Arduino ATMEGA368U powering over SCK (pin 19)?

Very rare case, I wonder if anyone could give me some insight...

Scenario #1: I have a Arduino Atmega368u chip. I power the chip over pin 7 and 20, for a few seconds, then I remove power but it stays on. I discovered that while I had 5.0v on pin 19 (SCK), it would remain on, but when I remove pin 19 (float) it will power down like it should.

Scenario #2: So during further research I discovered that if I power the Atmega368u only from pin 7 (pin 20 float) the chip will act just fine, power on and off as it should.

I notice too that the chip will return voltage over the power pins on scenario #1, 2.8v to be exact.

During scenario #2 I have 3.7v on power pins but the chip does power down fine. Confusing!

The Arduino is connected to a 7 Segment Common Cathode on pins 5, 6, 11, 12, 13, 14 and 17 which give these pins 3.7v at all times.

The issue is solved by disconnecting pin 20, yet I wonder why it was required to use Pin 20 for power if it works without it.

Any insights?

Scenario #1: I have a Arduino Atmega368u chip. I power the chip over pin 7 and 20, for a few seconds, then I remove power but it stays on. I discovered that while I had 5.0v on pin 19 (SCK), it would remain on, but when I remove pin 19 (float) it will power down like it should.

The Atmel datasheet for the AtMega specifies that the maximum voltage on any i/o pin is Vcc + 0.3V. When you remove Vcc, you violate this requirement and so the behaviour is undefined. It is also potentially harmful to the AtMega.

Best practice is to power the mcu and all peripheral cicuitry from the same power source if possible. It not, you should add circuitry to prevent scenarios such as you describe.

Atmel also states that the ADC internal circuitry is powered from a separate pin for better noise immunity. Again they do not offer an explanation to what will happend if we don’t power it at all, but I for one is willing to take their advice as is.

This sort of thing is a fault called parasitic powering or back powering. It is a bad thing to do and can damage the chip. It is sometimes a problem when you are trying to make very lower power systems in sleep mode. You should never apply any signal to a chip that has not got power attched to it. At best it can cause latch up and worst burn out.

Now I get it!

Thank You!