USB_OTG

No, this not the normal way to suspend USB traffic.  The USB specfication provides a method to suspend and resume traffic.  The condition of the bus during suspend is described in the specification, so please refer to the specification for these details.

Regards,

Paul 

Hi Paul,

I agree that this is not a normal way to suspend the USB traffic but due to the nature of my application it is important in certain mode the three lines USB0_DP, USB0_DM and USB0_VBUS be low.  In this mode I disable the USB port but my concern is to have the lines low.  May this condition be arranged by software?

Thanks,

Ali.

Which processor are you using?  Are you using a processor that requires an external USB PHY?  If so, which USB PHY are you using?

Regards,

Paul

Hi Paul,

Thank you for your help.

I am using the AM3517 and I am using USB_OTG port (USB0_VBUS, USB0_DP, USB0_DM, GND, USB0_ID floating).  I am running this port as a client not as host.  I do not believe that this port requires the USB PHY.

High Regards,

Ali.

You are correct the AM3517 USB0 port has an internal USB OTG PHY.

I have determined the internal PHY supports GPIO mode on the DP and DM pins but the chip team tied the GPIO control signals to UART3 which defines DP as UART3 transmit data and DM as UART3 receive data.  Therefore, it would not be possible to configure the DM pin as an output in GPIO mode.

I have sent an email to our design team asking if the Mentor controller could be forced into a mode that would drive a low level (SE0) on the DP and DM signals.

I have also asked if the Mentor controller could be forced into a mode to activate the “dischrgvbus” signal which would turn on the internal pull-down resistor on VBUS.

I think this may be the only way to drive these signals low when you are not using the USB port.

It may take several days to get a reply from the design team.  I will let you know when I get an answer.

Regards,

Paul

Hi Paule,

Would it be possible that I receive the response to the USB-OTG a little bit faster please.

High Regards,

Ali.

The AM3517 was designed to support modes of operation defined in the USB specification and what you are trying to do is not defined in the USB specification.  Therefore, AM3517 does not support your requirement.

A USB host will be sourcing VBUS so you would not want to hold VBUS low while connected to a host.  I assume your requirement to hold these signals low only applies when the AM3517 USB port is not connected to a USB host.

The detection of VBUS is how a USB peripheral knows it is attached to a host.  How do you plan to detect VBUS while holding it low?

You may be able to use three FETS and a couple of resistors to hold DP and DM low until VBUS is detected.  A circuit to hold VBUS low would be more complex since VBUS may be applied anytime a USB host is attached.

Regards,

Paul      

Hi Paul,

Thank you for your response.

I was just wondering if we can place a buffer  in between the AM3517 USB port (VBUS, DP and DM)  and the host to put them in low state while they are not in use?

Thanks,

Ali.

I still do not understand how you plan to hold VBUS low when it may be attached to a USB host which sources 5 volts to this signal.  If I knew more details related to this requirement and how you plan to detect and select this mode of operation, I may be able to offer suggestions. 

What impedance to ground is required when these signals are being pulled low?

A USB PHY is a very complex I/O with multiple receivers and multiple output drivers connected to DP/DM.  This complex I/O is required to support all of the USB signaling requirements.  I do not know of any buffers that can be inserted in the signal path. 

A USB Multiplexer/Demultiplexer switch similar to the TI TS3USB221 could be used if the parasitic effects of the switch did not degrade signal quality.  This is one of the best USB switches being sold, but it potentially adds too much series resistance to the DP/DM signals.  The maximum series resistance recommended for AM3517 DP/DM signals is 3.0 ohms, but the maximum ron of the TS3USB221 is 6.0 ohms.  However, the typical ron for the T3USB221 is 0.2 ohms. 

Are you willing to risk you never get a T3USB221 device that has too much series resistance?  If so, you could insert one of these in the signal path with one of the ports tied to ground and use a GPIO to select the grounded port when you are not using the USB signals.

Regards,

Paul      

Hi Paul,

This is how I am planning to use the USB port.  I will connect a 1M Ohm resister connected to the VBUS to ground.  This would assure me that the VBUS is low.  When it is connected to a host it will be +5V.  I am assuming that the VBUS pin on AM3717 is an input.

According to the AM3517 datasheet on page 23 of

http://www.armkits.com/download/AM3517_Datasheet.pdf,

the DP and DM are defined as an I/O and can be pull up or down.  Therefore while the USB port is not connected I will pull these pins low.

Would you agree with my way of handeling the issue please?

High Regards,

Ali.