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

Page 14

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

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Contents Abstract Info Content KeywordsDocument information Revision history Contact informationNXP Semiconductors Rev Date DescriptionIntroduction Features Register overview PCF8563 Address Register name BitEvent counter mode ComparisonFeatures Comparison of six real time clocksVoltage-low detector Power-on reset PORPower-on reset Oscillator-stop detection OscillatorVoltage-low detection Pierce Oscillator equivalent diagram Overview of internal and external oscillator capacitorsOscillator frequency determining components UM10301 + C Typical values for crystal and surrounding capacitors Parameter Value Unit SourceUsing an external oscillator Oscillation allowanceCrystal and crystal selection Effect of temperature Modes which don’t work− f nom Capacitors and capacitor selection Accuracy Influences on time accuracy Oscillator tuning Oscillator tuning 10.1 PCF2123 Offset register Year and leap year tracking Century and leap year, Daylight Saving TimeCentury tracking Daylight Saving Time DSTBlock Diagram PCF8563 Initialization and setting of alarm and timerInitialization of the RTC and setting the time Binary BCD AlarmSetting the alarm Register Comments AddressAlarm function Register Setting the timerSetting the timer Backup power supply Lithium Primary cellsBackup circuit using primary lithium cell Backup circuit using secondary cell NiCd or NiMH NiCd and NiMH secondary batteries13.3 Capacitors Charging the backup capacitor 1N4148 Diode selectionSome suggestions for diode D1 BAS716 BAS116 BAV170PCB layout guidelines PCB layout proposal for PCF8563 using leaded components Partial circuit switch down Hints to keep power consumption low Protection diodes0007 8473 ⋅ C b Rpmax as a function of bus capacitanceApplication diagram 1, I2C-bus interface Application diagram 2, SPI interfaceTimer Source clock frequency Delay for n = First period inaccuracy when using the timerTimer delays General countdown timer behaviourFirst period delay for timer counter value n Timer source clock Minimum timer period Maximum timer periodTiming requirements for I2C read and write Block diagram I2C interface and Time counters I2C interfaceSequence of events example Read Troubleshooting Oscillator startup time Checking for oscillationNo communication via I2C-bus References Wrong time and date, wrong clock speedDisclaimers Legal informationDefinitions TrademarksContents