Discussion
Why can a plain 555 astable not produce an exactly symmetrical square wave?
Answer: B
Charging goes through both resistors while discharging goes through only the second, so the high time always exceeds the low time and the duty cycle cannot reach a half. The thresholds at one third and two thirds of the supply are symmetrical, and the output stage is not the issue. The standard fix is a diode across the second resistor so that charging bypasses the first, which makes the two paths equal - a good example of solving an asymmetry by giving each direction its own path.
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