Assuming that it is used with a 10-MHz counter/timer, its resolution is roughly 10 µV per count ... or by U1 itself. Using a 555 in this self-resetting mode is unusual but is very handy here. This ...

Here’s an interesting implementation of a classic: the 555 timer as astable multivibrator for the noble purpose of making weird music. [pratchel] calls this a Morgenflöte or morning flute ...