3
Clock Generation
CSR8640 BGA requires a Bluetooth reference clock frequency of 16MHz to 32MHz from an externally connected
crystal.
All CSR8640 BGA internal digital clocks are generated using a phase locked loop, which is locked to the frequency
of the external reference clock source or safely free-runs at a reduced frequency if clock not present.
3.1
Clock Architecture
G-TW
-000018
9.3.3
Bluetooth
Radio
Auxiliary
PLL
Digital
Circuitry
Reference Clock
Figure 3.1: Clock Architecture
3.2
Input Frequencies and PS Key Settings
CSR8640 BGA is configured to operate with a chosen reference frequency. PSKEY_ANA_FREQ sets this reference
frequency for all frequencies using an integer multiple of 250kHz. The input frequency default setting for
CSR8640 BGA is 26MHz depending on the software build. Full details are in the software release note for the specific
build from
.
3.3
Crystal Oscillator: XTAL_IN and XTAL_OUT
CSR8640 BGA contains a crystal driver circuit that acts as a transconductance amplifier driving an external crystal
between XTAL_IN and XTAL_OUT. The crystal driver circuit forms a Pierce oscillator with the external crystal. No
external crystal load capacitors are required for typical crystals.
3.3.1
Crystal Calibration
The actual crystal frequency depends on the capacitance of XTAL_IN and XTAL_OUT on the PCB and the
CSR8640 BGA, as well as the capacitance of the crystal. Correct calibration of the Bluetooth radio is done on a per-
device basis on the production line, with the trim value stored in non-volatile memory (PS Key).
Crystal calibration uses a single measurement. The measurement finds the actual offset from the desired frequency
and the offset is stored in PSKEY_ANA_FTRIM_OFFSET. The firmware then compensates for the frequency offset
on the CSR8640 BGA. Typically, a TXSTART radio test is performed to obtain the actual frequency and it is compared
against the output frequency with the requested frequency using an RF analyser. The test station calculates the
offset ratio and programs it into PSKEY_ANA_FTRIM_OFFSET. The value in PSKEY_ANA_FTRIM_OFFSET is a
16-bit 2's complement signed integer which specifies the fractional part of the ratio between the true crystal
frequency, f
actual
, and the value set in PSKEY_ANA_FREQ, f
nominal
. Equation 3.1 shows the value of
PSKEY_ANA_FTRIM_OFFSET in parts per 2
20
rounded to the nearest integer.
For more information on TXSTART radio test see
BlueTest User Guide
.
Advance Information
This material is subject to CSR's non-disclosure agreement
© Cambridge Silicon Radio Limited 2011
Page 25 of 110
CS-209182-DSP1
CSR8640 BGA
Data Sheet
