Oscillator circuit frequency error
My very old CMOS documentation for this oscillator recommends a slightly different circuit, with an added resistor:



simulate this circuit &ndash; Schematic created using CircuitLab
The extra resistor R1 prevents any ESD diodes inside NOT1's input from clipping the RC waveform, whose voltage momentarily exceeds Vdd, and also extends below ground. With R1=0, the oscillator still works, but at a slightly higher frequency, because of the clipping caused by the protection network.
Out of curiosity, a simulation was run on the following LTSpice circuit, using a Philips 2005 model for HCMOS buffered NOT gate:


With R2=100k, oscillation frequency was 4666 Hz. Close to \${1}\over{2\ln(3)R_1C_1}\$
With R2=0 ohms, oscillation frequency was 6000 Hz.