BC41B143A-DS-002PD Datasheet, PDF (55/89 Pages) List of Unclassifed Manufacturers – BlueCore 4-ROM CSP EDR Single Chip Bluetooth v2.0 + EDR System

Device Terminal Descriptions
Figure 11.6 shows an electrical equivalent circuit for a crystal. The crystal appears inductive near its resonant
frequency. It forms a resonant circuit with its load capacitors.
C_m
L_m
R_m
C_o
Figure 11.6: Crystal Equivalent Circuit
The resonant frequency can be trimmed with the crystal load capacitance. BlueCore4-ROM CSP contains
variable internal capacitors to provide a fine trim.
Frequency
Initial Tolerance
Pullability
Min
Typ
8MHz
-
-
16MHz
±25ppm
±20ppm/pF
Table 11.4: Crystal Specification
Max
32MHz
-
-
The BlueCore4-ROM CSP driver circuit is a transconductance amplifier. A voltage at XTAL_IN generates a
current at XTAL_OUT. The value of transconductance is variable and may be set for optimum performance.
11.3.2 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
CSP 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 11.3: Load Capacitance
Where:
C_trim = 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.
11.3.3 Frequency Trim
BlueCore4-ROM CSP 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 PS Key
PSKEY_ANA_FTRIM (0x1f6). Its value is calculated as:
C_trim = 110 fF × PSKEY_ANA_FTRIM
Equation 11.4: 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.
BC41B143A-ds-002Pd
This material is subject to CSR’s non-disclosure agreement
Production Information
© Cambridge Silicon Radio Limited 2005
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