This question says "How much current can draw from the USB ports? ......The limit for Pi3 is 1200mA."
However, my Rosewill RNX-N600UB Wi-Fi dongle will only operate in the 5 GHz band when plugged into a powered USB hub. If it is plugged directly into the Pi3, hostapd gives the error
"Hardware does not support configured mode wlan1: IEEE 802.11 Hardware does not support configured mode (2) (hw_mode in hostapd.co nf) Could not select hw_mode and channel. (-2)"
When the dongle is plugged into a powered USB hub, which is then plugged into the same USB port on the Pi3, hostapd is able to initialize the 5 GHz band.
This behavior makes me suspect the issue is power-related. Perhaps the power consumption increases when the 5 GHz radio is enabled?
I have searched for the Raspberry Pi 3 schematics, but am unable to find any that are complete. Specifically, I'm looking for the power-control circuits supplying the 4 USB ports.
If the question referenced above is correct, I would think that 1.2A supplied to the USB port would be enough for any Wi-Fi adapter, but perhaps there are some other factors in play here? The supply for the Pi itself is adequate, with a 2.5 A rating.
I have a USB power meter, and inserted the adapter into it, and then into the powered hub. The current is a steady 130mA, at a voltage of 5.17. Current did not rise when the AP mode was activated in the 5 GHz band. (Understanding that the accuracy of these meters is limited, and they don't measure peaks)
Inserting the adapter and power meter directly into the Pi3 USB port, the current reads 120 to 130 (reading varies between 0.12 and 0.13). Voltage is 5.04.
Does anyone have any knowledge or insights regarding the USB power control circuits or limits on the Pi 3, and how they could affect a Wi-Fi adapter?