LVDS display bridges and automated measurements, Part 2
In Part 1, I described the LVDS signaling standards and the architecture of LVDS module used in a microcontroller. The signal specifications need robust characterization and functional tests to avoid field failures. In part 2, I’ll describe which instruments are needed for characterization and how to automate measurements.
Hardware used:
- SoC Development Board
- P&E Debugger: Lauterbach Debugger to load DUT Code into SoC
- Power Supply: Keysight E3648A
- Digital Multimeter for DC Characteristics. You can use a meter from any manufacturer
- UART Cable to run several test cases
- Source-measure unit for leakage current measurements
- Digital Serial Analyzer: Tektronix (50 GS/s with 16 GHz Bandwidth
- Differential probes
Software used:
- LabVIEW 2012
- Trace32 for LauterBach Debugger
- ARB Express Signal Generator
- TINA TI for simulations
- GHS Code Compiler
- P&E Debugger Software
Automation Process
LabVIEW based automation is used to sequence the test case and data acquisition routines. An oscilloscope acquires the LVDS signals. The instruments preferably should have GPIB for automation. Figure 1 gives a flowchart of the automation. Figure 2 shows the process in flowchart form.
Figure 1. This LabVIEW VI shows the automation process in code form.
Figure 2. Test automation flowchart.
The above automation process helps us automatically carry out the desired tests under various temperature and voltage conditions. Initially, a GPIB signal is sent to all the hardware devices, which establishes control through LabVIEW and sets up communication between the SoC and the test instruments. A temperature loop operates the SOC on various temperature levels. We also use a voltage loop to operate the SoC under various voltages. We then load DUT code into the SoC. The Digital Serial Analyzer measures the desired parameters using LabVIEW and stores the report with desired parameters. The data from the report is then imported into an Excel sheet for further Analysis. Both the DC and AC Characteristics are carried out using the Automation processes done using LabVIEW.
The DUT code in Listing 1 programs the SoC using the P&E Debugger and the code is compiled using GHS Compiler. We start by initializing the SRAM memory for the program code as there is no FLASH memory on the SoC. We initialize clocking of all the modules present in the SoC, then configure the interrupts with their starting addresses. The UART is configured for use in several test cases. Display content is defined in the SRAM memory in graphic format. Next, the LVDS Display Bridge is configured followed by the Control Descriptor registers, which gets the desired output signals.
Listing 1. Snapshot of the Test-case Code for LDB Settings.
Measurement Methods for OLDI Characterization
The DC characteristics tell us about the steady state parameters measured using a High precision Digital multimeter under normal room temperature. The 100 Ω termination resistor is removed to carry out DC measurements and single ended DC measurements are done individually on pad P and pad N (Figure 3). The DC characteristics were measured by driving a static high and a static low at the output. These are single-ended measurements, measured between the ground and PadP or ground and PadN.
Figure 3. A DMM measures the DC characteristics of PadP and PadN.
The 100 Ω termination has been removed for DC Characteristics. A high-precision DMM makes the automated measurements. The constant 1’s and 0’s are driven at the output by making changes in the DUT Code.
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Output Differential Voltage: This is the difference of the two single ended measurements, VpadP – VpadN.
Common Mode Voltage: This is average of the two single ended measurements i.e. (VpadP + VpadN) / 2.
The AC Characteristics give us information about rise time and fall time under several slew-rate trim conditions across a range of temperature and voltage. AC Characteristics were measured by driving a toggling signal at the output using eye diagrams. AC Characteristics were measured across the 100 Ω termination resistor. The Tektronix oscilloscope has an option called DPOJET that can automatically calculate the rise time and fall time across the 100 Ω termination resistor using an eye diagram (Figure 4. The test setup appears in Figure 5.
Figure 4. An eye diagram of the LVDS signal lets us measure rise time, fall time, and eye openings.
Figure 5. The test setup diagram shows a 100 Ω termination resistor across the transmission line.
We used the Digital Serial Analyzer to do the AC characterization of LVDS Display Bridge using DPOJET and the automation process designed in LabVIEW. The eye diagram lets us completely characterize the LVDS Signals of the Display Interface.
Leakage current measurement
Leakage current measurements are important with respect to the performance of the SoC, which may not behave as expected if the LVDS pad is multiplexed to perform as some other functionality. Leakage current will clamp the voltages and decrease the signal swings.
Figure 6 shows the LVDS Pad is configured as GPIO and a ramp voltage is applied to PadP. PadN is grounded through an ammeter, which measures leakage current. A high precision source-measure unit provides the source voltage and measures the leakage current. These measurements are single ended, so the 100 Ω termination resistor is removed.
Figure 6. A source-measure unit provides a test voltage from its voltage source and measures leakage current through its integrated ammeter.
Analysis of test results
Having gather the test data, we can analyze test results with respect to the TIA/EIA – 644 standards and check if results comply with the standard. If not, then there might be some setup issue or some problem with the design. We need to debug these issues to get proper waveform output. LVDS signaling is a high speed signal topology so we must be careful about the several noise components reflections due to electromagnetic interference due to differential signal pair. We must keep sources of interference out of its range.
The plots in Figures 7 and 8 depict the rise time and fall time of LVDS pads across a PVT characterization. The X-axes have temperature in degrees C, I/O pad voltage, and sample Numbers. 3V3 refers to a 3.3V IO. The Y-axes are the time durations in seconds.
Figure 7. Rise Time Characteristics across range of voltage and temperature.
Figure 8. Fall Time Characteristics across range of V and T.
Figure 9. Leakage current increases with applied voltage.
Conclusion
These two articles describe the LDB and its characterization at a sufficient detail level to enable a beginner to appreciate the interface and also characterize it.
Also see
LVDS display bridges and automated measurements, part 1
Automate current measurements when characterizing SoCs
LabVIEW 2015 makes productivity enhancements
The secrets of successful communications using LVDS
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