NXP Semiconductors | UM10301 |
| User Manual PCF85x3, PCA8565 and PCF2123, PCA2125 |
The frequency of a typical crystal at a specific temperature T is given by:
f = f0 [1 + B(T − T0 )2 ]
Further f0 can be considered to consist of two components as
f0 = fnom + foff
Here fnom is the nominal frequency as specified and foff the offset from this nominal frequency which is a result of production spread, both at room temperature.
| ⎛ |
| ⎞ |
|
|
|
|
|
| ||
| f = ⎜ fnom + | foff ⎟ ⋅ [1 + B(T − T0 )2 ] |
| ||||||||
| ⎝ |
| ⎠ |
|
|
|
|
|
| ||
For the frequency deviation | Δf |
| = | f − fnom | and expressed in ppm, this results in: | ||||||
fnom |
| ||||||||||
|
|
|
|
|
|
| fnom |
| |||
| Δf |
| = ⎨⎧B(T − T0 )2 + |
| foff | [1 + B(T − T0 )2 ]⎬⎫ ⋅106 | (7.1) | ||||
| fnom |
| fnom | ||||||||
| ⎩ |
|
|
|
| ⎭ |
| ||||
In these equations f is the frequency, f0 is the frequency at room temperature, B is the parabolic coefficient, T is the temperature and T0 is the turnover temperature where the apex of the drift versus temperature curve occurs.
Three variables in equation (7.1) influence the frequency as a function of temperature. These are the parabolic coefficient B, the turnover temperature T0 and the room temperature offset foff. The crystal manufacturer specifies these parameters and typical values are B = - 0.035 ppm/°C2 to - 0.04 ppm/°C2, T0 = 25 °C, ∆T0 = ± 5 °C and
foff = 30 ppm. The coefficient B has a very small spread for various crystals of one type, but it has the largest effect on the parabolic nature of the frequency deviation as a function of temperature. Variation in the turnover temperature T0 will shift the deviation curve left or right, variation in the offset at room temperature will shift it up or down. In practice the combination of variation in T0 and offset at room temperature easily results in a (lack of) accuracy of ±30 ppm at room temperature which equates to a time deviation of around 15 minutes per year.
Application note AN10652 “Improved timekeeping accuracy with PCF8563 using external temperature sensor” describes how accuracy over temperature can be improved using an external temperature sensor and a software algorithm. It can be used for the other RTCs in this manual too in conjuction with the respective datasheets.
PCF2123 contains an offset register which allows fine tuning of the clock. This can be used to compensate for crystal aging and temperature variations. See section 10.1.
Automotive RTCs PCA8565 and PCA2125 operate also at high ambient temperatures of 125 °C. Obviously also the crystal selected for these applications should be able to
UM10301_1 |
| © NXP B.V. 2008. All rights reserved. |
User manual | Rev. 01 — 23 December 2008 | 18 of 52 |