Enabling HDMI in the automotive segment
By Paul Slattery – Digital Video Products, Analog Devices Inc.
Playback of high definition content such as movies from Blu-ray discs requires significant processing power – not always found on existing processors in infotainment (in-car entertainment) systems today. By utilizing the HDMI industry standard, car makers can quickly and more easily integrate HD movie playback. Such systems may use an automotive qualified Blu-ray drive connected using Type-E cables and connectors listed in the HDMI specification. Since the HDMI standard offers content protection for synchronous audio (including multi-channel HD audio formats) and video transport, there is no need to have a separate connection to an audio amplifier over an alternative network or audio bus.
Playback from Blu-ray players is just one application for HDMI in the automotive segment. Increasingly not just passengers but also drivers want to connect their mobile devices to the in-car infotainment system. In the near future, smartphones can be envisaged as the entertainment hub greatly reducing infotainment processing requirements, complexity and cost. The design cycle of a smartphone is significantly shorter than an infotainment system ensuring the latest consumer developments can be quickly adapted into the infotainment system if the smartphone is the hub of the unit. HDMI is ideally suited to connect mobile devices to the infotainment system, as it provides a high quality uncompressed feed. Importantly for the automotive industry, HDMI cables have low noise coupling and are unlikely to be affected by other equipment in the car.
Figure 1 HDMI Application in an Automotive Design
Figure 1 shows an example HDMI application in a car. There are two HDMI inputs to the Head Unit (main controller of the car infotainment system), an audio output and a video output to the display. A HDMI Receiver (Rx) decodes the video from two HDMI inputs and extracts the audio. The audio is converted to analog and connected to the speakers. The first HDMI input is connected to an installed Blu-ray player, which may be located in the trunk of the car. The second HDMI input provides connectivity to portable media devices equipped with HDMI such as smartphones, tablet PCs, laptops, digital cameras and camcorders.
The HDMI interface is ubiquitous in the consumer industry, but how can it be integrated into automotive applications?
While HDMI interfaces are now found on a wide range of home entertainment equipment, automotive applications present new challenges for designing with the HDMI interface. Automotive applications could include either open or closed systems. Open systems allow connection of consumer devices for example, smart phones or tablet PCs. Closed systems allow inter-connection of specialized modules within the vehicle entertainment system.
Closed systems can be AC coupled to provide short to battery (STB) and short to ground (STG) protection. Although this is not described in the current HDMI specification, it provides an elegant solution to common automotive problems. If a fault condition develops in the HDMI cable, and a short occurs to the battery or to ground, the HDMI receiver and transmitter ICs should not be damaged. The signal levels used in an HDMI interface are 3.3V and 5V, so shorting to a car battery positive terminal (12V) would typically damage the HDMI receiver and transmitter. If the HDMI interface is AC coupled, the common mode voltage between the HDMI Rx and Tx cannot damage the ICs. In addition, an over-voltage protection circuit would be required to protect the HDMI DDC lines as these cannot be AC coupled. Figure 2 shows a high level example of how to implement STB and STG protection. The HDMI Tx and HDMI Rx AC coupling circuits are designed to work together as a pair.
Figure 2 HDMI STB and STG protection CCT Block diagram
PCB Design Considerations:
A major challenge in adding STB protection circuitry to the HDMI TMDS lines is to do so without affecting their performance or their matched impedance. In order to meet compliance requirements, it is recommended to keep the STB protection circuitry as close as possible to the HDMI TMDS traces. This minimizes the length of the stubs off the traces, and reduces the possibility of reflections due to the stubs.
How to Implement AC coupling on an HDMI TX?
One possible circuit to implement AC coupling on the HDMI Tx TMDS Lines is shown in Figure 3. The AC coupling capacitors provides the STB and STG isolation, and should be chosen to be a short circuit at the TMDS frequencies. They must be chosen to withstand a DC voltage of 18V. As car batteries typically output 12V and the alternators output approximately 14.5V, 18V is commonly used for STB testing to provide some safety margin.
The HDMI Tx switches in and out a current source and it needs to have pull-up resistors to 3.3V. This is the function of the 100Ω pull-up resistors shown. Each line of a differential pair of TMDS lines must have an AC impedance of 50Ω. It is not shown here, but there is a second 50Ω series internal pull-up resistor in the downstream HDMI Rx. Using the 100Ω pull-up resistors in parallel with two 50Ω resistors in series with each other, gives an overall impedance of 50Ω.
ESD protection is provided by the RCLAMP0524PA TVS (Transient Voltage Suppressor) devices. The RCLAMP0524PA has a pin to pin capacitance of only 0.3pF. This allows it to be used on circuits operating in excess of 3GHz without signal attenuation. It provides ESD protection up to +/-15Kv air and +/-8Kv contact discharge.
Figure 3 HDMI TX AC Coupling on the TMDS Lines
HDMI TX AC Coupling Testing:
The circuit shown in Figure 3 has been built and tested as follows: To verify the short to battery circuit, each of the TMDS lines was shorted to an 18V battery, and the HDMI Tx was functionally tested afterwards to ensure it was not damaged. This circuit was tested for HDMI compliance at 27MHz, 74MHz, 148MHz and 165MHz. The only issue seen was with the low level output voltage (VL) test. However, the ADV7612 in the HDMI Rx AC coupling circuit is robust to the higher VL level. This circuit is proven to work well beyond the 74MHz typically used with HDMI Type-E automotive connectors.
How to Implement AC coupling on an HDMI RX?
