Mega2560 - No USB, TX always half lit, stored sketch runs ok. Is it fixable?


Following on from topic I have a self-inflicted problem as follows.

Never done this before, but ... after running the board with a sketch powered by USB I turned off the PC. Then I decided to power up the board with a 12V wallwart I usually use (it checks ok). There was a puff of smoke and the AMS1117 5V reg oozed lava so I disconnected. Now it won't power with 12V with USB disconnected but will power ok via USB and the resident sketch runs ok but no USB comms to PC. Whilst connected via USB RX flashes regularly but TX remains on all the time and appears to be half bright compared to RX.

I there anything easy for a beginner to do to diagnose whether the USB is dead or repairable. As an alternative, can I simply use the ICSP port instead and what would I need to do to get it going?

I have ordered another board (this one is a chinese board as will be the replacement as they are about £5-6), but it would be nice to keep this board working if possible.

Thanks for any help that's given.

I think you trashed the USB serial adapter. It is probably not replaceable at home unless you have hot-air rework capability. You could cut the TX and RX traces, and the one from the cap to reset, and then connect TX, RX to RX, TX of serial adapter, and reset to one side of a 0.1uf cap, and the other side to the DTR line on said serial adapter, and that should work to program it.

Thanks for prompt reply.

Errr! sounds like I need a brain surgeon. However, I did manage to replace the regulator so I think I'll give it a go.

BTW, where do you get the knowledge from ... I could do with buying a bucket or 2. :)

On the subject of the ICSP, can that be used to upload programs to the board just like the USB port (I did by a gizmo and converter lead when I bought the board 'cos in ignorance, i thought that was the method of programming).

Thanks again for the help.

p.s. since replacing the ams1117 the board runs ok at 12V and behaves the same but the TX light flashes from half-bright to bright. Similar but different and still no USB comms so guess you're right about the driver chip.

DaveJJ, you have a number of options regarding performing the brain surgery on your Mega, and gaining the knowledge to do it will just take buckets of reading. You can do it! You might start with checking if your library has books on Arduino, and if they do, check them out and start reading. I don't have footnotes or references like a proper research paper, but here is a glob of stuff I've learned:

Once you know the terms surrounding a particular subject of Arduino, it should make it easier to dig more deeply and research a particular subject area for more details.

Some things specific to the Mega2560 you may find useful:

The Arduino brand Mega2560 uses an ATmega16U2 for its USB to serial chip, as do some other brands, derivatives, or counterfeits. Yours may have ATmega16U2 or may have some other chip, such as a CH340G. It would be good for you to become familiar with which one you have. If you have one with a ATmega16U2 chip, you can use the reference schematic as a guide:

If you have one without the ATmega16U2 USB-to-serial chip, you can probably still use that schematic as a general guide, but you are somewhat on your own when it comes to the USB-to-serial section of the board, and probably also on your own regarding figuring out the power supply section. You may already know this stuff, so sorry if I'm giving redundant info.

You may be able to just cut the reset enable jumper pad, and program the Mega with an external FTDI or FTDI-like adapter and 0.1uF capacitor as DrAzzy suggests. That may be the easiest way to continue using the board. You may or may not need to disconnect the RX and TX lines between the USB-to-serial chip and ATmega2560. If you do end up wanting to do that, the easiest way in my opinion would be to get out an ohm meter and find the 1K resistors which connect the traces between the USB-to-serial chip and ATmega2560, and use a soldering iron to just remove them. That will break the connection.

Depending on what ICSP programmer you have, it may or may not be able to successfully program an ATmega2560 through the ICSP connector. USBtiny and USBasp programmers may not be able to program it. If you read through posts in the Microcontrollers section of the forum you can find a lot of discussion about getting ICSP programmers, especially the USBasp, to work with the ATmega2560, and there is quite a bit of back and forth discussion about memory size limits, firmware upgrades for the programmer, upgrading to avrdude 6.1, fuse bytes for boot reset vector, and so forth. It will be enough to give you a brain workout (or headache). Get caffeinated before reading that stuff.


Thanks for my bucketful of info. I had intended asking DrAzzy where the TX and RX connections were that he was suggesting I connected the board TX/RX traces to. All is much clearer.

I have started to get together the references you have included, but in the meantime I have ordered some CH340G chips (yes it's a Chinese clone) and will attempt to do some brain surgery. If that fails I will follow DrAzzy's suggestion and hook it to a USB serial adapter that I use to program my ESP8266 boards. I figured that cutting all the legs of the defunct CH340G chip first will allow me to suck the pads free of bits ready to accept the new chip with less risk of tearing the pads off the board. If I fail, then as I say, there's still DrAzzy's advice.

In the meantime, it looks like I've got a little bit of swotting to do.

Thanks again, I'm sure I'll be back.

OH! It's a cheap-340-based board? That's much easier to replace the chip on than the official boards, or the ones with the converters in some TSSOP package. Just be careful not to munge the pads, and you may be able to get it back to life.


What do you mean .. Cheap :o

Seriously, I have managed to get the chip off and looking at the "Real" Mega2560, you are right ... it would have been very difficult to remove that chip by snipping legs and cleaning up. One pad has lifted a bit but I hope it will be fixed when the new chip is sat on top (5 for £2.99 is a bargain and some left over for other deserving causes). I'll let you know how it went when the new chips arrive. No further contact will probably mean embarrassment :blush:

Thanks again

Something useful for SMD rework is Chip Quik. It is fun to use and makes removal of components pretty effortless. There are videos on Youtube showing Chip Quik in action.

Well, the chips arrived this morning. I can hardly believe they were that fast especially at Christmas (ordered 23rd Dec at noon arrived 24th first post, soldered in before 11am) . They were free economy post but came with 1st class stamp, I'll keep the vendor in mind.

I was surprised how fast the chip was to solder in but positioning was tricky as the residual solder was a gentle rounded layer and the legs preferred to be one side or the other >:( . Wish I had known about ChipQuik before!

Thanks to both and the forum for being so helpful.

Have a good Christmas holiday.