Pwm frequency math challenge
From the datasheet, 13. transfer Characteristics

                      Vdd  min typ - min(full temp)
Vh hysteresis voltage 5 V 0.5 0.8 - 0.5 - V
                      10 V 0.7 1.3 - 0.7 - V
                      15 V 0.9 1.8 - 0.9 - V


Thus I interpolate for 12V between 1.3V ~ 1.8V or ΔV=Vh = 1.5V typ


The transition time, dT for a half cycle from Ic=C dV/dT
or dT = C dV/Ic  where the derivative slope dV=ΔV=Vh


since Ic= Vdd/R and dV= 1.5V from above , you can now calculate f=1/(2dT) from all this math.
just in case you are lost, the Cap current formula, Ic differentiates voltage dV/dT,  while the circuit Voltage from R integrates current is linear over a small interval ΔV, is the same thing.  
and Ic = (Vdd/2 + ΔV/2) * 1/R 
or Ic=(Vcc+ΔV)/2R  ( assuming waveform symmetry Vdd/2 almost...)
thus  dT = 2RC dV/(Vdd+ΔV) and dT * 2 = 1/f
thus \$f = \dfrac{{(Vdd+ΔV)}}{4RC \cdot ΔV}\$  (corrected)



for R=50 [kΩ] , C=0.1 [uf] , Vdd=12 [V] , Vh=ΔV=1.5 [V] , f= ?[Hz]




  if ΔV/(ΔV+Vdd) is 50% tolerance, then Δf will be ~ 50% tolerance.


For more precision use a comparator with &lt;=1% parts.






simulate this circuit &ndash; Schematic created using CircuitLab

I corrected my misteaks;)  

I now get 450 Hz but 396Hz on my Simulation. ...  error on mean threshold..