[Galaxy S III Accessories] Diim:
HW advices

Hello guys !

Very interesting thread out there.

I have already bought my smart dock, so I've not planned to try that, but I am still interested in a light USB OTG / MHL output split cable.

But I've got some electronics skills (as electronics engineer for a couple of years), so here are some basic rules that may help the ones who plan to build their own adapter (if not too late)...

First of all, avoid those awful cans of worms. Wires must be sorted out carefully, otherwise you'll probably get some EMC issues.

Connections must be made as straight as possible and as short as possible. Loops must be avoided.

Data+ / Data- wires are very high speed differential pair that carries either USB data or MHL audio/video data. Those pairs are the most critical signals and must be given a careful attention:

For each pair, try to keep the length of wires as matched as possible.

Keep each pair twisted as far as possible

Ground and shield: USB, HDMI, MHL, cable are shielded. At least there is 1 aluminium foil, plus probably a metallic braid. The shield is reasonably connected to the metallic part at each end of cables, and that seems also the case on the 11-to-5 pins adapter. So do not break the shield path. Reconnect the shield wherever it was connected. If you have removed a connector and replaced it by cable, if the shield was connected, then solder the shield on the ground plane of the PCB. In all cases, the shield connection must be as straight as possible and as wide as possible.

High speed communication cables are always designed with controlled line impedance. That applies to Phone lines, Ethernet, USB, HDMI, MHL, ... For instance, differential line impedance of data pairs is 90Ohms for USB, 100Ohms for Ethernet, 100Ohms for HDMI, 100Ohms for MHL. So using parts of HDMI cables for connections sounds a good choice. Because of the speed of data they carry, HDMI cables generally offer high quality level and good controlled impedance, and provides at least 4 twisted pairs plus additional wires. Besides, MHL standard was derived from the HDMI standard, so HDMI pairs should be suitable to carry MHL data, and the difference with the USB standard should not be a big issue.

The only concern I would have is the power supply. Are the thin HDMI wires able to carry 4 Amps (2 Amps + 100% safety margin). ?

If not, there could be cable overheating or at least unwanted voltage drop. Perhaps would be good considering connecting several wires in parallel ?

PCB: If needed to design a new PCB, high speed signals must be given particular attention. Traces must be sized and routed with proper matched and controlled impedance. Traces lengths among differential pairs must be matched. Common modes and Differential impedances can be calculated based on several key parameters (trace width, coper thickness, insulation distance with other traces and power/ground planes, dielectric material ...). Those HW design activities are a bit far in the past for me now, but as far as I remember, there are free tools on the web that allows to calculate that.

The key is to adjust geometrical dimensions to match the targeted impedance. That is often recommended to route high speed signal on external PCB layers and sides.

If needed to reproduce a smart dock by scratch, that would reasonably require a 4 layer PCB, perhaps 6 layers. That is a heavier job for sure. I just would say that since we are talking about high speed digital logic, all common EMC design rules must be taken care off (ground / power supply planes, power supply decoupling and filtering, ...)

Soldering:

Avoid to heat the pins to much. The eat is easily conducted by metallic parts inside the connector, and make plastic parts melt. However, PCBs are commonly manufactured with Pb free processes, that makes the melting of soldering joint harder (melting temperature is higher than with old soldering joint). So focus on the soldering spot and try to not apply the soldering iron to long. Using flux often help to ease the heat transfer from the soldering iron to the soldering joint.

Most of above recommendations are intended to reduce the EM interferences and preserve the signal integrity. Bad design would result in unstable data transmission, or untimely data loss. And the adapter may be sensitive to surrounding EM interferences.

Additional notes:

As for 3) the length of wire is supposed to be measured from the connector pin at one end to the connector pin at the other end, including PCB traces. However, do not untwist pairs to measure each wire ! Just try to match the length at each end of the pair. As for PCB traces, their lengths should be already matched, so you shouldn't have to care about that.

That is not recommended to cut a high speed transmission cable such as HDMI (and reconnect the wires after). Doing that creates a disturbance point on the transmission lines. When a signal reaches a disturbance on a transmission line, one part is forwarded, one part is returned toward the transmitter. That will result in signal distortion. So if really necessary to do that, above recommendation must be carefully followed. That applies also to the shield as well in order to minimize the impedance mismatch at that point.

Do not hesitate to do some web search about transmission lines, twisted pairs, if needed.

That's all I could say. Hope that will help.

Good luck !