AN95089 datasheet(3/19 Pages) RAMTRON

3 / 19 page

PSoC® 4/PRoC™ BLE Crystal Oscillator Selection and Tuning Techniques

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Document No. 001-95089 Rev. *A

Figure 3. Total Load Capacitance Including Parasitic Capacitance and Pin Capacitance

CPIN1

CPIN2

CPCB1

CPCB2

Inverting Amplifier

XTAL

CT1 = C1 + CPCB1 + CPIN1

CT2 = C2 + CPCB2 + CPIN2

Equation 1:

Where,

C1, C2 = Node Capacitance at XI and XO

CT1, CT2 = Total node capacitance (including pin capacitance and parasitic capacitance)

CPCB1 , CPCB2 = Parasitic Capacitance between PCB pads of the crystal

CPIN1, CPIN2 = Input capacitance of the oscillator pins

CL = Total load capacitance seen by the crystal

The load capacitance required to generate an accurate crystal frequency is specified in the crystal datasheet.

Capacitors C1 and C2 in Figure 1 should be chosen such that the value of CL from Equation 1 matches the datasheet

value. The oscillator of PSoC4/PRoC BLE devices is designed to work with an 8-pF load capacitance, which requires

that the chosen crystal has 8-pF load capacitance.

The crystal will not oscillate at the frequency specified in the crystal datasheet if the passive crystal load circuitry does

not provide the load capacitance (CL) that is required for the crystal. Too low a capacitive load will result in a crystal

oscillator frequency higher than the specified value, while too high a capacitive load will result in a lower oscillation

frequency. This frequency offset will be directly translated to an offset in the RF carrier frequency and symbol timing

accuracy of the device.

AN95089 Datasheet(PDF) 3 Page - Ramtron International Corporation