I was messing around with the board Sunday night and I managed to fry a board! Fortunately, I was then able to repeat the sequence and fry another two boards!
Forbidden operation:
-Remove the 5V DC power connector while keeping the USB peripheral connector (J6) connected to a non-conforming powered USB hub.
Result:
-USB port of the Parallella board gets fried if left on long enough.
Why it happens:
(this is based on the last 24 hrs, so feedback is welcome)
-When the 5V DC connector is removed, the SYS_5P0V signal drops to 3.9V which is just enough to allow the Intersil regulators to stay up.
-Since the rails stay up there is no reset triggered and the EN signal for the bidirectional switch (U13) stays up allowing significant current to flow back from the VBUS to SYS_5P0V keeping the circuit powered.
-The U13 circuit and the rest of this path was not designed to draw 1A+, so eventually the circuit gets fried.
Potential Immediate Workarounds:
RECOMMENDED:
-Connect a powered USB hub and Parallella to a common power strip to power both devices on/off at the same time.(tested)
OTHER OPTIONS:
-Only use un-powered USB hubs(tested)
-Use a conforming USB powered hub that prevents power flowing back to the Parallella board(untested)
-Modify the USB cable to the powered hub to remove the VBUS line(untested)
-Modify a USB cable to put a low forward-voltage-drop diode on USB VBUS line, preventing current from flowing back.(untested)
Long Term Fixes:
-We are working on a bullet proof board fix for to be implemented as soon as possible but we will not hold up production.
-Parallella Kickstarter backers will need to use one of the workarounds.
Any feedback regarding the severity of this issue would be much appreciated, especially if you have experience with similar issues and workarounds from other projects.
AndreasStatistics: Posted by aolofsson — Tue Dec 24, 2013 12:55 pm
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