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Siemens' DSCC-4 electrical addendum reached me when the boards were already produced. It says, that the 5V power line voltage may never be above the voltage on the 3.3V power line. Due to the big tantal capacitor at the output of the voltage regulator which takes some time to discharge at power off, latchup condition would have been fulfilled. This Schottky diode was added to discharge the cap via the 5V bus on power shutdown.

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