Enjoy "riding" success] Series 4: Tektronix solution resolves difficulties in smart cockpit HDMI display interface testing

HDMI can transmit video and audio data at the same time, with simple connection and good compatibility. It is widely used in consumer electronics products, such as TVs, set-top boxes, projectors, and car cockpit entertainment systems. The HDMI system can be divided into four categories, Source, Sink, Cable and Repeater. In order to ensure good compatibility of these devices, the specification sets signal integrity requirements for electrical signals.

The figure below is a schematic diagram of the HDMI interface, which is suitable for standard HDMI1.4b and HDMI2.0. The HDMI interface uses TMDS encoding technology. The interface has a total of 4 pairs of TMDS differential signals. TMDS.Clock.channel is used as an independent clock signal for synchronization and signal acquisition; TMDS.channel.0/1/2 is used as a data channel for transmission. Video and audio data. For example, HDMI2.0 defines a maximum rate of 6Gbps for each channel, and the total bandwidth of the interface is up to 3.channel.x. 6Gbps.=18Gbps, which just meets the 17.82Gbps bandwidth required by 4Kp60Hz.

Figure 1. HDMI interface diagram

In order to pursue better visual effects and experience, people are not satisfied with the 4Kp60Hz display resolution, but are also pursuing the experience of 8Kp60Hz and 4Kp120Hz. However, 8Kp60Hz requires about 64G of bandwidth (RGB/YCbCr.4:4:4 format), which far exceeds the support range of HDMI2.0. Therefore, the HDMI Association added the HDMI2.1.FRL (Fixed.Rate. Link) mode to increase the interface bandwidth and meet the needs of 8Kp60Hz. At the same time, it needs to be combined with the corresponding YCbCr.4:2:.0 encoding and video compression technology. There are two commonly used methods. Method one: increase the channel data rate; method two: increase the number of channels when the rate remains unchanged. The latest HDMI2.1 FRL mode uses both methods.

While keeping the HDMI physical interface unchanged, the rate supported by each channel is increased to 12Gbps. In addition, the original TMDS.Clock.channel is redefined as FRL. Lane3 (the clock is embedded in the data stream); TMDS.Data.0 /1/2 correspond to FRL.lane. 0/1/2 respectively, as shown in the figure below, there are a total of 4 data channels. This achieves a maximum bandwidth of 48Gbps. The signal encoding method is changed from TMDS 8b/10b to FRL.16b/18b format, and the encoding efficiency is higher.

FRL.mode. can be divided into two modes:

Figure 2. HDMI 2.1 FRL mode diagram

In 3 lanes working mode, only two rates of 3Gbps and 6Gbps are supported; unused Lane3, source and sink need to use differential 50Ω ~ 150Ω termination. In 4 lanes working mode, it supports four rates of 6/8/10/12.Gbps.

Solve the difficulties of source-side testing

There are 9 total test items for HDMI2.1, as shown in the table below, taking testing Lane0 as an example.

Table 1. HDMI 2.1 FRL test content

Ÿ The test signal is a fixed pattern. A total of 8 code patterns are defined for the test, Link training pattern 1 to 8, abbreviated as LTP1 to 8. Unlike HDMI1.4b/2.0, there are no requirements for code types.

Ÿ The test signal rate is fixed and does not need to change with the resolution.

Ÿ It is necessary to consider the interference from other lanes, such as the HFR1-1 project. When testing Lane0, Lane0 needs to send out LTP5 code patterns, and Lane1/2/3 need to send out LTP6/7/8 code patterns respectively. The test method is more complicated.

Realization of termination voltage

Tektronix oscilloscopes and probes do not require an external power supply. They can not only provide a standard 3.3V termination voltage for compliance testing required by the association. In the user-defined mode, an adjustable termination voltage is also provided, such as setting a termination voltage of 3.0V, which is used to verify the behavior of the source chip when the termination voltage changes.

Automatic acquisition of single-ended and differential signals

Corresponding to single-ended projects and differential projects, single-ended signals and differential signals need to be collected respectively during testing; in HDMI1.4b/2.0 testing, differential signals are collected through differential probes; after manually changing the probe hardware connection, single-ended signals are collected Signal. Changing connections is cumbersome and cannot be automated, resulting in low testing efficiency.  Tektronix Tri-mode probe (three-mode probe), under the control of the test software, alternately works in single-ended mode (A-GND and B-GND). It can realize the detection of 8 single-ended signals without changing the hardware connection. Collect and then automatically calculate the differential signal. This enables the automation of all projects. In addition to the three-mode probe solution, Tektronix also provides a cascade automation solution for two oscilloscopes, which enables simultaneous acquisition of 8 single-ended signals through 8 channels, resulting in higher testing efficiency.

