CYW20704
6.2 Crystal Oscillator
The CYW20704 can use an external crystal to provide a frequency reference. The recommended configuration for the crystal oscil-
lator, including all external components, is shown in Figure 6.
Figure 6. Recommended Oscillator Configuration
XOIN
0 ~18 pF*
Crystal
Oscillator
XOOUT
0 ~18 pF*
*Capacitor value range depends on
the manufacturer of the XTAL as well
as board layout.
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 ref-
erence frequency, the CYW20704 must have the reference frequency set correctly in order for any of the USB, UART, and PCM
interfaces to function properly.
The CYW20704 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 pin needs to be tied to ground when a dedicated Bluetooth crystal is used.
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 refer-
ence 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 CYW20704 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 meth-
ods used, the CYW20704 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 trim-
ming 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 CYW20704 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.
Document Number: 002-14786 Rev. *E
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