Building a stand alone ATmega2560 circuit DIY PCB

I'm basically trying to build my own Arduino Mega with as many shortcuts as possible.
I've had success building the basic circuit, but I had to use a Nano as ISP to upload sketches onto it. My goal is to have some kind of UART to serial circuit i could easily build next to the 2560 chip, so i can have it all on one pcb, where i only have to plug a USB cable into it to program it using Arduino IDE, like a Mega.

i've tried the FT232RL chip, i managed to get it to upload sketches onto a stand alone ATmega328 chip no problem, but not the 2560. I ended up frying my 2560chip and i'm just wondering if it's worth buying a new one.

I've not been able to find much online about how to upload sketches without using a Nano. Does anyone here know of any existing circuits that work? or any other helpful advice? sorry i don't have much info to go on.

So, basically, you want to build an Arduino Mega that programs like an Arduino Mega. Did you consider just buying an Arduino Mega? Or is this some sort of learning exercise?

As long as you have the SPI pins brought out, and nothing ‘hinky’ wired up to the other pins, it should be fine.

I have a complete working board with the ISP / SOI pins on a header, and it just works.

Yeah Yeah! Learning exercise, i want to be able to build a pcb with a Mega in the middle of it.

Then use the reference schematic for the Mega and USB chip.

I'm using a Arduino Mega r3 on a custom baseboard... And I can see some reasons to put a 2561 and a serial/usb directly on my board instead of using a mega. And cost is not even a real point here, jst an added bonus.

  1. Routing of the analog lines is.. let's say:"could be better" (crossing digital lines, no propper ground routing, AGND as opposite side of the board and not even directly routed to the AGND of the chip and Aref is also on the opposite side of the board)
  2. If you use an own 5V supply you are connecting two 5V supplies as soon as you plug in USB
  3. Space
  4. Access to MOSI/MISO

There is probably more. That said: the arduino is wonderful! It's doing exactly what it is made for! An easy tool for education. Absolutely great. But at some point you grow out of it and instead of bickering over the shortcomings (like I did above) we can (thanks to the open source nature) build our own custom boards and still use the tools. That's awesome!

So yes, there are good reasons, to spin your own board as soon as you start doing some more advanced stuff. That's just always the case because you almost never have a "one size fits all" situation. And arduino manages to get pretty far in the right direction to "one size covers an amazing amount of cases".

I just looked up the ref schematic and they don't even use a FT232RL chip ..??? There are versions with a Atmega16u2, or ch340g as the USB to UART-serial. I don't know why i was even thinking that it was FT232RL?? Apparently the Atmega16u2 is a bit better/easier. I can even use an UNO to burn the firmware onto the chip.

I'm gonna try that, i'll let you guys know how it goes!! thanks for all your help guys!!

It's not exactly obvious but I think the reason was to avoid using the FTDI chip. The same applied to the UNO and the Mega 2560. At one point the 8U2 (later 16U2) was cheaper, then the CP2102 and later CH340 (keep forgetting the other chip) became much cheaper than the ATMEL chips so that's what all the clones use.

What is better about the 16U2 is that it is re-programmable.

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