NXP Semiconductors PCF85x3 Troubleshooting, Oscillator startup time Checking for oscillation

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	48 of 52