The circuit in Figure 4 has AC coupled the HDMI RX to support STB and STG error conditions. The AC coupling capacitors block the common mode voltage and protect the HDMI Rx from STB and STG conditions. The upstream HDMI Tx must be AC coupled for this circuit to work. If the HDMI Tx was DC coupled it may not sense the correct termination of 100Ω per differential pair. The RCLAMP0524 devices provide the necessary ESD protection. The 2KΩ pull-up resistors are used as bleed resistors and must be correctly sized for the increased current in STB and STG conditions. For example, in an STB condition:
P = (V²)/R
= 0.162 Watts
Figure 4 HDMI RX TMDS AC Coupling Circuit
HDMI Rx AC Coupling Testing:
This is the preferred circuit for automotive applications; the only circuitry between the HDMI Rx and Tx AC coupling capacitors is the bleed resistor. This circuit has been verified and is immune to short to battery and short to ground conditions. It also passed HDMI TMDS compliance testing up to 165MHz, the upper limit of the ADV7511W upstream transmitter. The result from the differential impedance testing is shown in Figure 5 for data lane 1. The maximum differential impedance allowed is 115Ω and the minimum allowed is 85Ω.
Figure 5 TMDS Differential Impedance Testing
How to make the remaining HDMI Rx lines tolerant to STB and STG?
Figure 6 shows a circuit designed to give STB protection for the HDMI DDC lines, CEC and the +5V line. There is a MOSFET switch on each of these lines. Under normal operation, once the +5V is applied to the gate of the MOSFET, it behaves like a switch, connecting the source to the drain. If an STB condition occurs, the Drain voltage will begin to rise above the +5V at the gate of the MOSFET transistor switching it off. There are some important design considerations when selecting this N-Channel MOSFET:
- It must have gate-drain and drain-source breakdown voltages of greater than 18V.
- It must have a very small output capacitance.
- The DDC_SCL and DDC_SDA must have a line capacitance of less than 50pF to pass HDMI compliance. This capacitance includes the trace capacitance from the HDMI connector, the total capacitance seen because of the MOSFET switch and the capacitance of the HDMI Receiver DDC pin.
If the Drain to source voltage drop is greater than 0.5V the Vscl (Test ID 8-9) will also fail. The Vscl test measures the output voltage from the DDC SCL and must be in the range 4.5V to 5.5V.
Figure 6 HDMI RX DDC, CEC and +5V Line STB and STG Protection Circuit
HDMI Rx DDC CEC, and +5V Line STB Protection Circuitry… Design tips for passing HDMI compliance testing
This circuit has been built and tested using several different types of video sources. The HDMI Tx is able to read the E-EDID using the DDC SDA and SCL lines and the HDCP encryption worked correctly. The system designer also needs to carefully consider the following points before implementing this circuit.
v CEC Line Resistance (HDMI Compliance Test ID 8-13): This test measures the CEC resistance from the HDMI input to the HDMI outputs (if any) on the PCB. This test will fail because this test is done with the PCB power off. When the PCB power is off the MOSFET is off and leaves the CEC line disconnected.
v CEC Line capacitance (HDMI Compliance Test ID 8-9): This test may fail if there are a number of HDMI Rxs on the PCB depending on the choice of MOSFET. The test limit is less than 150pF for a CEC non-root device. The capacitance of each MOSFET switch on each CEC line will add as they will all be in parallel.
How to make the HDMI TX DDC, CEC and HPD lines tolerant to STB and STG?
Figure 7 shows the circuit required to make the DDC, CEC, and HPD lines tolerant to STB and STG conditions. It is similar to the HDMI Rx DDC circuit except for the handling of the +5V line and the Hot Plug Detect line. The diode on the +5V line will turn off if an STB event occurs on this line. The diode should be chosen to have a low turn on voltage, a fast turn on time and a reverse breakdown voltage of greater than 21V. The 4.7KΩ resistor and zener diode protect the Hot Plug Detect line from an STB situation.
Figure 7 HDMI TX DDC, CEC, HPD and 5V STB and STG Protection Circuit
HDMI Tx DDC Line STB Protection Circuit… Design tips for passing HDMI compliance testing
The above DDC Tx circuit has been verified and passed the HDMI Compliance Tests. The HDMI Tx could read the downstream E-EDID using the DDC SDA and SCL lines and the HDCP encryption worked correctly. One important design consideration is that the diode on the +5V line needs to have a forward voltage drop of less than 200mV while drawing 55mA, in order to pass the HDMI compliance test (Test ID 7-11).
The ADV7511W HDMI transmitter and the ADV7612W HDMI receiver were used to test all of the HDMI circuits shown in this article. These are automotive qualified I.C.s from Analog Devices.
The ADV7612W is a 225MHz low power dual-input HDMI receiver. It supports all mandatory 3D TV formats defined in HDMI 1.4A. It is also capable of extracting audio from the HDMI stream to its audio output port. It is specified for operation over the -40˚C to +85˚C temperature range.
The ADV7511W is a 165 MHz HDMI transmitter, which is intended for automotive infotainment applications. The ADV7511W supports HDMI v1.4a features including extended colorimetry, High Bit Rate audio and 3D video. The ADV7511W is specified over the -40°C to +105°C temperature range.
For more information on the ADV7511W: http://www.analog.com/en/audiovideo-products/analoghdmidvi-interfaces/adv7511w/products/product.html.
For more information on the ADV7612W: http://www.analog.com/en/audiovideo-products/analoghdmidvi-interfaces/adv7612/products/product.html
About the author
Paul Slattery is a senior applications engineer with the Digital Video Products Group at Analog Devices, Inc. He is responsible for defining, developing and marketing analog and HDMI video products used primarily in consumer, automotive and portable applications. Paul received a degree from the University of Limerick. He can be reached via email at firstname.lastname@example.org or via ADI’s EngineerZone community at http://ez.analog.com/people/PaulS.