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I have a custom board with an I2C sensor & FTDI + Arduino mini (powered from 3.3 Volt from FTDI IC which also powers the sensor).

Am not able to program via FTDI & also directly via a blank arduino UNO. But if I disconnect the arduino mini from the custom board I am able to program it (though with disfficulty, is reset an issue?)

Can some one help what might .

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  • \$\begingroup\$ You're going to have to be clearer about exactly what does and does not work. Does the FTDI device enumerate? Can you accomplish serial loopback though it? If you plug in a pre-programmed ATmega which generates serial output, do you get that? Also you really shouldn't use the FTDI chip's weak internal regulator to power other things. \$\endgroup\$ Commented Oct 7, 2020 at 19:43
  • \$\begingroup\$ Custom board based on Arduino: how is the initial firmware loaded? Arduino's come pre-loaded with bootloader firmware that allows new user firmware to be loaded through the TX/RX, but a truly blank ATmega328 needs to be programmed through its SPI pins during reset. Normally there is an in-circuit SPI programming header that connects MISO, VCC, SCLK, MOSI, RESET, and GND to an external programming board such as AVR-ISP or USBtinyISP. See arduino.cc/en/Hacking/Bootloader?from=Tutorial.Bootloader \$\endgroup\$
    – MarkU
    Commented Oct 7, 2020 at 22:48
  • \$\begingroup\$ @ChrisStratton - I can communicate with the FTDI. It works. BUt cannot program the microcontroller. It's loaded with bootloader already. If I disconnect the microcontroller circuit from Rest of the circuit then I am able to program the microcontroller. I want to program the microcontroller via FTDI preferrably plus why do I need t isolate it to program ? With a pre-programmed microcontroller for serial transmission: It doesn't flash the TX LED on FTDI nor there is data. For the regulator I agree : I thought it would save a few components and so used that. \$\endgroup\$
    – U7786
    Commented Oct 8, 2020 at 2:20
  • \$\begingroup\$ @jsotola - My bad. Apologies for that, will be careful next time. \$\endgroup\$
    – U7786
    Commented Oct 8, 2020 at 2:21
  • \$\begingroup\$ @MarkU - Bootloader is there on microcontroller. I forgot tto mention that. Thanks \$\endgroup\$
    – U7786
    Commented Oct 8, 2020 at 2:21

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The schematics are absolutely horrible to follow what wire goes where - but more importantly, what is not connected.

The AVR does not work when half of its power supply pins are left unconnected. They really must all be connected, all VCC pins including AVCC, and all GND pins.

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