BC352239A Datasheet, PDF (53/116 Pages) List of Unclassifed Manufacturers – BlueCore 3-Multimedia External

Device Terminal Descriptions
8.3.2 Load Capacitance
For resonance at the correct frequency the crystal should be loaded with its specified load capacitance, which is
defined for the crystal. This is the total capacitance across the crystal viewed from its terminals.
BlueCore3-Multimedia External provides some of this load with the capacitors C_trim and C_int. The remainder
should be from the external capacitors labelled C_t1 and C_t2. C_t1 should be three times the value of C_t2 for best
noise performance. This maximises the signal swing, hence slew rate at XTAL_IN, to which all on chip clocks are
referred. Crystal load capacitance, C_l is calculated with the following equation:
C_l
= C_int
+
C_trim
2
+
C_t1 ⋅ C_t2
C_t1 + C_t2
Equation 8.3: Load Capacitance
Where:
Ctrim = 3.4pF nominal (Mid range setting)
C_int = 1.5pF
Note:
C_int does not include the crystal internal self capacitance, it is the driver self capacitance.
8.3.3 Frequency Trim
BlueCore3-Multimedia External enables frequency adjustments to be made. This feature is typically used to
remove initial tolerance frequency errors associated with the crystal. Frequency trim is achieved by adjusting the
crystal load capacitance with on chip trim capacitors, C_trim. The value of C_trim is set by a 6-bit word in the
Persistent Store Key PSKEY_ANA_FTRIM (0x1f6). Its value is calculated thus:
C_trim = 110 fF × PSKEY_ANA_FTRIM
Equation 8.4: Trim Capacitance
There are two C_trim capacitors, which are both connected to ground. When viewed from the crystal terminals, they
appear in series so each least significant bit (LSB) increment of frequency trim presents a load across the crystal
of 55fF.
The frequency trim is described by Equation 8.5:
∆(F_x ) _{= pullability × 55 ×10−3(ppm / LSB)}
F_x
Equation 8.5: Frequency Trim
Where F_x is the crystal frequency and pullability is a crystal parameter with units of ppm/pF. Total trim range is 63
times the value above.
If not specified, the pullability of a crystal may be calculated from its motional capacitance with Equation 8.6:
∂(F_x )
∂(C)
=
F_x
⋅
C_m
4(C_l + C₀
)²
Equation 8.6: Pullability
Where:
C₀ = Crystal self capacitance (shunt capacitance)
C_m = Crystal motional capacitance (series branch capacitance in crystal model). See Figure 8..
Note:
It is a Bluetooth requirement that the frequency is always within ±20ppm. The trim range should be sufficient
to pull the crystal within ±5ppm of the exact frequency. This leaves a margin of ±15ppm for frequency drift
BC352239A-ds-001Pc
© Cambridge Silicon Radio Limited 2004
Production Information
Page 53 of 116