NXP Semiconductors PCF2123, PCF85x3 PCB layout proposal for PCF8563 using leaded components

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

(1) In practice the input capacitor CT will in most cases be a fixed capacitor

Fig 14. PCB layout proposal for PCF8563 using leaded components

Alternatively to the last point listed above and in order to achieve highest noise immunity, a guard ring can be placed around the crystal which must be tied to ground to isolate the crystal from unwanted noise pickup. It should be tied to VSS of the real time clock at one place only to avoid unforeseen currents running via the guard ring. In addition another local ground plane on an adjacent PCB layer can be added under the crystal. Also this ground plane should be isolated from the regular PCB ground plane and connected to VSS of the RTC. The dimensions of this ground plane shouldn’t be much larger than the perimeter of the guard ring, but optimally should include the complete crystal and RTC. Be aware that this ground plane will create parasitic capacitances on the OSCI and OSCO pins because on the adjacent outer layer the tracks to the crystal will run. A sketch of such a layout is given in Fig 15. Here the use of SMD components is assumed which is recommended because the smaller dimensions will result in smaller loop area’s.

Both examples use the PCF8563. The same principles apply when one of the other RTCs is used, with some small modifications due to differences in pinning.

UM10301_1		© NXP B.V. 2008. All rights reserved.
User manual	Rev. 01 — 23 December 2008	37 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