logic gates simplest square wave generator
It will work if you use an inverting Schmitt trigger gate:







simulate this circuit &ndash; Schematic created using CircuitLab

(Our circuit editor doesn't seem to support Unicode to full extent).

There are practical circuits based on the same principle which use a quartz crystal or an RC circuit to define the desired frequency, rather than relying on delays in logic gates which change from chip to chip and even for the same chip in different conditions like PSU voltage or temperature.