NXP Semiconductors PCF2123, PCF85x3, PCA8565 8473 ⋅ C b, Rpmax as a function of bus capacitance

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

RP(max)	=	tr
RP(max)	=	0.8473 ⋅ Cb
		0.8473 ⋅ Cb
		20			002aac883
		20	(1)
		Rp(max)	(1)
		(kΩ)
		16
		12
		8	(2)
			(2)
		4	(3)
			(3)
		0
		0	200	400	600
					Cb (pF)

(1)Standard-mode

(2)Fast-mode

(3)Fast-mode Plus

Fig 17. Rp(max) as a function of bus capacitance

Below Rp(max) is calculated for both Standard-mode and Fast-mode I2C bus. The result has been rounded down to E6 standard values.

	400 kHz I2C bus: RP−400kHz		=	t	r	=	300 ⋅10−9		= 22 kΩ
				0.8473 ⋅ Cb			0.8473	−
								⋅14.9 ⋅10 12
	100 kHz I2C bus: R	−	=	t	r	=	1	⋅10−6	= 68 kΩ
				0.8473 ⋅ Cb			0.8473 ⋅14.9 ⋅10−12
		P 100kHz

Two examples of an application diagram are shown in Fig 18 and Fig 19. In these examples the INT output is left unused. If used, a pull-up to VDD is required.

The first example is using PCF8563 (Fig 18), the second example is using PCA2125 (Fig 19). In both examples the optional backup supply is realized using a 1 F capacitor.

UM10301_1		© NXP B.V. 2008. All rights reserved.
User manual	Rev. 01 — 23 December 2008	41 of 52

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Contents Abstract Info Content KeywordsDocument information NXP Semiconductors Contact informationRevision history Rev Date DescriptionIntroduction Features Address Register name Bit Register overview PCF8563Comparison Event counter modeComparison of six real time clocks FeaturesVoltage-low detector Power-on reset PORPower-on reset Oscillator-stop detection OscillatorVoltage-low 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 DSTBlock Diagram PCF8563 Initialization and setting of alarm and timerInitialization of the RTC and setting the time Setting the alarm AlarmBinary BCD Register Comments AddressAlarm function Register Setting the timerSetting the timer 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