NXP Semiconductors PCF2123, PCF85x3, PCA8565, PCA2125 user manual + C

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

Now in order to determine the value of CL resulting from CIN, COUT (plus CT if mounted) and CSTRAY it is necessary to realize that seen from the crystal, CIN and COUT are effectively in series; the 32 kHz signal goes from OSCI through CIN to ground, via ground to COUT and then through COUT to OSCO. In parallel with this series circuit is CSTRAY. For the remainder of this discussion, whenever in formulas COUT is written this represents either the value of COUT only, or in case a trimming capacitor CT is present too, the sum of COUT and CT. Now the load capacitance CL is given by:

CL =	CIN ⋅ COUT	+ CSTRAY

	CIN + COUT

Since C0 is in parallel with CL the total capacitance in parallel with the motional arm

L1-C1-R1is given by

CPAR	=	CIN ⋅ COUT	+ CSTRAY + C0

		CIN + COUT

The motional arm is a series circuit, which forms a closed circuit because there is a capacitance CPAR connected in parallel to this series circuit. Of course the crystal itself can’t oscillate stand alone, but the equivalent capacitance C which determines together with L1 the resulting resonance frequency is now given by the series circuit of CPAR and C1. Thus C is given by

⎧

C

IN

⋅ C

OUT

⎫

C

⎪

+ C

+ C

⎪

1

⋅ ⎨

STRAY

0

⎬

⎪

+ C OUT

⎪

C =

⎩C IN

⎭

⎧

C

IN

⋅ C

OUT

⎫

C

⎪

+ C

+ C

⎪

1

+ ⎨

STRAY

0

⎬

⎪

+ C OUT

⎪

⎩C IN

⎭

Typical values for crystal parameters are given in Table 4. From these values it is clear that C1 is several orders of magnitudes smaller than the other capacitances in this expression and therefore C1 dominates. C will be in the order of magnitude of C1 but it will be a bit smaller as a result of CPAR in series.

With

ω =

1

and Q =

1

⋅

1

the resulting resonance frequency and quality

LC

ωC

R1

factor can be calculated.

Because C1 is orders of magnitude smaller than the other capacitances Q can be

approximated by

Q

=

1

⋅

1

a

ωC1

R1

UM10301_1

© NXP B.V. 2008. All rights reserved.

User manual

Rev. 01 — 23 December 2008

13 of 52

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Contents Document information Info Content KeywordsAbstract NXP Semiconductors Contact informationRevision history Rev Date DescriptionIntroduction Features Address Register name Bit Register overview PCF8563Comparison Event counter modeComparison of six real time clocks FeaturesPower-on reset Power-on reset PORVoltage-low detector Voltage-low detection OscillatorOscillator-stop detection Overview of internal and external oscillator capacitors Pierce Oscillator equivalent diagramOscillator frequency determining components UM10301 + C Parameter Value Unit Source Typical values for crystal and surrounding capacitorsOscillation allowance Using an external oscillatorCrystal and crystal selection Modes which don’t work Effect of temperature− f nom Capacitors and capacitor selection Accuracy Influences on time accuracy Oscillator tuning Oscillator tuning 10.1 PCF2123 Offset register Century tracking Century and leap year, Daylight Saving TimeYear and leap year tracking Daylight Saving Time DSTInitialization of the RTC and setting the time Initialization and setting of alarm and timerBlock Diagram PCF8563 Setting the alarm AlarmBinary BCD Register Comments AddressAlarm function Setting the timer Setting the timerRegister Lithium Primary cells Backup power supplyBackup circuit using primary lithium cell NiCd and NiMH secondary batteries Backup circuit using secondary cell NiCd or NiMH13.3 Capacitors Charging the backup capacitor Some suggestions for diode D1 Diode selection1N4148 BAS716 BAS116 BAV170PCB layout guidelines PCB layout proposal for PCF8563 using leaded components Partial circuit switch down Protection diodes Hints to keep power consumption low0007 Rpmax as a function of bus capacitance 8473 ⋅ C bApplication diagram 2, SPI interface Application diagram 1, I2C-bus interfaceTimer delays First period inaccuracy when using the timerTimer Source clock frequency Delay for n = General countdown timer behaviourTimer source clock Minimum timer period Maximum timer period First period delay for timer counter value nTiming requirements for I2C read and write I2C interface Block diagram I2C interface and Time countersSequence of events example Read Oscillator startup time Checking for oscillation TroubleshootingNo communication via I2C-bus Wrong time and date, wrong clock speed ReferencesDefinitions Legal informationDisclaimers TrademarksContents