NXP Semiconductors PCA8565, PCF85x3, PCF2123 Typical values for crystal and surrounding capacitors

NXP Semiconductors																							UM10301
									User Manual PCF85x3, PCA8565 and PCF2123, PCA2125
Taking the numbers from Table 4 yields for L1 and Q:
L1 =		1					=				1						= 11234 H
		(2π ⋅ f	0	)2 ⋅ C				(2π ⋅ 32768)2 ⋅ 2.1⋅10−15
					1
Q =				1					=				1										= 42053
	(2π ⋅ f			)⋅ C		⋅ R				( π ⋅	32768	)⋅	2.1	⋅	10	−15		⋅	55	⋅	10	3
			0		1	1			2

This L of around 11000 H resulting in a Q of around 42000 explains why starting up the oscillator as well as stopping it can easily take more than a second. An oscillating quartz crystal is actually a mechanical oscillation and starting or stopping this takes time.

Calculations of start up time and more in-depth theory about the oscillator and load capacitance are beyond the scope of this user manual, but can be found in AN10716 “Background information and theory related to Real Time Clocks and crystals”.

The use of AGC’s improve start up by high drive initially to get it going and then reduce drive for low power.

Table 4. Typical values for crystal and surrounding capacitors

Parameter	Value	Unit		Source
f0	32768	Hz	[2]
∆f / f0	±100	ppm	[2]
Aging; ∆f / f0	±3…±5	ppm	[2]
B, freq(T)	-0.035	ppm / °C2	[2]
C1	2.1	fF	[2]
C0	1.2…1.5	pF	[2]
CIN	25 ± 10	pF	[1]
CIN, temp co.	+47	ppm/°C	[1]
R1	50…80	kΩ	[2]
CT variable	4…25	pF	[3]
CT, temp co.	300	ppm/°C	[3]
CT fixed 0603	Any	pF	[4]
CT fixed, tc	±30 for C0G	ppm/°C	[4]

Sources for values in table 4:

[1]NXP, Datasheet PCF8563, February 2008.

[2]Product Data Sheets, MicroCrystal.

[3]Murata TZB04 trim capacitor

[4]Vishay Beyschlag, datasheet ceramic multilayer capacitor, C0G

UM10301_1		© NXP B.V. 2008. All rights reserved.
User manual	Rev. 01 — 23 December 2008	14 of 52