Figure 3. Schematic diagram of Tektronix three-mode probe

Solving test complexity issues

As the rate increases, the HDMI specification defines new equalization technologies and cable models, which also complicates the testing process. The specification defines two Cable modes: Category 3 Worst Cable Mode (WCM3) and Category 3 Short Cable Mode (SCM3). Two equalizations: CTLE 1 ~ 8dB and DFE 1-tap d1 value 25mV.

Tektronix solution has optimized the algorithm for the above situation and the test time is short.

Figure 4. Complex channels, equalization and crosstalk

Automatic switching of test rate and code type

In the past, testing required manually changing the resolution to achieve changes in the test signal rate. Now, through the cooperation of test software and EDID/SCDC simulator, Tektronix sets the test signal rate in FRL Rate in SCDC (Status and Control Data Channel) offset.0x31, and sets the pattern for each Lane in offset 0x41/42. It realizes automatic switching of the rate and code type required for testing, realizes complete automation of testing, and improves testing efficiency.

Figure 5. EDID/SCDC diagram

Tektronix HDMI2.1 FRL automation solution

Configuration 1: DPO70000SX oscilloscope cascade solution

Two DPO70000SX oscilloscopes are synchronously cascaded using UltraSync cable, which can adjust the skew of 8 channels to within 1ps to ensure the synchronization of all single-ended signal acquisition. After collecting 8 single-ended signals at the same time, it automatically calculates and generates 4 pairs of differential signals. The test process does not require changing the hardware connection, the signal path attenuation is small, the test speed is fast, and the efficiency is high. Paired with EDID emulator, automatic switching of rate and code type is achieved.

Figure 6. Schematic diagram of Tektronix HDMI 2.1 oscilloscope cascade test solution

Configuration 2: DPO70000SX oscilloscope with Trimode probe

Using the characteristics of the Tri-mode probe, under the control of the test software, it alternately works in the single-ended mode (A-GND and B-GND), and completes the collection of 8 single-ended signals in batches. The testing process also does not require changes to hardware connections. The oscilloscope will automatically de-embed the probe to eliminate the impact of the probe on the signal. Taking both cost and efficiency into consideration, automated testing is also implemented through the EDID emulator.

Figure 7. Schematic diagram of Tektronix HDMI2.1 three-mode probe test solution

Oscilloscope Bandwidth Considerations

In the HDMI2.1 specification, the recommended oscilloscope bandwidth is 23GHz or above. Due to cost considerations, you may ask, can a 16GHz or 20GHz bandwidth oscilloscope be used? On the one hand it can be seen from the perspective of rise time and bandwidth. The fastest rise time allowed by HDMI2.1 signal is 22.5ps20%-80%. You can see from the table below that the higher the bandwidth, the smaller the rise time measurement error.

Table 2 Oscilloscope rise time considerations

From a bandwidth perspective, the bandwidth definition of an oscilloscope is: the frequency at which the amplitude of the sine wave observed by the oscilloscope is attenuated by -3dB. In the actual test process, non-sinusoidal signals need to consider the 3rd to 5th harmonics. The highest signal rate of HDMI2.1 is 12Gbps, the fundamental frequency is 6GHz, and the third harmonic frequency is 18GHz. When measured by an oscilloscope with a bandwidth of 16GHz, the third harmonic component will be attenuated by more than -3dB.

On the other hand, if the maximum FRL rate of the tested HDMI2.1.DUT does not reach the upper limit of 12Gbps, the bandwidth requirements of the oscilloscope can be actually evaluated according to the above calculation method. Simply put, in order to ensure better measurement accuracy and test compliance, the higher the bandwidth of the oscilloscope, the better.

Summarize

Tektronix oscilloscopes use the channel's adjustable termination voltage, the single-ended characteristics of the Tri-mode probe/oscilloscope cascade characteristics, and cooperate with the EDID/SCDC simulator to achieve true automation of HDMI2.1 FRL source-end testing and improve the test efficiency. Optimization algorithm specifically for FRL signals to help customers quickly verify HDMI2.1 products and accelerate the marketization process of customer products.