Because HDMI (High Definition Media Interface) can transmit video and audio data at the same time, the connection is simple, and the compatibility is good, it is widely used in consumer electronic products (such as TVs, set-top boxes, projectors, etc.). The HDMI system can be divided into 4 types, Source, Sink, Cable and Repeater. In order to ensure the good compatibility of these devices, the specification has made signal integrity requirements for electrical signals.

At present, the mainstream HDMI standards on the market are 1.4b and 2.0, and the corresponding maximum resolutions/refresh rates are 4K@30Hz and 4K@60Hz, respectively. With the increasing demand for higher resolutions and refresh rates, the HDMI Association launched the HDMI 2.1 version in November 2017 , adding FRL (Fixed Rate Link) mode to support 8K@60Hz and 4K@120Hz.

FRL mode

The specific changes of FRL: 1) The independent clock signal of the HDMI interface is changed to be embedded in the data, and the clock recovery unit is used at the receiving end to recover the clock; 2) The maximum number of channels used to transmit audio and video data is 4, and each channel has the highest Support 12Gpbs rate; 3) The encoding method is changed from 8b/10b to 16b/18b, and the encoding efficiency is higher. The physical form of the interface has not changed. The original TMDS Clock channel is redefined as FRL lane3; TMDS Data 0/1/2 correspond to FRL lane 0/1/2 respectively, as shown in the following figure.

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

Link Training Procedure of HDMI2.1 Source and Sink

The source must first read the EDID of the sink (the EDID is the storage space with the I2C address of 0xA0/0xA1 and a length of 256 bytes, including the resolution and the highest speed supported by the sink), and output the resolution/speed supported by the sink. . Source cannot output resolutions/rates not supported by Sink EDID. Sink that supports HDMI2.1 must set SCDC_Present to 1 in the corresponding byte of HF-VSDB of EDID, Max_FRL_Rate setting range is 1 to 6, the number 0 means that FRL mode is not supported, and other values are reserved values. Set Sink version to 1 in SCDC.

The specific meaning of the Max_FRL_Rate value is as follows. The number 1 represents only the 3Gbps rate in the 3 lanes mode, and other rates and lane configurations are not supported; the number 3 represents the support for the 3Gbps and 6Gbps rates in the 3 lanes mode; in the 4 lanes mode, only the 6Gbps rate is supported. Other rates are not supported. The meanings of other numbers are analogous.

SCDC is the storage space whose I2C address is 0xA8/A9. It is used for Source and Sink to communicate and exchange status and control information. Some bytes of Source and Sink can be read and written. Unlike EDID, EDID is read-only to source. During the communication between HDMI2.1 Source and Sink, information such as Sink version, Update Flags, Sink Configuration, and Status Flags will be used.

The first connection between Source and Sink that supports FRL mode requires Link Training, and Link Training is also required to update the FRL rate.

The first training will go through 4 link training states in sequence:

The source TX test is completed in the LTS:3 state. The test equipment sets the SCDC FLT_no_timeout to 1, so that the source will not exit the FRL LT state when the source exceeds LT Tflt = 200ms. Test items requiring pattern and FFE level are also requested and issued in this state.

When the Link updates the FRL rate, it will use the state LTS:4 Update FRL_Rate; when the Link training fails or times out, it will enter the state LTS:L Legacy TMDS. Source and Sink work in TMDS mode, which is HDMI1.4b/2.0 mode. Link training state transition can also refer to the following figure.

HDMI2.1 FRL test project requirements and solution configuration

Since the FRL mode supports a maximum rate of 12Gbps, the bandwidth of the oscilloscope is required to be no less than 23G. There are two configuration methods for different customer needs; in addition, HDMI2.1 fixtures are required, and the EDID/SCDC controller also needs to support HDMI2.1 for automated testing.

Configuration 1: Using the SMA cable method

Two DPO 70000 SX oscilloscopes, synchronized using UltraSync cable, can adjust the skew of all channels to within 1ps, ensuring the synchronization of all single-ended signal acquisitions. The connection between the fixture and the oscilloscope uses SMA cable, which supports the acquisition of 4 lane FRL signals at the same time. The test process does not need to change the hardware connection, the signal path is not attenuated, and the test speed is fast.

Configuration 2: Use Tri-mode probe P76/77xx

P76/77xx probes can be connected to fixtures with SMA adapters.

Using the characteristics of the Tri-mode probe, the oscilloscope automatically controls the probe to work in the single-ended mode A-GND and B-GND, and completes the acquisition of 4 lane FRL signals in multiple times. The testing process also does not require changes to the hardware connections. The connection diagram is as follows, the oscilloscope will automatically de-embed the probe to eliminate the influence of the probe on the signal.

Tekexpress FRL Setup

The FRL mode test software is Tekexpress FRL, and the software interface is as follows. According to the technical specifications of the product under test, select the minimum and maximum rates supported. In the Multi-lane setup, select the lane to be tested. Select the corresponding test item on the Test Selection interface. The test item no longer distinguishes between single-ended and differential, because the test scheme can automatically realize the acquisition of all single-ended signals and the operation of differential signals. No matter which configuration is used, the verification of FRL signals can be completed quickly through the Tektronix automated test solution.

Keywords: HDMI2 1 oscilloscope