NXP Semiconductors PCF85x3 Troubleshooting, Oscillator startup time Checking for oscillation

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NXP Semiconductors

UM10301

 

User Manual PCF85x3, PCA8565 and PCF2123, PCA2125

chapter could occur. First, the data could change while a single register is being read. Second, the data could change during the time between reading two registers. Therefore in order to avoid this from happening it is necessary to read all time registers in one single read operation, using the auto-increment function. The same goes for writing all registers in one single write operation when time and date is set.

19. Troubleshooting

This chapter provides some tips to troubleshoot an application if problems are encountered, for example when a new design is made.

19.1 Oscillator startup time

Assuming that a proper crystal was selected and that the layout guidelines given in this user manual were followed, the oscillator should start up without problems. As already mentioned starting times are relatively long due to the very high value of L resulting in a very high Q-factor. The start-up will mostly take less than a second and should definitely be achieved within five seconds. Oscillator start-up times are highly dependent on crystal characteristics and PCB layout. High ESR and excessive capacitive loads are the major causes of too long start up times, or the oscillator not starting at all. Oscillator start up depends also on the ambient temperature.

19.2 Checking for oscillation

In order to check whether the oscillator is running, the initial thought may be to connect an oscilloscope to the oscillator out pin in order to observe the waveform. When dealing with an RTC this is the wrong thing to do. As pointed out before the oscillator is very sensitive to disturbance due to the low power it consumes. Adding the probe capacitance will detune the oscillator which usually will stop it. Sometimes what seems to be a useable waveform may be seen but it will precisely tell you nothing due to the capacitance added by the probe. In some cases applying a scope probe can even cause a faulty oscillator design to start up, hiding design issues.

The simplest way to check for oscillation is to use the CLKOUT. If the design does not include a pull-up resistor to the CLKOUT, then add one temporarily. Applying a scope probe to the CLKOUT should reveal a block signal with a frequency that depends on the settings in the control registers. Don’t forget to first enable CLKOUT. Refer to the datasheet for details on initializing the RTC.

Another good method is to read the real time clock as time advances and see time readings adjust accordingly by looking at the seconds register. These methods will however not work if communicating with the RTC doesn’t work due to problems with the serial bus.

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 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