Various sources I've read including AVR122 indicate typical uncalibrated accuracy of +-10C. I just tested an ATtiny85, and am getting raw readings of 254 at room temperature (~20C). Is it just bad luck, or is +-10C just the average accuracy, with +-40C outliers still being somewhat common?
Here's the guts of my code:
unsigned int temperature; ADMUX |= ((1<<REFS1) | 0x0f); // select temperature sensor // turn on ADC with prescaler of /128 ADCSRA = ((1<<ADEN)|(1<<ADSC)|(1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0)); while (ADCSRA & (1<<ADSC)); // wait for conversion to finish temperature = ADCW;