NXP Semiconductors UM10301 PCF2123 User Manual
Page 18
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
18 of 52
The frequency of a typical crystal at a specific temperature T is given by:
(
)
[
]
2
0
0
1
T
T
B
f
f
−
+
=
Further f
0
can be considered to consist of two components as
off
nom
f
f
f
+
=
0
Here f
nom
is the nominal frequency as specified and f
off
the offset from this nominal
frequency which is a result of production spread, both at room temperature.
(
)
[
]
2
0
1
T
T
B
f
f
f
off
nom
−
+
⋅
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+
=
For the frequency deviation
nom
nom
nom
f
f
f
f
f
−
=
Δ
and expressed in ppm, this results in:
(
)
(
)
[
]
6
2
0
2
0
10
1
⋅
⎭
⎬
⎫
⎩
⎨
⎧
−
+
+
−
=
Δ
T
T
B
f
f
T
T
B
f
f
nom
off
nom
(7.1)
In these equations f is the frequency, f
0
is the frequency at room temperature, B is the
parabolic coefficient, T is the temperature and T
0
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 T
0
and the room
temperature offset f
off
. The crystal manufacturer specifies these parameters and typical
values are B = - 0.035 ppm/°C
2
to - 0.04 ppm/°C
2
, T
0
= 25 °C,
∆T
0
= ± 5 °C and
f
off
= 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 T
0
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 T
0
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