MSC8251_11 Datasheet, PDF (57/68 Pages) Freescale Semiconductor, Inc – Single-Core Digital Signal Processor

Hardware Design Considerations
3.3 Clock and Timing Signal Board Layout Considerations
When laying out the system board, use the following guidelines:
• Keep clock and timing signal paths as short as possible and route with 50 Ω impedance.
• Use a serial termination resistor placed close to the clock buffer to minimize signal reflection. Use the following
equation to compute the resistor value:
Rterm = Rim – Rbuf
where Rim = trace characteristic impedance
Rbuf = clock buffer internal impedance.
3.4 SGMII AC-Coupled Serial Link Connection Example
Figure 39 shows an example of a 4-wire AC-coupled serial link connection. For additional layout suggestions, see AN3556
MSC815x High Speed Serial Interface Hardware Design Considerations, available on the Freescale website or from your local
sales office or representative.
SR[1–2]_TX[[1–2]
50 Ω
Transmitter
50 Ω
SGMII
SerDes Interface
SR[1–2]_TX[1–2]
SR[1–2]_RX[1–2]
50 Ω
Receiver
SR[1–2]_RX[1–2]]
50 Ω
C_TX ^{SR[1–2]_RX[1–2]}
SR[1–2]_RX[1–2]
50 Ω
Receiver
C_TX
50 Ω
C_TX
50 Ω
SR[1–2]_TX[1–2]
Transmitter
C_TX
50 Ω
SR[1–2]_TX[1–2]
Figure 39. 4-Wire AC-Coupled SGMII Serial Link Connection Example
MSC8251 Single-Core Digital Signal Processor Data Sheet, Rev. 6
Freescale Semiconductor
57