USB modem power issues
Since some months I have a 3G USB dongle from Huawei connected to my
Raspberry Pi. Together with Asterisk
it acts as GSM gateway using chan_dongle. A couple of USB devices
were already connected to the RPi: 2.5" HDD, WLAN stick, DVB-S
To supply all this and to power the Pi I energize a USB hub by a
Goobay 5 Volts/3 Amperes power supply. Since I had connected the USB
modem, the Raspberry Pi was in trouble: USB dropped out entirely a
few seconds or several hours after reboot and sometime the Pi
crashed and was not even accessible by serial console anymore. I
have tried two HUAWEI internet sticks: E160 and E1550. Also I have
tried three different USB hubs but nothing worked. If the RPi
survived the USB crash, I could collect a bunch of error messages
from kern.log (the messages differed from time to time, hub to hub,
stick to stick):
raspbian kernel: smsc95xx 1-1.1:1.0 eth0: Failed to read reg index
raspbian kernel: NYET/NAK/ACK/other in non-error case, 0x00000002
raspbian kernel: option1 ttyUSB0: option_instat_callback: error -71
raspbian kernel: smsc95xx 1-1.1:1.0 eth0: unexpected urb length 0
raspbian kernel: ERROR::handle_hc_chhltd_intr_dma:2591:
handle_hc_chhltd_intr_dma: Channel 2, DMA Mode -- ChHltd set, but
reason for halting is unknown, hcint 0x00000012, intsts 0x04000001
raspbian kernel: hub 1-0:1.0: port 1 disabled by hub (EMI?),
raspbian kernel: usb 1-1: USB disconnect, device number 2
raspbian kernel: usb 1-1.1: USB disconnect, device number 3
raspbian kernel: smsc95xx 1-1.1:1.0 eth0: unregister 'smsc95xx'
usb-bcm2708_usb-1.1, smsc95xx USB 2.0 Ethernet
raspbian kernel: Unable to handle kernel NULL pointer dereference at
virtual address 00000034
...followed by a lot of messages by device drivers complaining that
USB hardware disappeared.
Due to the 'EMI' message I connected the USB modem with a 5 meter
active extension cable but nothing changed.
Based on many power issues with USB on Raspberry Pi reported around
the world, I added one 1000 µF capacitor directly to the HUAWEI
stick to stabilize voltage during high current peaks. This was
advised in several USB modem threads ... but nothing changed.
Then I checked the voltage (better late than never): Only 4.86 Volts
arrived at the UMTS stick! So I grabbed an old wall power plug with
USB port, labeled with 5.1 Volts/1 Ampere, and divided power and
data path (see picture). To improve stability I added also 2 x
1000µF low-ESR capacitors (that might be overkill).
That's it! Now the system runs stable since weeks. Looks like the
power issues reported with USB modems are mainly caused by voltages
below 5 Volt and not by insufficient power supplies (even if small
power supplies causes volatage drops during current peaks). The USB
specification allows 4.75 - 5.25 Volts but my sticks are not capable
to deal with the lower end.
Note that in the picture all connections share a common ground. Even
if USB uses differential signaling on data lines, there is some
communication relatively to ground, too. It does not work if ground
is not connected at the upper cable. There is no need to
interconnect the shieldings, so you should not do it until you have
a good reason.
And do not use a long USB cable between Raspberry Pi and modem. Even
with a high quality cable I got USB disconnects every few seconds
when using a 5 meter long cable. This cable worked fine with my
notebook but not with the Pi. With a 3 meter cable it runs stable.