BC41B143A-ds-003Pc CSR, BC41B143A-ds-003Pc Datasheet - Page 51

• Manufacturer Part Number: BC41B143A-ds-003Pc
• Description: Blue Core ROM Plug-n-Go
• Manufacturer: CSR

10.3.3 Frequency Trim
BlueCore4-ROM Plug-n-Go 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 10.3.
Where Fx 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 10.4.
Where:
Note:
BC41B143A-ds-003Pc
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.
0
m
= Crystal self capacitance (shunt capacitance)
= Crystal motional capacitance (series branch capacitance in crystal model). See Figure 10.6.
trim
capacitors, which are both connected to ground. When viewed from the crystal terminals, they
∆
C
© Cambridge Silicon Radio Limited 2005
Equation 10.2: Trim Capacitance
F
( )
F
Equation 10.3: Frequency Trim
trim
X
X
trim
Equation 10.4: Pullability
=
=
. The value of C
110
pullabilit
Advance Information
∂
∂
fF
( )
( )
F
F
×
X
X
PSKEY
y
=
×
F
55
X
•
×
4
_
10
trim
(
C
AN A
−
I
C
3
+
is set by a 6-bit word in the PS Key
(
m
ppm
C
_
0
FT R IM
)
2
/
LSB
)
Device Terminal Descriptions
Page 51 of 94