NXP Semiconductors | UM10301 |
| User Manual PCF85x3, PCA8565 and PCF2123, PCA2125 |
value is really too high start up problems may occur, but up to 100 kΩ no start up problems are expected. See 6.1”Oscillation allowance”.
The frequency accuracy of the oscillator depends mainly on the accuracy of the crystal and on how well the crystal is matched to the oscillator capacitive load (CL). A too small capacitive load results in the oscillator running fast, if the capacitive load is greater than what the crystal was designed for the oscillator and thus clock runs slow. This initial error
is indicated in Fig 7 as ∆foff. The largest influence on accuracy is a result of the temperature dependence of the crystal.
7.1 Modes which don’t work
To keep time with an adequate accuracy it is necessary to use a quartz crystal and thus the use of a quartz crystal is always assumed in this application note. A very low power crystal oscillator as used in an RTC requires a different set of parameters compared to a universal oscillator accepting crystals, RC- and LC networks or a ceramic resonator. The oscillator circuit is not designed for operating with RC or LC networks neither for use with a ceramic resonator. Ceramic resonators have a much shorter start up time than crystals, about 100 times faster. However, they have lower frequency accuracy (initial tolerance, temperature variations, drift) and since in an RTC accurate timekeeping is the goal, ceramic resonators are not a good choice for an RTC. Use a crystal.
7.2 Effect of temperature
A tuning fork crystal is usually cut such that its frequency over temperature is a parabolic curve centered around 25 °C, see Fig 7. This means that a tuning fork crystal oscillator will resonate close to its target frequency at room temperature, but will slow down when the temperature either increases or decreases from room temperature.
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| ΔT0 |
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frequency |
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deviation Δf/f0 |
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−20 |
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| Δfoff |
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−60 |
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−100 |
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−140 |
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−180 |
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−40 | −30 | −20 | −10 | 0 | 10 | 20 | 30 | 40 | 50 | 60 | 70 | 80 | 90 |
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| T (°C) |
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Fig 7. The deviation in frequency vs. temperature of a typical 32.768 kHz crystal
UM10301_1 |
| © NXP B.V. 2008. All rights reserved. |
User manual | Rev. 01 — 23 December 2008 | 17 of 52 |