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Few days ago, while making my ic burner, I notice one thing very interesting and weird. It's that AVR MCU ISP interface really doesn't need to be connected to ground at all. Why does AVR-ISP work without being connected to the ground?

I have tested this with few same package size avr MCU: namely the atmeaga328, 48v, 88v, 88pa and 168. They all work fine without connected to the GND.

My personal guess would be that it's probably because the avr is grounded via the MISO or the MOSI pins during the ISP session, since those 2 I/O pins can be programmed to connected to VCC or Gnd.

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closed as unclear what you're asking by Rev1.0, pipe, Leon Heller, Lior Bilia, Harry Svensson Nov 27 '17 at 13:32

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    \$\begingroup\$ Show us your actual schematic, this photo is completely useless - it doesn't show anything that can aid in answering your question. \$\endgroup\$ – pipe Nov 27 '17 at 8:14
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    \$\begingroup\$ By GND do you mean the pad on the bottom of the device? (I'm assuming based on your socket you are programming QFN devices). \$\endgroup\$ – Tom Carpenter Nov 27 '17 at 8:47
  • \$\begingroup\$ @Tom Carpenter by gnd, I mean the gnd on the 6 pins icsp header totally disconnected! I have a button to the Gnd, and when it's disconnected, the isp function of the avr still works perfectly!! \$\endgroup\$ – Atmega 328 Nov 27 '17 at 9:35
  • \$\begingroup\$ @pipe It's a basic AVR ISP circuit with GND disconnected. You don't need a schematic, and I built it without using a schematic too. it's that simple. \$\endgroup\$ – Atmega 328 Nov 27 '17 at 9:37
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    \$\begingroup\$ Expanding on pjc50's comment below regarding "phantom supply": even if you do not connect ground and/or supply a chip can still work, it then powers through the ESD protection diodes see this EEVBlog video for an explanation: youtube.com/watch?v=2yFh7Vv0Paw&t=1s \$\endgroup\$ – Bimpelrekkie Nov 27 '17 at 10:51
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Probably you have a sneak ground through the earth in your circuit and the USB connector on your computer to earth.

Edit: Alternately, AC signals may be capacitively coupled and DC-restored via the clamp diodes on CMOS inputs, but it's obviously not advisable to operate in this mode in most cases.

As pointed out in the comments, any pin which is low will act to bring the ground potential within a diode drop of where it should be. RESET often doesn't have a series resistor, which makes it a good candidate.

In any case, passing current deliberately through the protection diodes is bad and can lead to latchup.

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  • \$\begingroup\$ I am 100% sure that it's not connected to the ground! \$\endgroup\$ – Atmega 328 Nov 27 '17 at 7:51
  • \$\begingroup\$ I don't think so. my guess: it's probably because the avr is grounded via the MISO or the MOSI pins during the ISP session, since those 2 I/O pins can be programmed to connected to VCC or Gnd. That will be my guess, but I don't really know. \$\endgroup\$ – Atmega 328 Nov 27 '17 at 9:44
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    \$\begingroup\$ Another candidate is RESET, which I think is held low during programming. You're almost certainly experiencing current flow through the protection diodes. See "phantom power" \$\endgroup\$ – pjc50 Nov 27 '17 at 10:25
  • \$\begingroup\$ @pjc50 of course. it only has 6 possibilities that it's connected to the Gnd, and the last possibility would be that it's doesn't need to be connected to the Gnd, which is probably less likely. The million dollar question would be which one is correct and why!? \$\endgroup\$ – Atmega 328 Nov 28 '17 at 8:46
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    \$\begingroup\$ Providing a schematic would have been a good start. \$\endgroup\$ – Spehro Pefhany Nov 28 '17 at 8:50

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