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reverse supply through output

Question asked by Tamas Novak on Dec 8, 2015
Latest reply on Dec 9, 2015 by John F.
My STM32F030 is SPI slave to another external MCU.  External host may turn the VCC of my STM32  off.  When this happens SPI_CLK and MOSI pins are still at +3.3V, and these signals back-supply my MCU through outputs' limiting diodes:   STM32's VCC rail goes up to abt +2V and weird things happen. Probably STM32 starts to boot, GPIO pins get initialised, what requires more power, so VCC drops below PDR level.
PDR pulls down RST, and  this oscillates..

Can you see a way to disable this occasional back-powering of STM32 by external pins at
+3.3V ?