satisfy the requirements, which can be arbitrary, depending on the use case.

Integration with Protoplaster

To automate the testing procedure, we integrated the DAC eyescan tool with Protoplaster, Antmicro’s platform testing framework. Protoplaster is able to perform the entire eyescan test procedure using the eyescan tool, and generate an eye diagram based on the data output by the tool. The Protoplaster test is also able to verify the eye width and height by checking if the resulting width and height are in the specified ranges.

To run the eyescan test with Protoplaster, it needs to be added to the Protoplaster config file, providing the following parameters:

  • ftdi_dev - FTDI device number.
  • bit - Number of bits that are checked in the eyescan test.
  • eyescan_output - Output file of the eyescan test in the TSV (Tab Separated Values) format.
  • eyescan_diagram - A static HTML file containing the eye diagram produced by the test.
  • min_width and max_width - Expected range of the eye width.
  • min_height and max_height - Expected range of the eye height.

For more information about using Protoplaster, refer to its README.

Testing a DAC38j8x device

An example Protoplaster eyescan test config for a DAC38j8x device is shown below:

tests:
  base:
    ti_dac38j8x_eyescan:
    - ftdi_dev: 0
      bit: 20
      eyescan_output: "output.tsv"
      eyescan_diagram: "eye_diagram.html"
      min_width: 28
      max_width: 32
      min_height: 540
      max_height: 560

This test will produce output.tsv with raw, tab-separated eyescan data, and eye_diagram.html, the resulting static HTML containing the eye diagram, as shown below.

Eyescan results

For an interactive version of the diagram, visit the desktop version of the website.

Automated signal quality verification with Antmicro

Eyescan analysis provides valuable information not only for DACs, but also high-speed interfaces such as PCIe or NVMe, and across various industries, including telecommunications, automotive and space. By combining eyescan verification with our Protoplaster framework, commercial projects developed with Antmicro benefit from a comprehensive workflow for automated, CI-driven signal quality testing at scale. We also employ a signal integrity simulation workflow with eye diagrams during hardware development, ensuring comprehensive testing of our complex, multi-layer PCB designs before they’re manufactured.

If you would like to learn more about Antmicro’s engineering services or are interested in integrating the eyescan tool and Protoplaster into your workflow, reach out to us at contact@antmicro.com.