NXP Semiconductors PCA2125, PCF85x3, PCF2123 Oscillation allowance, Using an external oscillator

Page 15

NXP Semiconductors

UM10301

 

User Manual PCF85x3, PCA8565 and PCF2123, PCA2125

6.1 Oscillation allowance

Fig 4 shows the Pierce oscillator schematic with the external crystal. For an oscillation to take place the real component of the oscillator impedance has to be larger than the motional resistance R1 (sometimes called RS or ESR). If R1 is too large no oscillation will take place since no operating point can be reached.

Similarly, if the supply voltage is too low or the temperature is too low, no oscillation can build up.

A method to test how much margin the design has is to include a resistor RX in series with the crystal. The value of the resistor is changed (a trimmer is useful here) to see at which values of RX oscillation starts and stops. Starting from a large value of RX the resistance is lowered until oscillation starts. This value of RX is called RX-start. Now the value is increased again until oscillation stops, RX is called RX-stop.

The oscillation allowance OA is defined as:

OA = RX-start+ R1

As a rule of thumb, the motional resistance of the crystal chosen should be

 

R ≤

OA

 

 

 

 

 

 

1

5

 

 

 

 

 

 

 

This test can be done in the lab under room temperature. This should give enough safety

 

margins to allow for production spread of IC and crystal and to deal with the increasing

 

value of R1 under influence of increased temperature.

 

 

6.2 Using an external oscillator

 

 

It is possible to supply a clock signal from an external oscillator instead of using the

 

internal oscillator if for some reason it is desired to not use the internal oscillator. In this

 

case no crystal will be connected to the OSCI and OSCO pins. Instead the external

 

oscillator must be connected to OSCI while OSCO must be left floating.

 

 

The signal may swing from VSS to VDD. However, with a crystal attached the signal

 

 

amplitude at the oscillator input pin would be about 500 mV, swinging around a 250 mV

 

bias i.e. never going negative (not for PCF8583 and PCF8593, see below). For the

 

 

PCF85x3 supplying a signal with amplitude between 500 mV and 1000 mV is a good

 

starting point, with the bias such that the signal doesn’t go negative and operates in the

 

same region as would have been the case with a crystal. Square or sine wave is both ok.

 

For the PCF2123 the amplitude should be somewhat smaller. If the oscillator amplitude

 

is larger than the supply voltage to the RTC it is advisable to use a resistive divider for

 

the oscillator signal to bring its amplitude within the supply voltage of the RTC. Without

 

such a divider it will work too and nothing will be damaged (as long as the currents via

 

the clamping diodes don’t exceed the maximum limits) because the device has internal

 

clamping diodes from VSS to OSCI and from OSCI to VDD (not on PCF2123). However,

 

performance will be better if the oscillator amplitude is brought within the range from 0 V

 

to the actual VDD used for the RTC. This will first prevent periodic currents flowing via the

 

upper clamping diode to the decoupling capacitor on the supply pin. Secondly the signal

UM10301_1

 

 

© NXP B.V. 2008. All rights reserved.

User manual

 

 

Rev. 01 — 23 December 2008

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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
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