Nxp Semiconductors PCA8565 用户手册

 

 

UM10301_1 

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The values used in practice will be a bit smaller than the theoretically required values due 
to parasitic capacitances present in the application which add to the external physical 
capacitor. 

For the PCF2123 the integrated C

IN

 and C

OUT

 are dimensioned for a crystal which 

requires a load capacitance of 7 pF. If a crystal with required load capacitance of 12.5 pF 
is used still a small external capacitor is required, otherwise the clock will run too fast. 
For the other types the input capacitor C

IN

 is external and needs to be mounted on the 

printed circuit board. The power consumed by the oscillator circuit is through the amplifier 
and losses in R

1

 of the crystal. Oscillation will start if the loop gain at 360° phase shift is 

higher than one. The oscillator amplitude increases until the over-all loop gain is reduced 
to exactly 1 through either non linear effects of the amplifier (self limiting Pierce) or 
through some form of AGC (Automatic Gain Control) designed in into the amplifier. 

The resonating frequency can be pulled by changing the value of the capacitor at OSCI 
or by adding a variable capacitor C

T

 at OSCO as shown in Fig 5. External capacitors at 

OSCI and OSCO should be connected to GND, except for PCF8573, PCF8583 and 
PCF8593. For the latter three it is better to connect these external capacitors to V

DD

 

instead because these devices are manufactured in a process that has the substrate 
connected to V

DD

 (n-substrate). In the other RTCs the substrate is at V

SS

 (p-substrate). 

 

R1

L1

C1

C0

Cstray

Cin

Cout

CT

C

L

crystal

OSCO

OSCI

 

(1)  For PCF8573, PCF8583 and PCF8593 connect C

IN

 and C

OUT

  (and C

T

 if applicable) to V

DD

  

 

The reactive components indicated in Fig 4 and Fig 5 determine the oscillating 
frequency. Near the resonance frequency the equivalent circuit of the crystal consists of 
the motional inductance L

1

, the motional capacitance C

1

 and the motional resistance R

1

 

(in various literature also called series resistance R

S

). In parallel with this series circuit is 

the static or shunt capacitance C

0

. It is the sum of the capacitance between the 

electrodes and the capacitance added by the leads and mounting structure. If one were 
to measure the reactance of the crystal at a frequency far away from a resonance 
frequency, it is the reactance of this capacitance that would be measured. 

When a crystal is chosen, such a crystal has a specified load capacitance C

L

. During 

production the crystal manufacturer has adjusted the resonance frequency of the crystal 
using exactly this capacitance as the load for the crystal. The actual value of C

L

 as seen 

by the crystal in the application is determined by the external circuitry and parasitic