NXP Semiconductors UM10301 PCF2123 - Manual (Page 10)

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 10 of 52
C
STRAY
is a result of parasitic capacitances due to PCB traces, IC pins etc. and is directly
in parallel with C
0
of the crystal. In a practical situation care needs to be taken to keep
these parasitic capacitances as low as possible since it will add to the load capacitance
and this load capacitance must meet the specified value for the crystal that is being used.
If the load capacitance presented to a crystal is smaller than what the crystal was
designed for, the oscillation frequency will be too high and thus if used with an RTC, the
clock will run too fast.
001aah846
C1
C0
Cstray
R1 L1
Cin Cout
C L
crystal
RTC-IC
OSCO OSCI
Fig 4. Pierce Oscillator equivalent diagram
The inverting amplifier (with feedback resistor, and drive resistor which are not included
in Fig 4) is incorporated within the integrated circuit device. On the other hand, the quartz
crystal is a discrete device external to the integrated circuit. In the PCF85x3, PCA8565
and PCF2123, PCA2125 the output capacitor C
OUT
is integrated on the integrated circuit.
PCF8563A, PCA8565A and PCF2123 also include C
IN
, see Table 3 for overview.
Table 3. Overview of internal and external oscillator capacitors
PCx85x3 family PCx212x family Features
PCF8563 PCA8565 PCF8583 PCF8593 PCF2123 PCA2125
Integrated oscillator capacitor 1 at OSCO 1 at OSCO 1 at OSCO 1 at OSCO 2 1 at OSCO
Targeted crystal load capacitance 12.5 pF 12. 5 pF 12.5 pF 12. 5 pF 7 pF
[1]
12.5 pF
Value of integrated C
IN
, typ. - - 14 pF -
Value of integrated C
OUT
, typ. 25 pF 25 pF 40 pF 25 pF 14 pF 25 pF
Theoretically required at pin OSCI 25 pF 25 pF 18 pF 25 pF 0 pF 25 pF
[1] Can be used with 9 pF and 12.5 pF as well if external capacitance is added