NXP Semiconductors PCA2125, PCF85x3 Hints to keep power consumption low, Protection diodes

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

Fig 16. Protection diodes

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16. Hints to keep power consumption low

For some applications low power consumption is a key requirement. Power consumption can be minimized by applying several measures:

•Select PCF8563 (I2C) or PCF2123 (SPI-bus). The PCF2123 has the lowest power consumption which is further decreased when a crystal is selected that has a load capacitance of 7 pF instead of the more standard 12.5 pF (a lower load capacitance results in a lower power consumption, but not all RTCs allow to select a crystal that requires a CL in that range);

•Select a crystal with a low motional resistance. The higher the resistance the higher the losses. For lowest power consumption, aim to find a crystal with R1 < 40 kΩ;

•The timer source clock frequency influences power consumption which is highest at 4096 Hz. The difference in power consumption between the other three clock frequencies is negligible;

•Use lowest possible VDD. See the application examples given in Fig 18 and Fig 19. During normal operation a diode charges a super cap of for example 0.47 F or 1 F. During standby the RTC is supplied by this super cap. General purpose small signal diodes may have leakage currents in the order of magnitude of the current consumption of these low power RTCs. In order to maximize the RTC backup time it is advisable to select a low leakage current diode. See section 13.4;

UM10301_1		© NXP B.V. 2008. All rights reserved.
User manual	Rev. 01 — 23 December 2008	39 of 52

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Contents Info Content Keywords Document informationAbstract Rev Date Description Contact informationNXP Semiconductors Revision historyIntroduction Features Address Register name Bit Register overview PCF8563Comparison Event counter modeComparison of six real time clocks FeaturesPower-on reset POR Power-on resetVoltage-low detector Oscillator Voltage-low detectionOscillator-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 Daylight Saving Time DST Century and leap year, Daylight Saving TimeCentury tracking Year and leap year trackingInitialization and setting of alarm and timer Initialization of the RTC and setting the timeBlock Diagram PCF8563 Register Comments Address AlarmSetting the alarm Binary BCDAlarm 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 BAS716 BAS116 BAV170 Diode selectionSome suggestions for diode D1 1N4148PCB 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 interfaceGeneral countdown timer behaviour First period inaccuracy when using the timerTimer delays Timer Source clock frequency Delay for n =Timer 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 ReferencesTrademarks Legal informationDefinitions DisclaimersContents