Nxp Semiconductors PCA2125 User Manual

 

 

UM10301_1 

© NXP B.V. 2008. All rights reserved.

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 R

1

 (sometimes called R

S

 or ESR). If R

1

 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 R

X

 in series 

with the crystal. The value of the resistor is changed (a trimmer is useful here) to see at 
which values of R

X

 oscillation starts and stops. Starting from a large value of R

X

 the 

resistance is lowered until oscillation starts. This value of R

X

 is called R

X-start

. Now the 

value is increased again until oscillation stops, R

X

 is called R

X-stop

. 

The oscillation allowance OA is defined as: 

 

  OA = R

X-start

 + R

1

 

 

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

 

5

1

≤

 

 

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 R

1

 under influence of increased temperature. 

 

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 V

SS

 to V

DD

. 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 V

SS

 to OSCI and from OSCI to V

DD

 (not on PCF2123). However, 

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

DD

 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