NXP Semiconductors UM10301 PCF2123 - Manual (Page 20)

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 20 of 52
to +80 %. Its only redeeming feature is its high permittivity which allows high capacitance
values, typically ranging from 1 nF to 4.7 µF. Good for bypass and coupling applications.
It has low price, small size and low temperature stability.
Conclusion: For the oscillator only C0G types should be used. This will almost always
automatically be the case since the other types are usually not available in such small
values. For the decoupling of the RTC, use a capacitor with X7R dielectric. Using SMD
packages results in the lowest parasitic inductances and the small dimensions enable the
smallest loops which reduces sensitivity to EMI.
A more expensive alternative for the decoupling X7R capacitor is a film capacitor using
Polyethylene naphthalate (PEN), another form of polyester. It has very good heat
resistance, but is otherwise much like polyester. It is available in larger sizes than C0G
ceramic, lower temperature drift than polyester, and lower leakage than X7R. PEN
capacitors are available to 125 °C. It is commonly found in SMD capacitors, including
large values (>1
μF).
9. Accuracy
How accurate is accurate??
The international System of Units (SI) has defined the second as the duration of
9,192,631,770 cycles of radiation corresponding to the transition between two energy
levels of the ground state of the cesium-133 atom. This definition makes the cesium
oscillator (often called an atomic clock) the primary standard for time and frequency
measurements. Its accuracy is extremely high with deviations of only a second per
several million years. This equates to accuracy in the order of 10
-8
ppm or better.
In dealing with RTCs it is common to express accuracy in ppm, parts per million. But
what does it mean in more human units? A clock going too fast 1 s/day has an accuracy
of 1 / (number of seconds in a day) = 1 / (24 x 3600) = 11.57 ppm. The other way around,
20 ppm is about 1 minute per month.
1 s/week = 1.65 ppm, 1 s/month = 0.4 ppm and 1 s/year = 0.031 ppm. In contrast, a good
mechanical watch has a deviation of less than 12 s/day or 1300 ppm.
Fig 8 compares the magnitude of the different spreads and variations.
The RTC accuracy dominantly depends on the parameters of the resonating crystal. The
initial frequency tolerance f
off
can be compensated by tuning the external capacitance.
The temperature coefficient of the external capacitances has almost no effect. The main
contribution comes from the temperature coefficient of the crystal. In contrast to AT-cut
crystals tuning fork crystals have the parabolic temperature dependence indicated in
Fig 7 which results in a slow down of the clock if the temperature is lower or higher than
T
0
which is in the range of 25 to 28 degrees. The same type of crystal is also used in
wrist watches and the turnover temperature of the crystal matches well with the
temperature at the wrist which is typically about 28 °C and quite stable.