Oscillator made from an RC network
If you are trying to control the oscillator externally use a sync pulse as shown in the data sheet: -



This is also described here: -



There are several ways that the oscillator can work and this is probably the basis: -



Picture from here. If it has an offset duty cycle i.e. it produces more of a pulse than a square wave it may have an internal diode and resistor to forcibly shorten the time taken to charge or discharge the capacitor like this: -



Picture from here. \$R_d\$ and its series diode may be inside the chip. There isn't a requirement for \$R_c\$'s diode if \$R_d\$ is >> \$R_d\$.

There are a few other ways this can be done too.