AM4379: [2nd] AM4379: RNDIS USB device mode not working after hot swapping from USB host mode under OTG configuration

Hi Matt,

Thanks for the update.

I have a few work for the next couple of days, but I will review your original E2E thread early next week and get back to you.

This issue occurs only when hot swapping from USB Host to USB device. Do you have any advice or suggestions for further investigation? 

Hi Matt,

Since the issue only happens on your custom board, but not on the EVM, I am routing your query to our USB hardware expert for comments. I expect you will be asked to send over the board schematics for review.

Tomorrow will be my last day in office for 2024, if I get some time tomorrow, I will look for the USB controller registers for you to dump. But this probably only tells the state of the controller, but likely won't tell why it gets into that state.

Hi Matt,

Can you please share the custom implementation schematics to review if there are any issues from the hardware side.

Regards,

Sreenivasa

Hi Kallikuppa, 

I'm assisting Matt with this issue on the hardware end. As Matt has mentioned in this post, we have removed the 100 uF capacitor on the VBUS line and saw the same issue. VBUS capacitance is below 10 uF on our PCB. We also noticed an anomaly when swapping btw host and device. We didn't see any voltage on the USB_ID pin when the uP acted as a device (USB_ID pin is left open). We didn't see any voltage when we swapped to host mode (USB_ID is pulled low). However, when swapping back to device mode, the USB_ID pin is seeing something btw 2.0 - 3.0 V. When connecting as a host mode, USB_ID pin is pulled low. Please see the attached image of our setup. Do let us know if we have done something incorrect. 

Hello Oscarz,

Thank you for the inputs.

Let me review the inputs and comeback.

Regards,

Sreenivasa

Hi Oscar,

Can you please remove R4 and the FET on the VBUS line for USB0_VBUS_DET_N to see if the problem still exists? I didn't see this type of design before and am not sure if it has any interfere on VBUS which might cause problems on the ID pin.

We removed R4 during our initial debug effort and encountered the same issue. Since the SW team was not using USB_VBUS_DET_N (Matt told me that they didn't use that feature on the EVAL board as well), we also removed Q1. Same issue with swapping btw host and device. 

We spent some time yesterday monitoring the USB_VBUS on our custom board using an oscope. After disconnecting the USB cable as either a host or device, the USB_VBUS line fell from 5.0V to about ~ 2.5V. Since we had our USB cable connected from our custom board to our PC via a USB hub, we decided to check if USB_VBUS would fall to 0V by disconnected the USB Hub from the PC. We noticed that the USB_VBUS line fell to 0V. We then decided to connect our custom board to our PC using the same USB cable, therefore eliminating the USB hub. For this attempt, we only connected to our custom board acting as a device only. Strangely, the USB_VBUS line was still stuck around 2.5 V after disconnecting the USB cable from our custom board. Any idea as to why it would be held up at 2.5V after disconnecting? 

Hi Oscar,

Can you please share the schematic in pdf format including the processor interface.

Regards,

Sreenivasa

I apologize for the delay response, Kallikuppa. 

 USB_Schematic_for_TI.pdf

I can only upload this due to security clauses on our end. Apologies for that. 

Kallikuppa, 

I have caught something that is different btw our design and yours, however, I am not sure as to why this would impact USB host-device swapping. On the AM437x Starter Kit EVM schematic, USB0_ID is connected to the gate of the Q9. There is a 0 ohm jumper resistor (R706) that pulls ID to ground, but the BOM calls it as a DNI. We have USB_VBUS connected to an NFET that controls VBUS_DET. We don't use VBUS_DET and Matt confirmed that VBUS_DET isn't used on the Eval Board. Also, we don't have a cap after the 1K resistor (R553). Is that capacitor necessary apart from a time delay/noise filtering?  

Hello Oscars, 

Thank you.

Did you add a cap and do some check.

The addition of cap may have layout dependency.

Did you any time remove the common mode choke, add bypass resistors and do a check.

Regards,

Sreenivasa

Hello Oscars, 

Did you have a chance to review the below checklist for USB

https://www.ti.com/lit/an/sprace0a/sprace0a.pdf

Regards,

Sreenivasa