CASIO fx-3600P scientific calculator from 1985

[jebem]

(05-29-2018 05:16 PM)Csaba Tizedes Wrote:  I am not sure how the resistor is set the frequency itself. This resistor and capacitor works like an oscillator? As I can remember (and Googled) this system is not oscillating

Sure it works as an oscillator.
Look for "RC oscillator" in google.

There are several RC Oscillator variants, from a basic single transistor feedback loop, to more sophisticated operational amplifier designs, or even a astable flip-flop design.

In many of these basic calculator designs using just one chip, it is common to have the capacitor integrated inside the chip.
This capacitor capacitance is usually very small, in the order if hundreds ot pF only, therefore the resistor value must be in the order of several hundreds kiloohms to achieve the nominal low oscillator frequency..
These Casio series runs at frequencies around 400 to 800 KHz only. These machines are slow and in this way they save the battery that will last years.

The lower the resistor, the higher the frequency will be. However the rc oscillator will not work with any range of resistor values and tbat is why there is the need to try and test.