AN-1058 Datasheet, PDF (1/7 Pages) Cymbet Corporation – Using CBC348xx EnerChip RTC in High Accuracy Applications

AN-1058
Application Note
Using CBC348xx EnerChip RTC in High Accuracy Applications
Introduction
This application note addresses issues relating to overall timing accuracy of the final design of systems utilizing
the Cymbet CBC34803 and CBC34813 (collectively known as CBC348xx) Self-Powered Real-Time Clock (RTC)
devices. This ap note also includes tips on crystal selection.
Typical Crystal Parameters of Interest
Typical tuning fork crystals are specified in terms of:
• Load Capacitance (pF)
• Frequency Tolerance /Initial Frequency Error (parts per million = ppm)
• Series Resistance(Ω)
• First-Year Aging (ppm)
• Frequency vs. Temperature (ppm)
• Turnover Temperature, which is usually around 25°C
In addition to these parameters, there are parasitic capacitances and inductances in the circuit board layout
and assembly that need to be considered. With the exception of aging, the CBC348xx has the ability to
calibrate out or compensate for all of these parameters. The lesser exceptions of Load Capacitance and Series
Resistance are relatively easy to specify in the specific crystal chosen. Crystals with frequency tolerances of
±20ppm or so can easily be used, because they can be calibrated to 0ppm at initial test.
How the CBC348xx Parts Compensate for Crystal Parameters
The CBC348xx includes offset registers to allow temperature compensation for thermal drift of the crystal. The
part can calibrate out all initial frequency variations at assembly test time by measuring an output clock and
loading several registers in the part based on simple calculations derived from the measured time. The register
values should be kept in non-volatile memory somewhere in the system to reload these calibration registers
whenever system power is recovered after a shutdown. The only parameters that are not controlled are the
crystal aging parameter and the crystal temperature variation, but these can be controlled at the system level
if necessary. The aging parameter is typically ±2ppm to ±4ppm for the first year. Most aging occurs in the first
few months of operation. See the section on temperature compensation for information on crystal temperature
variations.
Factors Affecting Overall System Accuracy
The final accuracy of a timing system using a CBC348xx involves: the CBC348xx itself, the thermal use-case of
the system, the crystal, and the parasitic loads that result with a printed circuit board implementation. Issues
to be considered are:
1. Temperature compensation of the crystal.
2. Thermal use-case of the system.
3. Trimming the initial frequency of the mounted crystal.
4. Aging of the crystal.
5. The following selectable operating modes of the RTC offer a trade-off of power for overall accuracy:

a. Crystal (XT) mode.

b. RC Autocalibrate mode.

c. RC only mode.
©2014 Cymbet Corporation • Tel: [phone] • www.cymbet.com
Doc AN-72-1058 Rev B
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