I made a standalone Mega (2560) board.
- It uses a 16 MHz resonator.
- Serial (RX-TX) pins are connected to a FTDI FT232RL, over 1K resistors.
- The FTDI chip is only powered when USB is connected.
- The FTDI DTR pin is connected to RESET through a 100nF.
- USB Ground is connected to GND.
- All SPI slaves are disconnected from the MISO, for now.
- Reset is pulled-up to 5V, as should be.
- RX pin is pulled-down to GND with 20k, just in case it gets garbage and confuses the bootloader.
- I can burn the bootloader, via Arduino ISP (A Duemilanove as ISP).
- I can upload a blink sketch using the Arduino ISP.
- I can’t upload a sketch using FTDI (FTDI is recognized by Windows 7).
So, the problem is, when I upload the sketch, it doesn’t run. Something stops the 2560 from starting execution.
2560 was a factory new chip. I am experimenting with different fuse options without success.