NXP Semiconductors PCF85x3, PCF2123, PCA8565, PCA2125 user manual UM10301

Page 12

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

capacitances. The external components of the oscillator have to be chosen such that the actual value of CL matches the specified value of CL. If there is mismatch the crystal will not run exactly at its specified frequency resulting in the clock running slow or fast.

The crystal manufacturer can manufacture crystals for any load capacitance, but in practice some standard values are used. For use in real-time clocks you may find crystals specified for load capacitances of 7 pF, 9 pF and 12.5 pF with 12.5 pF the most common value.

(1)Frequency on the left scale and the equivalent deviation from the nominal frequency in ppm on the right scale

Fig 6. Graph of oscillator frequency as function of load capacitance CL

Fig 6 depicts the influence of the load capacitance applied to the crystal on the oscillator frequency. The lower curve represents a crystal with a specified CL of 7 pF, the upper curve represents a crystal with a specified CL of 12 pF. From this graph it is obvious that the 7 pF crystal is more sensitive to deviations from the specified CL. If the applied CL is 1 pF lower than specified, the frequency deviation will be 18 ppm, whereas the 12.5 pF crystal will only show a frequency deviation of 6 ppm if the applied CL is 1 pF below the specified value. This is not surprising since the same absolute change in load capacitance is a larger relative change if the load capacitance is smaller. A lower load capacitance however will result in lower power consumption and in cases where this is an important requirement a crystal with lower required CL could be selected.

UM10301_1		© NXP B.V. 2008. All rights reserved.
User manual	Rev. 01 — 23 December 2008	12 of 52

Image 12

Contents Info Content Keywords Document informationAbstract Contact information NXP SemiconductorsRevision history Rev Date DescriptionIntroduction Features Register overview PCF8563 Address Register name BitEvent counter mode ComparisonFeatures Comparison of six real time clocksPower-on reset POR Power-on resetVoltage-low detector Oscillator Voltage-low detectionOscillator-stop 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 Century and leap year, Daylight Saving Time Century trackingYear and leap year tracking Daylight Saving Time DSTInitialization and setting of alarm and timer Initialization of the RTC and setting the timeBlock Diagram PCF8563 Alarm Setting the alarmBinary BCD Register Comments AddressAlarm function Setting the timer Setting the timerRegister 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 Diode selection Some suggestions for diode D11N4148 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 interfaceFirst period inaccuracy when using the timer Timer delaysTimer Source clock frequency Delay for n = 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 speedLegal information DefinitionsDisclaimers TrademarksContents