Problem with the ATTiny25 and Spence Konde core

OK, I am experiencing a problem that is, no doubt, simply do to my own ignorance. I am pretty experienced with Arduinos but this is my first foray into ATTiny-land. I have a sketch that runs fine on a Leonardo, but has a different output if run on the ATTiny.

int raw_adc = 128;
void setup() { 
Serial.begin(9600);
//  pinMode(3, INPUT);
} 

void loop() { 
//  raw_adc = analogRead(3);
  Serial.print(raw_adc);        //This line works on an Arduino, but on the ATTiny25 prints a bunch of 8's and spaces!
//  Serial.print("Hello World");     //this line works fine  
  Serial.print(",");
  delay(25);
}

The purpose of the code is to read a voltage and send the value via serial port. I have commented out the analogRead parts to troubleshoot. When run on the Leonardo it outputs 128,128,128,128,128... as expected. On the ATTiny I get a bunch of 8's and blanks! It would seem that there is something different about how Serial is handled in Spence Konde's core but I just can't figure it out.

Any help is much appreciated!

Mysterious. I will investigate.

"Serial" on a tiny x5 with my core (well, actually any serial implementation for tiny x5) is a software implementation, and my cores use a special version that uses the analog comparator interrupt instead of a PCINT to improve library compatibility.

DrAzzy, sorry didn't mean to cross post. Just wanted to see if anyone here had experienced like problems or could point out some obvious mistake on my part! :-)

AAHA! Here's that issue I made the board up to test a while back... I knew it was somewhere but wasn't able to locate it when I had a chance to test the issue.

This is why issues should always be in github... otherwise it gets lost.

Issue created to track this bug report. https://github.com/SpenceKonde/ATTinyCore/issues/186

Cannot reproduce, code above works fine on my ATTiny85, and there are no relevant differences between Tiny85 and Tiny45 here.

I suspect the issue is down to the factory calibration of the internal 8mhz (or what passes for it) - it is not guaranteed to be accurate enough for UART communication. On most individual chips it is, at normal voltages and at room temperature, but this isn't guaranteed. You can user-calibrate it (sorry, no resources available to help - this is on my list but haven't had time), but that's a pain in the ass. This is why I try to always use a crystal when I'm relying on UART working (though at 3.3v the tiny841, 1634R and 828R have the oscillator calibrated precisely enough for that.