USB Signals do not work - expect when using a multimeter (Custom PCB based on KL27)

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USB Signals do not work - expect when using a multimeter (Custom PCB based on KL27)

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oliviermartin
Contributor I

I have done my own custom PCB using Kinetis KL27. The board should be powered by USB 5V and appeared as a USB device when plugged to the computer.

So far, the board seems to correctly be powered by 5V and the MCU by 3.3V (converted by a LDO). The KL27 MCU can run and be flashed through the ROM Bootloader using UART.

My issue is the board never appear has a USB device (either from the Bootloader ROM or from my own Flash code) when plugged to a computer.

While I was measuring the various USB signals. I realize when I measure the voltage on USB D+ (with the GND) using a multimeter, the board is magically recognized as a USB device when it runs either from the Bootloader ROM or from my own Flash code).

Any idea?

Here is a reduce schematic and layout of my custom PCB.

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renato_kiss
NXP Employee
NXP Employee

Hello Olivier,

Analyzing hw by picture is not ideal, but I can suggest some improvements based on your files.

For USB diff pair you have to keep in mind that the shortest path is the best for signal integrity (and immunity). The picture shows a very long trace for USB data tracks. Keep them short.

It seems to have variable distance between differential tracks. Use the minimum gap your PCB manufacturer can provide and try to not change it during path. Every time you deviate (open or close the gap) you change diff impedance. It is specially important for high speed devices/modes. Don't worry to much in keep 90Ohms diff/45ohms Com impedance. Stress to keep traces parallel and with same impedance all the way. Two vias per trace, as you have, are ok (the ideal is no vias).

Provide a efficient return path. This is the most important for you board. You have a very poor grounding, specially for USB. If you can, in your prototype, increase the grounding using a copper tape. For two-layer board, with low density, it is recommended to ground analog/critical signals and them poor ground on whole board. It will give a good chance of having good return paths for all the signals.

Why does your USB is working after you probe it with multimeter ? I guess you are adding a parasite capacitance, and it is improving your eye diagram. I thought a good layout (diff traces + grounding) will increase the robustness of board.

Regards,

Renato Kiss

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oliviermartin
Contributor I

Hi,
I actually found my issue! After some time of reflexion, I was thinking the side effect of the multimeter might not be due by an electronic effect but might come from the fact I was pressing the multimeter probe on the USB D+ pin. I resold the USB D+ pin of the USB connector and it magically works!

The real layout (I removed the components and routes that were not USB specific to simplify the schematic and layout) has a GND plane.

But thanks a lot Renato for all these advices. I do not understand them all (this PCB was actually my first PCB) now but I will carefully read them and try to understand them for my next PCB!

Thanks again,
Olivier

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renato_kiss
NXP Employee
NXP Employee

Thanks a lot for let us know the solution.

Have a nice day.

Regards,

Renato

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