AN95089 Datasheet, PDF (3/19 Pages) Ramtron International Corporation – BLE Crystal Oscillator Selection and Tuning Techniques

PSoC® 4/PRoC™ BLE Crystal Oscillator Selection and Tuning Techniques
Figure 3. Total Load Capacitance Including Parasitic Capacitance and Pin Capacitance
Inverting Amplifier
R
CT1 = C1 + CPCB1 + CPIN1
CT2 = C2 + CPCB2 + CPIN2
Equation 1:
XI
CPIN1 CPCB1
C1
XTAL
XO
C2
CPCB2 CPIN2
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.
www.cypress.com
Document No. 001-95089 Rev. *A
3