NXP Semiconductors UM10301 PCF2123 - Manual (Page 48)

NXP Semiconductors
UM10301
User Manual PCF85x3, PCA8565 and PCF2123, PCA2125
UM10301_1 © NXP B.V. 2008. All rights reserved.
User manual Rev. 01 — 23 December 2008 48 of 52
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.