Weird serial behavior on atmega2560 EDIT
This seems to be something to do with the oscillators. Switching to the internal oscillator and writing straight avr-c allows me to run a basic blink program. Writing the same basic blink in arduino and flashing with the IDE (through serial or usbasp) results in a non functional program. I'd like to be able to use the oscillator because it doesn't look like I can get everything done in time at 8MHZ
I have an arduino mega 2560 which a few weeks ago was working fine. Now, I'm trying to output some data from it, and 've broken it down to a very, very basic proof of concept:
void setup() {
// put your setup code here, to run once:
  Serial.begin(9600);
}

void loop() {
   // put your main code here, to run repeatedly:
   Serial.println(&quot;hello&quot;);
}

This prints:
hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh

indefinitely on the screen. If I change the baud to 115200, I get the &quot;hello&quot; and the newline, and slowly start getting garbage afterwards. If I switch to 234500, I get a few prints, and then nothing.
Adding a delay after the print does not change the frequency of the serial prints:
void loop() {
// put your main code here, to run repeatedly:
    Serial.println(&quot;hello&quot;);
    delay(100000);
}

They still come in immediately.
I have a feeling this has something to do with timing of an interrupt, but I'm not setting anything up. I plan to reflash and fix the fuses, but my usbASP cable is broken right now. Any other input on what might be going on here?
Edit:
This occurs on two different boards. I originally had some i2c devices (mag/gryo/barometer) on board, but those have since been removed. I have tried two different usb cables in two different USB ports. This might be on the host side?
Edit:
Both the TX and Rx lights are solid this whole time. Almost as if there's a loopback. My other laptop won't even boot right now. I'm concerned a surge from a storm last night might have taken out both of these machines
More testing:
Looking at the output on a scope, the board is actually only outputting an &quot;h&quot; character. If I do:
Serial.println('U');
delay(1000);

I get a square wave, so the delay doesn't run at all.
If I do:
Serial.println('U');
Serial.println('U'); //or any other character
delay(1000);
I get a U character over the scope, and then an empty serial frame. The delay still doesn't run, but it looks like the second println is exiting.
Even burning a new bootloader fails on these boards
Answering my own question - a bit of a walk of shame here.
I had disabled the internal TIMER0_OVF_vect in the arduino core code, so that I could write my own (for another project). I wrapped in in an #if 0 so I could go and bring it back when I needed it.
Today, I learned that the atmega interrupts aren't filled with an IRET by default, but instead hit the reset vector, resetting the board. Anything I compiled on this library would reset whenever TIMER0 overflowed, because my ifdefs didn't cover the TIMSK setting in the arduino core either
The &quot;Serial.println isn't exiting&quot; was actually &quot;the board is resetting before it can complete&quot; and explains why faster bauds could print more data.
Not much smarter than hairless apes, sometimes.