NXP Semiconductors
UM10699
User manual for SPI-bus RTC demoboard OM11059
UM10699
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© NXP B.V. 2013. All rights reserved.
User manual
Rev. 1 — 04 April 2013
8 of 12
5. RTC tuning
5.1 Frequency tuning
The 32 kHz quartzes are typically sold with a tolerance at room temperature of either
±10 ppm or ±20 ppm. 11.5 ppm corresponds to 1 s/day.
The quartzes feature a characteristic load capacity of either 7 pF or 12.5 pF. Oscillators
utilizing 7 pF quartzes feature slightly lower power consumption, where the quartzes of
12.5 pF have largest production quantities. The tracks between quartz and RTC
represent also some parasitic capacitances and must be kept short.
The PCF85063 has a tuning facility where above tolerances can be compensated.
Tuning procedure:
•
Measure the 32xxx Hz (f) signal at the CLKOUT pin.
•
The offset is calculated in ppm as
Δf
[ppm]
= 10
6
× (f - 32768) / 32768
•
Consult the offset table in the data sheet. Take the correction value and write it into
the register 02h.
•
The correction is done by means of inhibition or addition: the oscillator runs at
constant speed, then every 2 hours (mode 0) 1 second is corrected to by making it
shorter or longer. This is not easily visible at the CLKOUT.
•
Corrections can also be applied every 4 minutes by using mode 1. This mode will
consume slightly more power.
The 32 kHz quartzes are of the type tuning fork and feature a parabolic frequency
response over temperature. When the application is dominantly used over a limited
temperature range, it is often helpful to tune the frequency to be slightly higher at the
turn-over point. The error around 25 °C (clock goes too fast) is then compensated during
the time when temperature is lower or higher. For example, for operation between 5 °C
and 45 °C, tune the clock 8 ppm faster than the value for 25 °C would be. (See Fig
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.)