BLUENRG-MSCSP datasheet(31/42 Pages) STMICROELECTRONICS

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BlueNRG-MS

Electrical specification

10.5.1

High speed crystal oscillator (HSXOSC)

The BlueNRG-MS includes a fully integrated, low power 16/32 MHz Xtal oscillator with an

embedded amplitude regulation loop. In order to achieve low power operation and good

frequency stability of the Xtal oscillator, certain considerations with respect to the quartz

load capacitance C0 need to be taken into account. Figure 13 shows a simplified block

diagram of the amplitude regulated oscillator used on the BlueNRG-MS.

Figure 13: Simplified block diagram of the amplitude regulated oscillator

Low power consumption and fast startup time is achieved by choosing a quartz crystal with

a low load capacitance C0. To achieve good frequency stability, the following equation

needs to be satisfied:

��0=��1′∗��2′

��1′+��2′

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.

DocID027103 Rev 6

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