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The main points design of PCB layout and routing of HDMI interface
1. Definition
The full name of HDMI is "High Definition Multimedia", namely: High Definition Multimedia Interface. HDMI is pin-compatible with DVI, but uses a different package. Compared with DVI. HDMI can transmit digital audio signals, and adds support for HDCP, while providing better DDC optional functions. HDMI can transmit up to 15 meters, which is enough to cope with a 1080p video and an 8-channel audio signal. HDM1 supports EDID and DDC2B, so HDMI devices have the "plug and play" feature, and the signal source and display device will automatically "negotiate" to automatically select the most appropriate video frequency multiplication format. One pair of clock differential, three pairs of data transmission differential.
2. Wiring design requirements
Keep the electrical length between the HDMI connector and the device as short as possible to minimize attenuation. In order for the differential signal to be transmitted normally, the distance between the two traces of a differential pair must be kept consistent on the entire trace. Otherwise, the change of the distance will cause the magnetic field coupling to be unbalanced, thereby reducing the effect of magnetic field elimination and increasing EMI. The recommended line-to-space ratio is 1:1
a. Winding is performed where the end points of the differential traces do not match.
b. The gap width of the differential equal-length winding meets 4W, and the length of A is 3 times the line width.
C. Adjacent to the GND layer, carry out the grounding treatment if there is enough space. The distance between the ground wire and the differential wire is 3 times the width of the differential wire (center to center), and the copper is 20 mil away from the differential wire.
Place some grounded vias near the vias of the signal change layer to provide the nearest return path for the signal.
3. Impedance design requirements
The differential impedance is controlled to 100 ohms (+/-10%), and the single-ended wiring is controlled to 50 ohms.
The purpose of PCB design is to minimize the discontinuity impedance as much as possible, thereby eliminating reflections and keeping the signal intact. The remaining inevitable discontinuities should be concentrated together, that is to say, the area of this area should be kept small and placed as close as possible. The idea is to concentrate the reflection points in a certain area instead of distributing them in the entire signal path (see the isometric correction method for details).
The locations where discontinuity may occur are:
A. The place where the HDMI connector pad meets the signal trace;
B. The signal trace touches the via hole, resistor component board or IC pin;
C. The place where the signal pair is separated to be routed around an object;
The matching resistance of the signal line is used to prevent ESD and fine-tune the impedance. It is usually placed close to the socket, but the two resistors must be placed side by side, not one behind the other.
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