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”
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”.