USB modem causes system reset when no signal

[dmt]

Brief description:
When there is no proper 3G signal for my 3G modem, the system resets when I do usb_modeswitch. The modem is usually interrogated at boot time, so when there is no signal, the system goes into a constant reboot cycle until a 3G signal becomes available.

Long description:
From what I can gather, there seems to be a problem with the dwc_otg usb driver. It seems that some patches have already been applied to the driver, but that some issues may still persist. My RPi is fine when there is a good 3G signal or when I remove the SIM card from the modem. But, if the SIM card is inserted and there is no 3G signal, my RPi resets as soon as I try to switch the modem or the system becomes non responsive. This problem appears with both ArchLinux and Raspbian. Both distro's were up to date with the latest packages on 19-06-2014.

I do not know if this is a know error or if a patch is under way. Slowing down the USB speed down tot USB1.1 is not an option because I need to add an HD webcam later on. If anyone can shed some light on this problem it would be greatly appreciated.

Hardware setup:
---------------------------------------
RaspberryPi Model-B Rev.2 (I have two boards, same problem)
4GB Class10 SD card with ArchLinux updated with latest packages (19-06-2014)
Ethernet cable connected
Huawei E173 USB 3G modem connected directly to top USB port
Targus 4-port USB hub connected to bottom USB port
TRENDnet TEW-648UBM WiFi adapter (RTL8188CUS chipset) connected to USB hub (not connected to a network)
Dell USB Keyboard connected to USB Hub
Raspicam connected
HDMI Monitor.
DS3231 realtime clock on i2c-1
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A note on the power supply:
The RPi is powered from a 25W, 5V power supply that is connected directly to the pin2 (+5V) and pin6 (gnd) on the pin header. I have bridged a wire directly from pin2 (the +5V pin) to the +5V pins of the USB socket. I also placed a 330uF capacitor between the +5V pins and ground at the bottom of the USB socket to make sure that this is not power related. The RPi and USB port therefore has a stiff 5V power supply. I therefore regard this problem as software related.

A note on the 3G modem:
The 3G modem works fine on my PC running ubuntu as well as window 7. Where there is no 3G signal, it simply does not connect and the system runs normally.

[dmt]

[SOLVED]
Get a separately powered USB hub and run it off it's own power supply. If the hub and and RPi runs from the same 5V supply, add a stiffening capacitor in the order of 6800uF (bigger is better). If you do not have/want a separately powered USB hub, solder the 6800uF capacitor to the bottom of the USB power socket (between +5V and ground) and power the RPi directly from the header pins and not from the micro-USB power port. WARNING!: There is no fuse on the header pins so do this only if you know what you are doing.