BCM20705 Datasheet, PDF (40/67 Pages) Cypress Semiconductor – Single-Chip Bluetooth Transceiver and Baseband Processor

BCM20705 Data Sheet
Frequency Trimming
Frequency Trimming
The BCM20705 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 BCM20705 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 BCM20705 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.
LPO Clock Interface
The LPO clock is the second frequency reference that the BCM20705 uses to provide low-power mode timing
for sniff. The LPO clock can be provided to the device externally, from a 32.768 kHz source or the BCM20705
can operate using the internal LPO clock.
The LPO can be internally driven from the main clock. However, sleep current will be impacted.
The accuracy of the internal LPO limits the maximum sniff intervals.
Table 5: External LPO Signal Requirements
Parameter
Nominal input frequency
Frequency accuracy
Input signal amplitude
Signal type
Input impedance (when power is applied or power is off)
External LPO Clock
32.768
±250
200 to 3600
Square-wave or sine-wave
>100
<5
Units
kHz
ppm
mVp-p
–
kΩ
pF
Broadcom®
Bluetooth Transceiver and Baseband Processor
November 13, 2014 • MCS20705-DS104-R
Page 39
BROADCOM CONFIDENTIAL