MPC8308 datasheet(31/88 Pages) FREESCALE

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MPC8308 PowerQUICC II Pro Processor Hardware Specification, Rev. 1

Freescale Semiconductor

High-Speed Serial Interfaces (HSSI)

Figure 22 shows the SerDes reference clock connection reference circuits for LVPECL type clock driver.

Since LVPECL driver’s DC levels (both common mode voltages and output swing) are incompatible with

MPC8308 SerDes reference clock input’s DC requirement, AC-coupling has to be used. Figure 22

assumes that the LVPECL clock driver’s output impedance is 50

Ω. R1 is used to DC-bias the LVPECL

outputs prior to AC-coupling. Its value could be ranged from 140

Ω to 240 Ω depending on clock driver

vendor’s requirement. R2 is used together with the SerDes reference clock receiver’s 50-

Ω termination

resistor to attenuate the LVPECL output’s differential peak level such that it meets the MPC8308’s SerDes

reference clock’s differential input amplitude requirement (between 200 mV and 800 mV differential

peak). For example, if the LVPECL output’s differential peak is 900 mV and the desired SerDes reference

clock input amplitude is selected as 600 mV, the attenuation factor is 0.67, which requires R2 = 25

Ω.

Consult clock driver chip manufacturer to verify whether this connection scheme is compatible with a

particular clock driver chip.

Figure 22. AC-Coupled Differential Connection with LVPECL Clock Driver (Reference Only)

SD_REF_CLK

Clock Driver

100

Ω differential PWB trace

SerDes Refer.

CLK Receiver

Clock Driver

CLK_Out

LVPECL CLK

Driver Chip

MPC8308

10nF

10 nF

MPC8308 Datasheet(PDF) 31 Page - Freescale Semiconductor, Inc