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Frequency Selection

BCM20705 Data Sheet

BROADCOM CONFIDENTIAL

Broadcom®

Bluetooth Transceiver and Baseband Processor

November 13, 2014 • MCS20705-DS104-R

Page 38

TCXO Clock Request Support

If the application utilizes an external TCXO as a clock reference, the BCM20705 provides a clock request output

to allow the system to power off the TCXO when not in use.

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 BCM20705 must have the

reference frequency set correctly in order for any of the USB, UART, and PCM interfaces to function properly.

The BCM20705 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 BCM20705.

• 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

BCM20705 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 BCM20705 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 BCM20705 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 BCM20705 automatically detects the reference frequency and

programs itself to the correct reference frequency. In order for auto-frequency detection to work properly, the

BCM20705 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.

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