NXP Semiconductors PCA8565, PCF85x3, PCF2123, PCA2125 Pierce Oscillator equivalent diagram

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

CSTRAY is a result of parasitic capacitances due to PCB traces, IC pins etc. and is directly in parallel with C0 of the crystal. In a practical situation care needs to be taken to keep these parasitic capacitances as low as possible since it will add to the load capacitance and this load capacitance must meet the specified value for the crystal that is being used. If the load capacitance presented to a crystal is smaller than what the crystal was designed for, the oscillation frequency will be too high and thus if used with an RTC, the clock will run too fast.

RTC-IC

OSCI

crystal

L1 C1 R1

C0

Cstray

OSCO

CL

Cin

Cout

001aah846

Fig 4. Pierce Oscillator equivalent diagram

The inverting amplifier (with feedback resistor, and drive resistor which are not included in Fig 4) is incorporated within the integrated circuit device. On the other hand, the quartz crystal is a discrete device external to the integrated circuit. In the PCF85x3, PCA8565 and PCF2123, PCA2125 the output capacitor COUT is integrated on the integrated circuit. PCF8563A, PCA8565A and PCF2123 also include CIN, see Table 3 for overview.

Table 3. Overview of internal and external oscillator capacitors

Features		PCx85x3 family			PCx212x family
	PCF8563	PCA8565	PCF8583	PCF8593	PCF2123	PCA2125
Integrated oscillator capacitor	1 at OSCO	1 at OSCO	1 at OSCO	1 at OSCO	2	1 at OSCO
Targeted crystal load capacitance	12.5 pF	12. 5 pF	12.5 pF	12. 5 pF	7 pF [1]	12.5 pF
Value of integrated CIN, typ.	-	-			14 pF	-
Value of integrated COUT, typ.	25 pF	25 pF	40 pF	25 pF	14 pF	25 pF
Theoretically required at pin OSCI	25 pF	25 pF	18 pF	25 pF	0 pF	25 pF

[1] Can be used with 9 pF and 12.5 pF as well if external capacitance is added

UM10301_1		© NXP B.V. 2008. All rights reserved.
User manual	Rev. 01 — 23 December 2008	10 of 52