BC41B143A-ds-001Pe CSR, BC41B143A-ds-001Pe Datasheet - Page 58

• Manufacturer Part Number: BC41B143A-ds-001Pe
• Description: Blue Core ROM
• Manufacturer: CSR

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
capacitors labelled C
maximises the signal swing, hence slew rate at XTAL_IN, to which all on chip clocks are referred. Crystal load
capacitance, C
Where:
Note:
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
PSKEY_ANA_FTRIM (0x1f6). Its value is calculated thus:
There are two C
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:
Where F
times the value above.
If not specified, the pullability of a crystal may be calculated from its motional capacitance with Equation 10.6:
Where:
Note:
BC41B143A-ds-001Pe
Ctrim = 3.4pF nominal (Mid range setting)
C
C
C
C
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.
int
int
0
m
= Crystal self capacitance (shunt capacitance)
= Crystal motional capacitance (series branch capacitance in crystal model). See Figure 10.7.
= 1.5pF
does not include the crystal internal self capacitance, it is the driver self capacitance
x
is the crystal frequency and pullability is a crystal parameter with units of ppm/pF. Total trim range is 63
l
is calculated with the following equation:
trim
capacitors, which are both connected to ground. When viewed from the crystal terminals, they
t1
and C
t2
. C
t1
This material is subject to CSR’s non-disclosure agreement
should be three times the value of C
Δ
F
( )
Equation 10.3: Load Capacitance
C
Equation 10.4: Trim Capacitance
F
x
Equation 10.5: Frequency Trim
x
trim
© Cambridge Silicon Radio Limited 2005
trim
=
Equation 10.6: Pullability
C
=
pullabilit
l
. The value of C
∂
110
∂
=
( )
( )
F
trim
C
C
x
Production Information
int
fF
and C
=
+
×
y
F
PSKEY_ANA_
x
×
C
⋅
55
trim
2
4
int
(
. The remainder should be from the external
C
×
+
10
l
C
trim
+
m
C
C
−
C
1 t
3
1 t
is set by a 6-bit word in the Persistent Store Key
0
(
ppm
+
)
⋅
2
C
C
2 t
FTRIM
2 t
/
t2
LSB
for best noise performance. This
)
Device Terminal Descriptions
Page 58 of 102