NXP Semiconductors PCA2125, PCF85x3, PCF2123, PCA8565 Oscillator frequency determining components

NXP Semiconductors	UM10301
	User Manual PCF85x3, PCA8565 and PCF2123, PCA2125

The values used in practice will be a bit smaller than the theoretically required values due to parasitic capacitances present in the application which add to the external physical capacitor.

For the PCF2123 the integrated CIN and COUT are dimensioned for a crystal which requires a load capacitance of 7 pF. If a crystal with required load capacitance of 12.5 pF is used still a small external capacitor is required, otherwise the clock will run too fast. For the other types the input capacitor CIN is external and needs to be mounted on the printed circuit board. The power consumed by the oscillator circuit is through the amplifier and losses in R1 of the crystal. Oscillation will start if the loop gain at 360° phase shift is higher than one. The oscillator amplitude increases until the over-all loop gain is reduced to exactly 1 through either non linear effects of the amplifier (self limiting Pierce) or through some form of AGC (Automatic Gain Control) designed in into the amplifier.

The resonating frequency can be pulled by changing the value of the capacitor at OSCI or by adding a variable capacitor CT at OSCO as shown in Fig 5. External capacitors at OSCI and OSCO should be connected to GND, except for PCF8573, PCF8583 and PCF8593. For the latter three it is better to connect these external capacitors to VDD instead because these devices are manufactured in a process that has the substrate connected to VDD (n-substrate). In the other RTCs the substrate is at VSS (p-substrate).

crystal

L1 C1 R1

OSCI

Cin

C0

Cstray

OSCO

CL

Cout CT

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(1) For PCF8573, PCF8583 and PCF8593 connect CIN and COUT (and CT if applicable) to VDD

Fig 5. Oscillator frequency determining components

The reactive components indicated in Fig 4 and Fig 5 determine the oscillating frequency. Near the resonance frequency the equivalent circuit of the crystal consists of the motional inductance L1, the motional capacitance C1 and the motional resistance R1 (in various literature also called series resistance RS). In parallel with this series circuit is the static or shunt capacitance C0. It is the sum of the capacitance between the electrodes and the capacitance added by the leads and mounting structure. If one were to measure the reactance of the crystal at a frequency far away from a resonance frequency, it is the reactance of this capacitance that would be measured.

When a crystal is chosen, such a crystal has a specified load capacitance CL. During production the crystal manufacturer has adjusted the resonance frequency of the crystal using exactly this capacitance as the load for the crystal. The actual value of CL as seen by the crystal in the application is determined by the external circuitry and parasitic

UM10301_1		© NXP B.V. 2008. All rights reserved.
User manual	Rev. 01 — 23 December 2008	11 of 52