I just read through question AVR Random Number Generator and encountered a number of ways to generate random seeds in an AVR:
- Use a special-purpose "Secure AVR"
- Use an internal temperature sensor
- Read unwritten EEPROM
- Measure time intervals between user input
- Use a floating ADC pin.
Why not just a single digital pin, configured as input without pull-up and floating? In theory that should generate a stream of random bits. Why not use this? Is the state too slowly changing? Does it tend to stuck at 1 or 0? Any other problems?