This FAQ presents a general guideline to laying out HDMI traces on circuit boards. Many of the problems we see in the forum threads can be traced back to routing and layout issues. The rules and notes I present here have been gleaned from various boards I have designed and seen over the years. I can not reference back to the original sources for this information however simple web searches will reveal many documents covering this topic.
HDMI 1.4b specification defines signaling up to 3 GHz requiring board designers to be very careful with HDMI signal integrity during layout. Following is a good set of rules that will allow you to transmit 3 GHz across FR-4 PCBs.
First Some Math
Let's assume we have a transmitter routed to a connector and we want to transmit a format with 297MHz clock rate.
Layout is key. The following is a general set of rules that has become part of my layout specification for boards. They are a bit tighter then the math above indicates but are easily met during layout. These rules can be added directly as constraints to your schematic or as part of the layout specification document.
The following is a general set of instruction notes placed on the PCB fabrication drawing. They allow the PCB fabricator to tweak the gerbers to match their process and materials.
How to handle reference plane transitions
Analog Devices HDMI components are designed so routing is a straight shot from the device pin to the connector pin. However sometimes you need to change layers the HDMI routing is done on, often changing the reference ground plane in the process. The transition vias should be symmetrically placed in the path and ground to ground vias to handle the signal integrity of reference plane jumping as illustrated below
This particular example is from an eight layer board, the red trace is on the top layer, the blue trace is on the bottom layer and the dark green layers are the solid ground planes adjacent to the top and bottom layers. Note that the vias are place symmetrically in the path and separated by 1.14mm. Also note the 2 ground vias adjacent to the 2 trace vias.
In this example if the differential pair jumped from layer 1 to layer 3, no ground to ground vias would need to be added since you are not jumping reference ground planes.
It is best to keep the number of differential pair vias to a minimum since each via introduces impedance changes.
It is also a good idea to stitch ground to ground vias along the trace path every 2 cm or so. This aids in general signal integrity control.
How far can you go?
Normally HDMI traces on boards are very short, directly from the connector to the receiver/transmitter. Many times we get the question of how long can I make HDMI traces. The HDMI specification does not define maximum length, only expected impedance. As long as you follow the layout rules above you should be able to run HDMI traces over 20-30cm easily. Longer if you are careful.
ESD protection devices are often added to increase ESD protection levels. I suggest devices with signal flow through like Semtech RClamp0524. They allow HDMI signals to flow directly under the part while providing low capacitance ESD protection. There are many vendors producing these types of low capacitance TVS arrays.
HDMI Differential Routing Example
Here's an example HDMI routing from a horizontal HDMI connector on the left to an ADV7850 receiver port.
Things to Note:
The Big Picture
Designing a HDMI transmitter or receiver board is just part of the of the whole system. You must always keep in mind that you are often connecting through a cable to a source/sink. The connection from the board to the cable and from the cable to the source/sink are transition points. Most HDMI connectors are 100 Ω ± 15% and cables have attenuation proportional to the length. Poor transition points can cause reflections causing poor signal integrity. Long cables will attenuate signals decreasing the swing voltage at the receiver dropping the received signal out of specification. All these problem become more prevalent at higher resolution like 1080p or 4K.
Analog Devices Inc provides many reference board designs from which you can copy layouts from.
I recommend using signal integrity tools to verify HDMI trace layouts meet specifications.
This is a pretty good guide, although there is something that confuses me.
HDMI1.4b supports data rates up to 3.4Gbps, this would create a signal with max frequency of 1.7Ghz
HDMI2.0 supports data rates up to 6Gbps, this would create a signal with max frequency of 3Ghz
So, 6.67ps/mm is for a signal of 3GHz(6Gbps HDMI2.0) or 3.4Gbps(1.7Ghz HDMI1.4b)?
Do you have a plot ps/mm vs GHz?
From CEA861E, 1080p120 => 297Mhz pixel rate =? TMDS rate = 10 * 297MHz = 2.97GMhz (3G).
Yes, 1080p60 deep color only goes to 2.25GHz
Where in the spec do you see the 3.4Ghz limit?
For HDMI2.0 4Kp60 => pixel rate at 594MHz => 10 * 594MHz => 5.94GHz tmds rate but this is for HDMI 2.0
This guide was originally done for 3G designs. I'll have to update it for 6G in the future.
Ideally you'd want intra-pair skew to be less than (7.57 / 2) mm
Thanks for your prompt response.
Category 2 cables support TMDS clock frequencies up to 340MHz, supporting 3.4Gbps. Section 4.2.6 or table 4-1
But this is no what confuses me.
a 3.4Gbps signal will need a 3.4GHz clock to recover the data(or 1.7GHz if working in dual data rate), but the signal itself will generate a maximum of 1.7GHz this would be in the case a sequence of ones and zeros is transmitted(1010101)
So, if 6.67ps/mm is for 3GHz signals, this is the delay a 6Gbps signal will have.
If 6.67ps/mm is for a 3Gbps signal, then it is the delay for a 1.5GHz signal.
Gbps = 2*GHz
6.67ps/mm is for 3GHz or 1.5GHz?
That cable assembly just specs 340MHz for the cable. Receivers and transmitters are designed for 3G max.
When you talk about transmitting a byte (01010101), the transmitter is doing 8b10 encoding. This is where the 10x comes from. What's important is the differential delay between different pairs of channels. All 4 tmds pairs could be delayed 100ns without problems. But they all must be delayed together. Differential delay between pairs are limited to 0.15Tbit so the 10 bit symbols can be decoded by the receiver. If one tmds pair gets delayed by 1 bit, the 10bit symbol decoding will be out of sync and broken.