PCI7510 datasheet(71/234 Pages) TI

71 / 234 page

3–31

1 M

Ω

0.001

µF

0.01

µF

Outer Cable Shield

Chassis Ground

Figure 3–19. Typical Compliant DC Isolated Outer Shield Termination

Outer Cable Shield

Chassis Ground

Figure 3–20. Non-DC Isolated Outer Shield Termination

3.10.2 Crystal Selection

The PCI7510 device is designed to use an external 24.576-MHz crystal connected between the XI and XO terminals

to provide the reference for an internal oscillator circuit. This oscillator in turn drives a PLL circuit that generates the

various clocks required for transmission and resynchronization of data at the S100 through S400 media data rates.

A variation of less than

±100 ppm from nominal for the media data rates is required by IEEE Std 1394-1995. Adjacent

PHYs may therefore have a difference of up to 200 ppm from each other in their internal clocks, and PHY devices

must be able to compensate for this difference over the maximum packet length. Large clock variations may cause

resynchronization overflows or underflows, resulting in corrupted packet data.

The following are some typical specifications for crystals used with the PHYs from TI in order to achieve the required

frequency accuracy and stability:

•

Crystal mode of operation: Fundamental

•

Frequency tolerance @ 25

°C: Total frequency variation for the complete circuit is ±100 ppm. A crystal with

±30 ppm frequency tolerance is recommended for adequate margin.

•

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.

•

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 (CL) is a function of not only the

discrete load capacitors, but also board layout and circuit. It is recommended that load capacitors with a

maximum of

±5% tolerance be used.

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