CD4521B: Clarification on datasheet

Hello,

The circuit you're looking at is a crystal oscillator (Figure 11). Here's a standard crystal oscillator circuit, used in many devices:

R1 provides bias for the unbuffered inverter. This is typically a large value of resistance - 1 Mohm is typically used (in this datasheet 18 Mohm is recommended).

R2 is a damping resistor to reduce power input to the crystal, and then C1 and C2 are the load resistors for the crystal (typically defined in the crystal datasheet, commonly about 20pF).

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The oscillator circuit shown in the CD4521B datasheet is a bit different. Here's all the oscillator circuitry shown in the datasheet:

And here's the external components from the datasheet specific to the crystal oscillator:

I redrew this to be a bit less convoluted:

You can see from Figure 11 in the datasheet that only pins 4 and 6 connect to the crystal oscillator circuit.

This is basically the same circuit I showed above (first image), but instead of connecting the crystal resonator circuit across the inverter, it's instead connected to ground.

Roland Melad said:

Q1. Does the oscillator frequency calculation formula given under fig.10 also apply to the oscillator circuit in fig11?

No, figure 11 is a crystal oscillator -- the frequency of oscillation is set by the crystal.

Roland Melad said:

Q2. How to calculate the capacitor "Cs" value shown in fig.11?

Crystal oscillator operating frequency will be dependent on the capacitive loading of the crystal. The crystal datasheet should give you the load capacitance of the crystal, and may also provide a suggested configuration for the capacitance.

Roland Melad said:

Q3. In fig. 11, an unannotated resistor between pin 4 and pin 6 is shown. Is this the resistor "Rt"?

This is the bias resistor, which is different from the timing resistor in the RC loop oscillator circuit.

Roland Melad said:

Q4. Is the resistor "R0" shown in Fig.11 has the same functionality as resistor "Rs" in fig.10?
If so, then, since "R0 = 18 MOhm" (as shown in fig.11), can one use the relationship "Rs >= 2Rt" (as given in fig. 10) to calculate the value of the unannotated resistor in fig.11 (presuming, it is "Rt")?

No, Rs is part of an RC loop oscillator circuit -- this functions very differently from a crystal oscillator.