BCM4325
Preliminary Data Sheet
6/30/09
FREQUENCY SELECTION
Any frequency within the ranges specified for the crystal and TCXO reference may be used. These include not only the
standard handset references frequencies of 12, 13, 14.4, 16.2, 16.8, 18, 19.2, 19.44, 19.68, 19.8, 20, 24, 26, 38.4, and
52 MHz, but any other frequency between these as desired by the system designer. The BCM4325 must have the reference
frequency set correctly in order for any of the UART or PCM interfaces to function correctly, since all bit timing is derived
from the reference frequency.
The reference frequency for the BCM4325 may be set in one of the following ways:
•
•
Specify the frequency in the nvram.txt file.
Auto-detect the standard handset reference frequencies using an external LPO clock.
The BCM4325 is set at the factory to a default frequency of 26 MHz. For a typical design using a crystal it is recommended
that the default frequency be used.
For applications such as handsets and portable smart communication devices, where the reference frequency is one of the
standard frequencies commonly used, the BCM4325 automatically detects the reference frequency and programs itself to
the correct reference frequency. In order for auto-frequency detection to work correctly, the BCM4325 must have a valid and
stable 32.768-kHz LPO clock present during power-on reset.
FREQUENCY TRIMMING
The BCM4325 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 BCM4325 changes the frequency using a fully digital implementation and is
much more stable and unaffected by either the crystal characteristics or the temperature. The input impedance and loading
characteristics remain unchanged on either the TCXO or the crystal during the trimming process and are unaffected by
process and temperature variations.
The option of whether to use frequency trimming would be determined by a cost trade-off between the cost of the crystal and
the added manufacturing cost associated with frequency trimming. Frequency trimming value can be stored in the host and
written back to the BCM4325.
Broadcom Corporation
Page 8
Frequency Selection
Document 4325-DS04-R
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