[avr-chat] Help: PWM output pin at half Vcc, no load, with other firmware OK

[Rick Mann]

We've suddenly got this bizarre behavior. We've got an ATmega128 that  
controls our robot, and drives a brushless motor. It's been working  
very well, but suddenly there's a strange roughness in the BLDC at  
low speeds.

However, a previous version of the code works fine.

The only difference we've been able to detect is that one of the 3  
PWM outputs is going from 0 to 2.5 V, rather than 0 to 5V (the other  
two go from 0 to 5V). I've disconnected the PWMs from the half  
bridges to ensure that the pin isn't being brought down by an  
excessive load, but nothing changes. My first thought was that the  
'128 was damaged, but older firmware doesn't exhibit this behavior.

Since the PWM outputs drive an IR MGD, I don't think the low output  
is causing the problem, but it may be a symptom of the problem.

What (firmware) could cause an unloaded PWM output pin to drive at  
2.5 V instead of 5?

TIA,

--
Rick