User's Manual

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 14 of 52
Taking the numbers from Table 4 yields for L
1
and Q:
() ()
H
Cf
L 11234
101.2327682
1
2
1
15
2
1
2
0
1
=
⋅⋅⋅
=
⋅⋅
=
−
ππ
()
()
42053
1055101.2327682
1
2
1
315
110
=
⋅⋅⋅⋅⋅
=
⋅⋅⋅
=
−
π
π
RCf
Q
This L of around 11000 H resulting in a Q of around 42000 explains why starting up the
oscillator as well as stopping it can easily take more than a second. An oscillating quartz
crystal is actually a mechanical oscillation and starting or stopping this takes time.
Calculations of start up time and more in-depth theory about the oscillator and load
capacitance are beyond the scope of this user manual, but can be found in AN10716
“Background information and theory related to Real Time Clocks and crystals”.
The use of AGC’s improve start up by high drive initially to get it going and then reduce
drive for low power.
Table 4. Typical values for crystal and surrounding capacitors
Parameter Value Unit Source
f
0
32768 Hz [2]
∆f / f
0
±100 ppm [2]
Aging; ∆f / f
0
±3…±5 ppm [2]
B, freq(T) -0.035 ppm / °C
2
[2]
C
1
2.1 fF [2]
C
0
1.2…1.5 pF [2]
C
IN
25 ± 10 pF [1]
C
IN
, temp co. +47 ppm/°C [1]
R
1
50…80 kΩ [2]
C
T
variable 4…25 pF [3]
C
T
, temp co. 300 ppm/°C [3]
C
T
fixed 0603 Any pF [4]
C
T
fixed, tc ±30 for C0G ppm/°C [4]
Sources for values in table 4:
[1] NXP, Datasheet PCF8563, February 2008.
[2] Product Data Sheets, MicroCrystal.
[3] Murata TZB04 trim capacitor
[4] Vishay Beyschlag, datasheet ceramic multilayer capacitor, C0G