I used a 555 once in my life, to run a little charge pump to make some
>negative voltage from +12. I ran the output back to the timing cap
>through one resistor, avoiding the usual extra parts in the
>charge/discharge path to get a square wave... something that I thought
>was obvious but not on the datasheet. I still don't see that
>configuration very often.
--- It's fairly common, and Maxim shows it on page 6 of their 17 year old CMOS 555 data sheet at:
formatting link
It's not so common with bipolar 555s because its output doesn't swing symmetrically about Vcc and GND, so it's not easy to get a square wave out of it.
Plus, its output doesn't swing to the positive rail, so if Vcc gets low enough, the output won't swing high enough to get to 2Vcc/3, THRESHOLD will never be triggered, and the timing cap will never discharge.
JF