BC41B143A-DS-001PE Datasheet, PDF (58/102 Pages) List of Unclassifed Manufacturers – BlueCpre 4-ROM Single Chip Bluetooth v2.0 System with EDR

Device Terminal Descriptions
10.3.2 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. BlueCore4-ROM
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 10.3: 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
10.3.3 Frequency Trim
BlueCore4-ROM 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 10.4: 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 10.5:
Δ(F_x ) _{= pullability × 55 ×10−3(ppm / LSB)}
F_x
Equation 10.5: 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 10.6:
∂(F_x )
∂(C)
=
F_x
⋅
C_m
4(C_l + C₀
)²
Equation 10.6: Pullability
Where:
C₀ = Crystal self capacitance (shunt capacitance)
C_m = Crystal motional capacitance (series branch capacitance in crystal model). See Figure 10.7.
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
with ageing and temperature. A crystal with an ageing and temperature drift specification of better than
±15ppm is required.
BC41B143A-ds-001Pe
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© Cambridge Silicon Radio Limited 2005
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