User's Manual

9/29/2004 OV4F2
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User Manual BCM2035
04/23/03
Broadcom Corporation
Document 2035-UM100-R System Clock Configuration Page 23
The Internal/External LPO selection allows a design to take advantage of a precise external 32.768 kHz
oscillator if one is present in the design. The internal LPO may be used in a design in which no external 32.768
kHz oscillator is available. The advantage of using an external LPO when possible is that the higher precision
will allow longer intervals to be used in the power savings modes (Park, Sniff, Hold).
See
“Low Power Oscillator” on page 24 regarding LPO connections for more details about how the BCM2035
I/O should be connected for internal or external LPO designs.
The XTAL_PD pin is an output that is used by the BCM2035 to indicate when the main clock may be shut off
in a design that is feeding a clock signal to the
BCM2035 rather than using the build in crystal oscillator circuit
with an external crystal. The mode pins are used to select the polarity of the XTAL_PD pin.
The XTAL_PD signal is not needed in a design using the BCM2035 oscillator circuit with an external crystal,
because the
BCM2035 shuts down the oscillator circuit internally when it is not needed.
SYSTEM CLOCK CONFIGURATION
The
BCM2035 uses a fractional-N synthesizer to generate the radio frequencies, clocks, and data/packet
timing. This enables it to operate from any of a multitude of frequency sources. This may either be an external
source such as a TCXO or a crystal interfaced directly to the
BCM2035.
The default frequency reference setting is 15.36 MHz crystal or TCXO, a widely used frequency reference for
3G handset platforms.
CRYSTAL OSCILLATOR
The
BCM2035 can use an external crystal to provide a frequency reference. The recommended configuration
for the crystal oscillator including all components is shown in
Figure 6.
Figure 6: BCM2035 Crystal Oscillator Configuration
XTAL_IN
XTAL_OUT
BCM2035
Crystal
C1
C2