BCM43907 Preliminary Data Sheet
External 32.768 kHz Low-Power Oscillator
The reference frequency for the BCM43907 may be set in the following ways:
•
Set the xtalfreq=xxxxx parameter (in Hertz) in the nvram.txt file (used to load the driver) to correctly match
the crystal frequency.
•
Auto-detect any of the standard handset reference frequencies using an external LPO clock.
For applications such as handsets and portable smart communication devices, where the reference frequency
is one of the standard frequencies commonly used, the BCM43907 automatically detects the reference
frequency and programs itself to the correct reference frequency. In order for automatic frequency detection to
work correctly, the BCM43907 must have a valid and stable 32.768 kHz LPO clock that meets the requirements
listed in Table 4 on page 27 and is present during a power-on reset.
External 32.768 kHz Low-Power Oscillator
The BCM43907 uses a secondary low frequency clock for low-power-mode timing. Either the internal low-
precision LPO or an external 32.768 kHz precision oscillator is required. The internal LPO frequency range is
approximately 33 kHz ± 30% over process, voltage, and temperature, which is adequate for some applications.
However, one tradeoff caused by this wide LPO tolerance is a small current consumption increase during power
save mode that is incurred by the need to wake-up earlier to avoid missing beacons.
Whenever possible, the preferred approach is to use a precision external 32.768 kHz clock that meets the
requirements listed in Table 4.
Table 4: External 32.768 kHz Sleep Clock Specifications
Parameter
LPO Clock
Units
Nominal input frequency
Frequency accuracy
Duty cycle
32.768
kHz
ppm
%
±200
30–70
Input signal amplitude
Signal type
200–3300
mV, p-p
–
Square-wave or sine-wave
a
>100k
<5
Ω
Input impedance
pF
Clock jitter (during initial start-up)
<10,000
ppm
a. When power is applied or switched off.
Broadcom®
March 12, 2016 • 43907-DS104-R
Page 27
BROADCOM CONFIDENTIAL
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