CYW20702
6.4 Frequency Selection
Any frequency within the range specified for the crystal and TCXO reference can be used. These frequencies include standard handset
reference frequencies (12, 13, 14.4, 15.36, 16.2, 16.8, 18, 19.2, 19.44, 19.68, 19.8, 20, 24, 26, 33.6, 37.4, and 38.4 MHz) and any
frequency between these reference frequencies, as desired by the system designer. Since bit timing is derived from the reference
frequency, the CYW20702 must have the reference frequency set correctly in order for any of the USB, UART, and PCM interfaces
to function properly.
The CYW20702 reference frequency can be set in one of the following ways.
■ Use the default 20 MHz frequency by leaving the CFG_SEL pin unconnected or by strapping it high. The CFG_SEL pin is internally
pulled up in the CYW20702.
■ Use the 26 MHz option by setting CFG_SEL low.
■ Designate the reference frequency in external NVRAM
■ Auto-detect the standard handset reference frequencies using an external LPO clock
For PC and embedded applications where there typically won’t be an LPO clock, if autobaud is enabled, the CYW20702 will use XTAL
to sync up with the UART, thus allowing a user to download a firmware configuration without having to make a crystal frequency
assumption. Alternatively, the CYW20702 will upload the firmware configuration from an attached NVRAM (if one is attached) and
use it to make a crystal frequency assumption. Finally, if neither of the above is true, then the CYW20702 will look at the status of the
CFG_SEL pin and decide whether to choose 20 or 26 MHz. Autobaud is only valid for UART applications.
The 20 MHz choice is only a default in the sense that if the pin is left floating, it will be pulled up internally to support the 20 MHz option.
If the application requires a frequency other than the default, 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.
For applications such as handsets and portable smart communication devices, where the reference frequency is one of the standard
frequencies commonly used, the CYW20702 automatically detects the reference frequency and programs itself to the correct
reference frequency. In order for auto-frequency detection to work properly, the CYW20702 must have a valid and stable 32.768 kHz
external LPO clock present during POR. This eliminates the need for NVRAM in applications where the external LPO clock is available
and an external NVRAM is typically not used.
6.5 Frequency Trimming
The CYW20702 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 external frequency source such as a TCXO. Unlike the typical crystal-
trimming methods used, the CYW20702 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 TCXO or
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 CYW20702 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: 002-14773 Rev. *L
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