Document Number: 002-14790 Rev. *A

Page 21 of 50

CYW20706

6.2 Crystal Oscillator

The CYW20706 can use an external crystal to provide a frequency reference. The recommended configuration for the crystal oscillator,

including all external components, is shown in Figure 5.

Figure 5. Recommended Oscillator Configuration

6.3 Frequency Selection

Any frequency within the range specified for the crystal and frequency reference can be used. Since bit timing is derived from the

reference frequency, the CYW20706 must have the reference frequency set correctly in order for any of the USB, UART, and PCM

interfaces to function properly.

The CYW20706 reference frequency can be selected by using BT_XTAL_STRAP_1. The typical crystal frequencies of 20 MHz and

40 MHz are supported.

The GPIO_2 needs to be tied to ground.

Clock (MHz): XTAL_Strap_1 (Pin-F2)

40:Low

20:High

If the application requires a frequencies other than these, the value can be stored in an external NVRAM. Programming the reference

frequency in NVRAM provides the maximum flexibility in the selection of the reference frequency, since any frequency within the

specified range for crystal and external frequency reference can be used. During power-on reset (POR), the device downloads the

parameter settings stored in NVRAM, which can be programmed to include the reference frequency and frequency trim values.

Typically, this is how a PC Bluetooth application is configured.

6.4 Frequency Trimming

The CYW20706 uses a fractional-N synthesizer to digitally fine-tune the frequency reference input to within ±2 ppm tuning accuracy.

This trimming function can be applied to either the crystal or an reference frequency source. Unlike the typical crystal-trimming

methods used, the CYW20706 changes the frequency using a fully digital implementation and is much more stable and unaffected

by crystal characteristics or temperature. Input impedance and loading characteristics remain unchanged on the crystal during the

trimming process and are unaffected by process and temperature variations.

The option to use or not use frequency trimming is based on the system designer’s cost trade-off between bill-of-materials (BOM) cost

of the crystal and the added manufacturing cost associated with frequency trimming. The frequency trimming value can either be

stored in the host and written to the CYW20706 as a vendor-specific HCI command or stored in NVRAM and subsequently recalled

during POR.

Frequency trimming is not a substitute for the poor use of tuning capacitors at an crystal oscillator (XTAL). Occasionally, trimming can

help alleviate hardware changes.

0  ~ 18 pF*

0  ~ 18 pF*

XOIN

XOOUT

Crystal 

Oscillator

*Capacitor value range depends on 

the manufacturer of the XTAL as well 

as board layout.