USB device not recognized without resetting the board

On our RAM board, I have a weird issue : the board needs external power (in addition to USB) for Windows to recognize it as an USB device.

But… without that external power, the board is still working as expected, with at least two Click boards on it (bargraph) and 3 leds blinking, so it has enough power.

The board is using the SC20260E SOM.

What could cause this issue ?

What GHI board did you use as a basis for your power design?

Kind of hard to guess what you have done or not done?

Sounds like your power management has an issue?

Does it work fine with powered hub?

I did not use any GHI board as a basis. I know you may argue on that :wink:

The power has two “options” : external power with two OKI-78SR modules (one for 5V and one for 3.3V) and USB power with a MC33269 to get the 3.3V.

I say it’s weird because if I boot in bootloader mode, then I can see the GHI bootloader device.

@Gus_Issa : I will have to buy such a hub but I’ve made some other tests and the issue is not really the same now and seems partly caused by my PC.

On two ports of my PC the board alone (without external power) is not seen by Windows. On those ports, if I add external power, then the board is seen by Windows.

On other ports, I have to reset the board (reset button) once plugged for Windows to see the USB devices.

On another PC, same behaviour : I have to plug the board and then reset it to see the USB device.

Once resetted and USB device seen by Windows, everything is working fine (without external power) : I can debug or use TinyCLR config.

Kind of strange behaviour, isn’t it ?

Not strange

How much current at 5V is your board taking?

From your post, I assume that when you plug in your board, it actually runs the software installed on it, just that the USB does not get recognised? How quickly does the board boot on USB power compared to external DC power? Could it be a slow power up issue? This quote would indicate a power up issue

“I have to plug the board and then reset it to see the USB device.”

How can I measure the 5V current the board is taking ? Should I break an USB cable to insert current-meter ?

Regarding the startup time, I cannot notice any difference between with/without external power. Visually, the system is starting in less than half a second in both cases. If there are any differences, they are minimal.

I too suspect a power-up issue but when I compare the SC20260 dev board and our board, the only difference on the USB power rail is the 3.3V regulator. We use MC33269 and GHI is using AZ1117TE. So unless the SC20260E SOM is eating more than 800mA at startup, I don’t see how this component could be the cause of the behaviour. But I may be wrong, I don’t know.

Anyway, since the board is running the software as expected, I have inserted a software fix while waiting for the hardware issue to be resolved.
Now the board is seen by Windows each time I plug it on my PC.

@Gus_Issa : this week-end I will buy a powered hub. I will come back to you about this at that time.

Post your schematic. Maybe someone will see an issue?


@Gus_Issa : are the SC20260D and SC20260E SOM identical except the form factor ?

Correct. Theu are the same

GHI uses resistors on the D- and D+ pins of the USB connector. Might be worth putting them in and seeing if it makes a difference.

Resistors are not needed but you can try

If Gus uses resistors, I use resistors!

VCC is too low because of the voltage drop thru D5?

VCC is too low because of the voltage drop thru D5?

I had that though also, but GHI uses the same component in the same manner…

No it doesn’t, there ain’t no diode on vcc

Yes, they have no diode because 3.3V is given by the 5V supply. My 3.3V comes from two sources : 5V USB or external power via the OKI parts, hence the diode on my design.

My biggest problem here is that it seems to occur at startup, during the USB initialization (it’s only a guess at the moment). Because the software still runs fine after that. No USB device on Windows but device fully functionnal.

If it’s a voltage drop at startup then why would the MCU still boot up ? Why would it prevent only the USB thing ?

I just can’t see what’s going on :roll_eyes:

Remove D5 and short it out and see if it works without external power

Also, why are you using the second oki to make VCC as well?

Because those are (well) designed in terms of regulators. So I did not want a very good 5V only and a potentially less good 3.3V.
I understand that it has a cost and that I could go with only a 3.3V regulator and 2 capacitors but I wanted to be 100% sure of both 5V and 3.3V lines.

And btw, 6 Click modules at the same time would have needed a “stronger” power design on the 3.3V line, which is exactly what is done on those OKI parts. I don’t want to reinvent the wheel :wink: