I am working on a pcb that has an on board AVR microcontroller. I am providing a 6 pin programming header for the chip but I want to be able to program the chip without powering the rest of the board. What is the normal setup for something like this? I am thinking I can throw a Zener Diode on the power line to prevent power to the chip from flowing back onto the board and the rest of the components. During normal operation it is just powered from the power plane. Is this a good setup?

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    \$\begingroup\$ 1-position DIP switches can work wonders. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 11 '13 at 2:40
  • \$\begingroup\$ @IgnacioVazquez-Abrams - make this an answer. It is better than the provided. \$\endgroup\$ – johnfound Oct 11 '13 at 4:47

You could use a Diode OR to select the highest voltage supply. You will want to use a schottky diodes as they drop less voltage then regular silicon diodes.


simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ @Dabloons - You should be aware that you will have some drop across the diodes (0.6V for silicon, 0.3-0.2V for schottky). If you're close to the minimum allowable voltage to run the AVR, this could cause issues. \$\endgroup\$ – Connor Wolf Oct 11 '13 at 5:25
  • \$\begingroup\$ @ConnorWolf - Very true. It makes more sense in this case to just disconnect the rest of the circuit by way of a 2-pin jumper or a 1-position dip switch. However, both of those require the user to remember to something whereas the supplied answer is "automatic." Pros and cons... I guess it just depends on the user. \$\endgroup\$ – Kurt E. Clothier Oct 11 '13 at 21:10

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