for USB OTG Slave mode, the system may not power USB VBUS, while in Master mode it has to. Some kind of detection/switch/power enable is needed.
A similar quesion has been asked here:
However, I am currently comparing the MX7 and MX6 reference designs regarding the power supply of USB OTG.
The MX6 design uses a "USB_OTG_MODE" signal, for the MX7 this signal is called "USB_OTGx_PWR".
In both cases, these signals are used to cut the power supply using a transistor circuit or a power switch IC.
In my case, USB OTG1 on the MX7 shall be used for both flashing the linux image and attaching a USB stick.
So the default should be to NOT power the port and detect USB slave dvices based on a pin toggle in software?
If I understand correctly, the USB_OTGx_PWR signal has to be triggered in software (means: there is no hardware based detection)?
I assume the MX6 and MX7 use the same USB OTG hardware inside the chip. What external circuit would you recommend? The power switch based solution of the MX7 or the transistor based solution of the MX6?
Thanks for your help,
Ive also found this
For OTG-device compatibility, four voltage detectors are required to watch VBUS for (greater than) 4.4V, (0.8V to) 4.0V, (0.8V to) 2.0V, and (0.2V to) 0.8V. These levels correspond to the different states of VBUS as seen by an A-device or B-device
But i cannot find anything like this on the reference designs.
I can not find any VBUS capacitors either on the MX7 sabre design (From what i know, 1..6.5 uF is necessary).