Nxp Semiconductors PCA2125 User Manual

 

 

UM10301_1 

© NXP B.V. 2008. All rights reserved.

levels can be tuned such that they are similar to those when the internal oscillator is 
used.  

Suppose that the RTC is supplied with 3.3 V and that the amplitude of the external CLK 
is 5 V (from 0 V to 5 V). Using 1 M and 220 k resistors the signal could be reduced to 
(220 / 1220) x 5 V = 0.9 V. This is better in line with the signals that the internal circuitry 
handles when an external crystal is used as is the case in the standard application. This 
reduced signal can then be applied to the OSCI pin directly or via a small capacitor of 
e.g. 22 pF - 100 pF. This is a lower power option, where bias from the resistive devider 
and oscillator will be lost and will be determined by the oscillator input. This option is also 
more susceptible to noise. 

If PCF8583 and PCF8593 are used together with a crystal, the signal would swing 
around a bias of some 100 mV below V

DD

. If these RTCs are fed with an external signal, 

it should be either AC coupled, or swinging with amplitude of around 1 V below V

DD

, 

where the lower value may be lower than 1 V below V

DD

 as well. For example, swinging 

from (V

DD

 – 1 V) to V

DD

 would be ok, but also swinging from V

SS

 to V

DD

. 

: Values mentioned here are guidelines only. For every application correct 

operation must be verified. 

 

 

Select a crystal of the tuning fork type with a nominal frequency of 2

15

 Hz = 32768 Hz. 

The allowed tolerance depends on the requirements for the application and on whether a 
trimming capacitor will be used. If a trimming capacitor will be used even a tolerance of 
±100 ppm is ok since it can be compensated. Either through hole or surface mount 
crystals can be used where the latter provide the smallest dimensions which makes the 
circuit less susceptible to noise pick up. 

As previously pointed out crystals used for RTCs come in three versions, optimized for 
three standard values for C

L

 with 12.5 pF the most common. Generally, an RTC using a 

12.5 pF crystal has a timekeeping current of about 1.6x more than an RTC using a 7 pF 
crystal. If lowest power consumption is a key consideration, a 7 pF crystal (some 
manufacturers use 6 pF) should be selected. The PCF2123 has been optimized for use 
with such a crystal. The other RTCs include load capacitance optimized for a 12.5 pF 
crystal. Using a 7 pF crystal would require an external capacitor of about 9.7 pF and thus 
the capacitances at OSCI and OSCO would not be balanced. In general this may have a 
detrimental influence on start-up behaviour but no problems are expected when a 7 pF 
crystal is used in combination with the PCF8563 because it uses an AGC in its oscillator. 

An oscillator using a 12.5 pF crystal will be more stable and less susceptible to noise and 
parasitic capacitances. One reason for this is that the capacitors on the input and output 
will have higher values and therefore create a higher load for noise. Further these higher 
values make the parasitic capacitance relatively smaller for the same PCB. 

Besides technical considerations there are also procurement issues. Crystals designed 
for a 12.5 pF load capacitance are readily available through many distributors. Crystals 
designed for a load capacitance of 7 pF or 9 pF are not as readily available and may 
have longer lead times or require a minimum quantity to be purchased. 

The series resistance R

1

 should ideally remain below 50 kΩ. If higher values are used 

(up to 100 kΩ is ok) the current consumption of the oscillator will increase a bit. If the