We have an issue with the iMX6 USB OTG.
For now, it’s more of an annoyance than anything… But we would like to make sure there’s not a bigger issue behind the behavior we have noticed.
Behavior:
Our Board is un-powered, and the Software group is plugging their laptops in the USB OTG port.
It seems that most of our boards behave as unpowered devices… So no response until we power up.
However, some of our boards “try” to operate from the 5V the Laptop places on USB_OTG_VBUS. i.e. The Laptop recognizes a “unidentified” USB device.
So it’s as if the USB OTG PHY gets enough power to recognize the connection, but no logic is awake to manage it.
So as you can imagine, we have worries that a reverse leakage current may damage some of the iMX6’ inner circuits.
Even though at this time we NEVER noticed any “warmer than normal” iMX6 during this behavior, and once the iMX6 is Powered, all behaves well…
Our USB OTG is like in the Nitrogen schematics (see page 5/12 of attached pdf).
Meaning that the USB_OTG_VBUS signal goes from the connector, directly (via EMI filter) to the iMX6 pin E9.
If you look at the Saber schematics (see page 11/25 of attached pdf), Q514 stands in between the connector and the iMX6 (page 3/25).
By my understanding of the circuit (since Q514 is a P-FET and Q515 is an N-FET) Q514 is pretty much “unconditionally on” as soon as the Saber is powered…
Has Q514 been included for the sole purpose of preventing the OTG_VBUS from reaching the iMX6 when it is un-powered ?
Or is it part of the USB_OTG_CHGR circuit ?
We need to ask because the last answers were conflicting…
So here’s the summary of the questions:
- Is it safe to apply 5V on the iMX6 USB_OTG_VBUS pin E9 when the chip is unpowered.
- How do we explain the difference in behavior between boards ? i.e. how come some try to power up the PHY and some don’t.
- On the Saber design, Has Q514 been included for the sole purpose of preventing the OTG_VBUS from reaching the iMX6 when it is un-powered ?
This one is CRITICAL and kind of URGENT, our Customer wants to go to production in the upcoming month, and the USB OTG will be used to Flash the Boot code in the SD-Card… So it will be used on ALL the boards.
