TSB41AB1_14 Datasheet, PDF (30/71 Pages) Texas Instruments – IEEE 1394a-2000 ONE-PORT CABLE TRANSCEIVER/ARBITER

TSB41AB1
IEEE 1394aĆ2000 ONEĆPORT CABLE
TRANSCEIVER/ARBITER
SLLS423I − JUNE 2000 − REVISED MARCH 2005
APPLICATION INFORMATION
crystal selection (continued)
The following are some typical specifications for crystals used with the physical layers from TI in order to achieve
the required frequency accuracy and stability:
D Crystal mode of operation: Fundamental
D Frequency tolerance at 25°C: Total frequency variation for the complete circuit is ± 100 ppm. A crystal with
± 30-ppm frequency tolerance is recommended for adequate margin.
D Frequency stability (over temperature and age): A crystal with ± 30-ppm frequency stability is recommended
for adequate margin.
NOTE:The total frequency variation must be kept below ± 100 ppm from nominal with some
allowance for error introduced by board and device variations. Trade-offs between frequency
tolerance and stability may be made as long as the total frequency variation is less than ± 100 ppm.
For example, the frequency tolerance of the crystal may be specified at 50 ppm and the temperature
tolerance may be specified at 30 ppm to give a total of 80 ppm possible variation due to the crystal
alone. Crystal aging also contributes to the frequency variation.
D Load capacitance: For parallel resonant mode crystal circuits, the frequency of oscillation is dependent
upon the load capacitance specified for the crystal. Total load capacitance (C_L) is a function not only of the
discrete load capacitors, but also of the board layout and circuit. It may be necessary to select discrete load
capacitors iteratively until the SYSCLK output is within specification. It is recommended that load capacitors
with a maximum of ± 5% tolerance be used.
As an example, for the OHCI + 41AB1 evaluation module (EVM), which uses a crystal specified for 12 pF
loading, load capacitors (C9 and C10 in Figure 15) of 16 pF each were appropriate for the layout of that particular
board. The load specified for the crystal includes the load capacitors (C9, C10), the loading of the PHY terminals
(C_(PHY)), and the loading of the board itself (C_(BD)). The value of C_(PHY) is typically about 1 pF, and C_(BD) is
typically 0.8 pF per centimeter of board etch; a typical board can have 3 pF to 6 pF or more. The load capacitors
C9 and C10 combine as capacitors in series so that the total load capacitance is:
CL + ^ƪ(C9 C10 ) ń (C9 ) C10 )^ƫ ) C(PHY) ) C(BD)
(1)
C9
XI
24.576 MHz
X1
C(PHY) + C(BD)
XO
C10
Figure 15. Load Capacitance for the TSB41AB1 PHY
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