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Raspberry Pi USB Hotplugging

19 08 2014   //   Written by Yannick   //   Classified in : HAB  //  Tags : USB, HotPlugging, capacitor

The payload had a backup transmission using a USB 3G/SMS dongle (Second hand / 10€) to send SMS with coordinates in case of failure of the NTX2.

As it is useless and waste of battery to power the USB dongle during all the flight, I used a script to power it (using a GPIO and a relay) only when the payload was back to the ground (altitude bellow 2 Km).

But when testing my script and the “hot plugging”, I noticed the Raspberry PI A failed (freeze / reboot) everytime the USB 3G/SMS dongle was plugged.
After some investigations, I found the folowing links explaining about the “inrush current” and the Voltage Droop”

http://theiopage.blogspot.com.es/2012/06/increasing-raspberry-pis-usb-host.html
http://thestuffsido.blogspot.co.uk/2013/05/fixing-raspberry-pi-hotplugging.html
http://therandomlab.blogspot.fr/2013/01/raspberry-pi-mod-to-avoid-shutdown-on.html

The USB.org whitepaper explain:

I n accordance to the USB Specification Revision 2.0, the VBUS power lines must be bypassed with no less than 120µF capacitance of low-ESR capacitance per USB port.

So I soldered a “Low ESR” 220uF and this fixed the issue \o/ !



Note: this problem was known by the RaspberryPi foundation as it seems to have been fixed with the new version B+ that have “better hotplug and overcurrent behaviour”.