AN4630

Components dimensioning

Equation 1

Where C1’=C1+CPCB1+CPAD, C2’= C2+CPCB2+CPAD, where C1 and C2 are external

(SMD) components, CPCB1 and CPCB2 are PCB routing parasites and CPAD is the

equivalent small-signal pad-capacitance. The value of CPAD is around 0.5 pF for each

pad. The routing parasites should be minimized by placing quartz and C1/C2 capacitors

close to the chip, not only for an easier matching of the load capacitance C0, but also to

ensure robustness against noise injection. Connect each capacitor of the Xtal oscillator to

ground by a separate via.

Regarding the low-frequency crystal oscillator the same consideration has to be done.

It is important to underline that the BlueNRG and BlueNRG-MS integrate an internal low-

frequency RC oscillator that can be used without external quartz. The customer can choose

to use the internal or an external one. The BlueNRG and BlueNRG-MS also integrate an

internal high-frequency RC oscillator, but it is disabling after an initial system bootstrap, and

it is necessary to use an external quartz for radio operations.

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