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I have a project and I need use ISP to send codes to ATMEGA328P, the problem is that my ISP programmer works with 5V and my system works with 3.3V. I know ATMEGA328P supports 5V but other components in my system (like a radio) not, and I like run it in 8MHz .

I used a LM1117-3.3 to reduce ISP 5V to 3.3V but I have a problem when power the system with external 3.3V, the regulator is in short because its VOUT is getting 3.3V. to solve it just put a diode as shown in the picture below, but the diode has a 0.7V voltage drop which forces my circuit to work with 2.6V, this is causing problems on my radio.

Does anyone have any suggestions to solve this? I did not want use more jumpers ....

My circuit

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  • \$\begingroup\$ If you don't use +5V, why don't you simply disconnect that pin and power the system with 3.3V? \$\endgroup\$ Jul 4 '16 at 14:07
  • \$\begingroup\$ During the prototype stage I would like to feed the project by USB (ICSP) instead of batteries. But if anyone suggest something I will remove the 5V and use batteries even during upload \$\endgroup\$ Jul 4 '16 at 14:12
  • \$\begingroup\$ @Dmitry Grigoryev - his ISP(ICSP) uses 5V. \$\endgroup\$
    – zeffur
    Jul 4 '16 at 14:15
  • \$\begingroup\$ @Renato Tavares - can't you connect your ext 3.3V supply at the output of your LM1117 chip to bypass it? You don't have to use a diode there, do you? \$\endgroup\$
    – zeffur
    Jul 4 '16 at 14:17
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    \$\begingroup\$ ultra low VF Schottky like PMEG1020EA ~ 150 mV voltage drop at 100 mA \$\endgroup\$
    – matzeri
    Jul 4 '16 at 14:21
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It's a little unclear to me why you can't just feed 3.3 volts into the point you indicate, so this is a long shot. The traditional way to prevent problems where the output of a regulator is higher than the input is to connect a diode like this:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Specifically a schottky diode so that its voltage drop is lower than that of the parasitic diode. \$\endgroup\$ Jul 4 '16 at 15:07
  • \$\begingroup\$ @IgnacioVazquez-Abrams I didn't think they existed in CircuitLab, but I managed to figure out how to make one, thanks! \$\endgroup\$
    – pipe
    Jul 4 '16 at 15:13

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