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I bought Gboard module with Atmega328P inside: https://wiki.iteadstudio.com/Gboard

I don't want to use Arduino at all. I just want to upload my own code into Atmega328P and make a phone calls without external devices. The problem is GBoard works in 3,3V level but I want to upload my software with 5V USBasp (Barion copmany, V2 of USBasp). There is a jumper to work with USB 5V OR not to supply the device that is programmed. I want to supply my programmed Atmega328P with 5V so the jumper is on.

There are ISP pins available in GBoard but is it safe to connect my USBasp?

  1. Once I connect my USBasp there will be 5V in the output of GBoard ASM1117 voltage regulator (in the output pin; is reverse voltage safe?).
  2. Once I connect my USBasp there will be 5V in the LED with 1k resistor.
  3. Will it break my SIM900 module? In general there is no direct connection with power supply of SIM900. Could anyone advise?

Will it break my GBoard module? Can I safely connect USBasp via ISP pins with 5V? enter image description here enter image description here enter image description here

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I don't want to use Arduino at all. I just want to upload my own code into Atmega328P and make a phone calls without external devices.

Use "ISP1" in the page #2.

The problem is GBoard works in 3,3V level but I want to upload my software with 5V USBasp (Barion copmany, V2 of USBasp). There is a jumper to work with USB 5V OR not to supply the device that is programmed. I want to supply my programmed Atmega328P with 5V so the jumper is on.

It is an incorrect approach.

  1. Use 3V, if you have to choose one.
  2. However, I would prefer powering GBoard direct from own supply,the ISP pin#2 (VDD33) not connected.

There are ISP pins available in GBoard but is it safe to connect my USBasp?

Best case scenario: Yes, except pin#2 (the VDD pin).

  1. Once I connect my USBasp there will be 5V in the output of GBoard ASM1117 voltage regulator (in the output pin; is reverse voltage safe?).
  2. Once I connect my USBasp there will be 5V in the LED with 1k resistor.
  3. Will it break my SIM900 module? In general there is no direct connection with power supply of SIM900. Could anyone advise?
  1. Supplying 5V through ISP will definitely cook the board nicely, if you give enough current.

  2. LED will still survive.

  3. If you supply 3.3V through ISP (never 5V), Ti's LM1117 datasheet, page 14, "8.4.1 Protection Diodes", suggests something you need to do with the reversed direction of the supply voltage. From there, IC2 will try to take the reversed power (you need to study the datasheet).

Will it break my GBoard module? Can I safely connect USBasp via ISP pins with 5V? enter image description here

Yes, that should break your GBoard, and probably USBasp as well.

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  • \$\begingroup\$ Thank you for quick answer. To sum up I cannot supply Atmega328P from USBasp. "1. Use 3V, if you have to choose one. 2.However, I would prefer powering GBoard direct from own supply,the ISP pin#2 (VDD33) not connected." I can remove jumper so GBoard/Atmega328P will be supplied from it's own source (3.3V) as you suggested. USBasp will be supplied from USB 5V. Will it work fine? \$\endgroup\$
    – sticker
    Nov 7, 2021 at 15:42
  • \$\begingroup\$ I still feel like data pins (MISO, MOSI etc.) from USBasp will use 5V level. Can I supply Atmega from it's own source (3.3V) and send data from USBasp with 5V? Or the only possibility is to buy another USBAsp with 3.3V level support? \$\endgroup\$
    – sticker
    Nov 7, 2021 at 15:42
  • \$\begingroup\$ @sticker Yes, and yes. I understand your concerns. Power the GBoard direct from own supply. USBasp takes 5V from the USB host connection, while keep compatible signal with 3.3V on target. That is my impression from reading both GBoard & USBasp schematics. You should verify that with the one designed USBasp. ---- for the second question: I understand your concern. I exactly do question that when I plug debug headers first time. \$\endgroup\$
    – jay
    Nov 7, 2021 at 16:12
  • \$\begingroup\$ BTW, if you are worried about power sequencing, Try to power up GBoard first, However, I assume the USBasp should have enough consideration to handle the random powering sequence. \$\endgroup\$
    – jay
    Nov 7, 2021 at 16:19
  • \$\begingroup\$ thank you very much for help. It was very useful. \$\endgroup\$
    – sticker
    Nov 7, 2021 at 19:22

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