Sundance SMT123-SHB - Manual (Page 10)

User Guide SMT123-SHB Page 10 of 13 11/06/2004
4 CLOCKs
The board contains two crystals to allow data transfer through the fibre module and/or the
SHB connector to other boards. The fibre module operates at 53.125MHz (half speed) or
106.25 MHZ (full speed). This corresponds to the REF signal mentioned in section 2 of this
document. There are 3 resistors that play a key role to achieve proper operation of the
VSC7146 according to the value of this crystal (X1 on the component side):
Resistor Speed Position
HALF 0 R19
FULL 1
HALF 0 R20
FULL 1
HALF 1 R25
FULL 0
The SHB interface is usually clocked at 50 or 100 MHz (X2 on the bottom of the board) and it
is connected to the FPGA pin A8.
5 PCB Layout Details
5.1 Component Side
Figure 5 – PCB Layout, Top