Multiple Arduinos seem to be bricked!

Today I came into my office to do some more work on a UNO project.
Plugged the USB cable in to the UNO. No COMx port was enumerated. I spent over an hour looking for USB problems but turned up nothing. I did everything but could not get a COMx port enumerated when I plugged the UNO in. Yes, I tried man other USB ports and cables.
Here's where it gets interested. I also have and Arduino MEGA. It's a known good board. It also fails to enumerate a COMx port when plugged in.
Next I tried my MKR1010 Arduino and it works
Just about any other my USB devices I could get my hands on, all would work as expected, just not my UNO and the MEGA.

One last item. The last sketch loaded on the the UNO still works. When I plug the USB cable in the LCD display the sketch was written for shows what it's supposed to.

Gonna stop short for now and see what the community has to say about my situation.

While waiting I'm gonna assume the Arduinos are bricked and the bootloaders are brain dead. So on your responses please recommend a couple of good programmer for reprogramming the bootloader. Hopefully not too expensive.

Thanks Al

One of the most universal programmers (in my and some others opinion) is the Pololu USB AVR Programmer v2.1. It's a programmer as well as a USB-to-TTL converter that can be used to program e.g. a ProMini.

If the COM ports are not recognised, it's more than likely not the main processor (328P / 2560) but possibly the USB-to-TTL converter; that is, if it's board related. So flashing the main processor will not help.

So are you suggesting that no matter what state the processor is in the board should always enumerate a COMx port when plugged into a USB port? I'm trying to understand your response.

I've ordered another UNO to use as an AVR programmer in hopes of recovering the MEGA and the UNO I have. Your response suggests this is an effort in futility.

Correct me if I'm wrong.

The main processor has indeed no function when it comes to connectivity on boards like your problematic ones. It's e.g. what the loopback test is based on; you keep the main processor in reset and run the test.

I think your MKR works differently, but I have no experience with them.

Any yellow triangles in Windows device manager? No unknown devices?

No yellow indicators in DM.
Just weeks earlier I was uploading sketches to the UNO just fine.
So to come in this AM to pick up where I left off and see no COMx port in the DM when plugged in when I've done nothing different is a bit bewildering.
I just need some way to determine whether this UNO and the MEGA are really toast. I find it really hard to believe both have failed in this way when neither have been subjected to anything that warrants this failure.

One more item. I just read up on how to do loopback test. It fails. This suggests the 16U2 chip needs to be reprogrammed I guess. I found the writeup on that also. I guess the 16U2 also has some basic code (bootloader) on it to perform the serial to USB functions.
Got any nuggets of wisdom on that also before I proceed?


You can't run a loopback tests if you don't have COM ports.

I've never investigated programming the 16U2. I suspect it's a command line exercise using avrdude. I've also heard of Atmel Flip to reprogram, but not sure if it applies to your current situation.

Question is what you did to your boards? What was all connected the last time. 16U2s usually seem to die due to abuse of power rails (12V).

If you don't need a full functioning board (meaning no USB functionality) you can upload your code via ICSP using the new Arduino you ordered or a dedicated programmer.

If you need USB functionality (e.g. for serial monitor), you can buy a TTL-to-USB converter and hook it up to the RX and TX pins of the board; it will basically replace the 16U2 (though your board will identify itself as a serial port and not an Arduino xx).

If you want to be able to also upload using the boot loader, that adapter needs one additional line for the reset when the upload starts; this is just an example robotdyn USB-Serial adapter that seems to provide that functionality.

Thanks for all the info.
However ... 12 volt rails ???
Does this board also have a DC to DC convertor to generate +/- 12v DC?


No, the power rails are 5V and GND. But accidentally putting 12V somewhere will kill the board.

Well I did NOT do that!


Update. It appears some of my trouble was the USB cable. Why it worked on the MKR1010 and not the UNO or the MEGA I can't say.

So with a new cable, the only Arduino I have trouble with is the UNO.

I received my new UNO today and set it up as a programmer to reload both bootloaders. I can reload the main 328 CPU but not the 16U2. To make a long story short the 16 mhz crystal to the 16U2 is not oscillating. I scoped it. Does this mean the crystal really is bad, or is it possible there is a 16U2 fuse config wrong that would prevent the crystal from oscillating.
Should that crystal oscillate no matter what the config is for the 16U2?
If it's the latter how do I fix it. How do I reconfig the 16U2 to change it's config?


I can't help you with this; but crystals (in my experience) don't fail unless you physically abuse them.

Well I can assure you this UNO board was NOT abused. I take care of my stuff.
Anyway, shortly after I posted the last message I thought I'd get my scope busy and probe around on the board some more.
I got the scope and UNO ready before powering up. Scope attached to crystal. Plugged the USB cable into the UNO and the crystal is oscillating!!!! AND I hear the familiar Windows beeps as it enumerates the COMx port for the UNO. I haven't heard that in weeks because I thought it was bad. For reasons I can't explain it just suddenly decided to start oscillating. ?????

I have nothing further to add to this mystery ????


I never implied that you abused your boards. To break a crystal, it needs brute force and that is usually intentionally.

Oh I know you were not going after me on the abuse thing. You just had to cover that base.
We're good.


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