IDT EB8T5A Eval Board, Руководство пользователя

Страница: 14 / 48

IDT Installation of the EB8T5A Eval Board
EB8T5A Eval Board Manual 2 - 2 July 23, 2009
Notes The source for the onboard clock is the ICS9FG104 clock generator device (U8) connected to a 25MHz
oscillator (Y1). When using the onboard clock generator, the EB8T5A allows selection between multiple
clock rates and spread spectrum settings via DIP switches as described in Tables 2.2 and 2.3 respectively.
Spread Spectrum technology reduces peak EMI emissions by modulating the frequency to spread the peak
energy over a wider bandwidth.
If the Clock Spread Spectrum is used to modulate data rate, then both ports must use same modulated
clock source. Therefore, if your system uses SSC, the on-board clock generator must be disabled and the
upstream reference clock should be used instead.
The output of the onboard clock generator is accessible through two SMA connectors located on the
Evaluation Board. See Table 2.4. This can be used to connect a scope for probing or capturing purposes
and cannot be used to drive the clock from an external source.
Figure 2.1 illustrates the clock distribution block diagram for the EB8T5A evaluation board.
Clock Frequency Switch - S3[1]
S3[1] Clock Frequency
OFF 125MHz
ON 100MHz (Default)
Table 2.2 Clock Frequency Selection
Clock Spread Spectrum Switch - S3[2]
S3[2] Spread Spectrum
OFF Enable Spread Spectrum
ON Disable Spread Spectrum (Default)
Table 2.3 Spread Spectrum Clock Selection
Onboard Reference Clock Output (Differential) – J18, J19
J19 Positive Reference Clock
J18 Negative Reference Clock
Table 2.4 SMA Connectors - Onboard Reference Clock