MPC8640DEC datasheet(66/140 Pages) FREESCALE

66 / 140 page

MPC8640 and MPC8640D Integrated Host Processor Hardware Specifications, Rev. 1

Freescale Semiconductor

High-Speed Serial Interfaces (HSSI)

Figure 45 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

MPC8640D SerDes reference clock input’s DC requirement, AC-coupling has to be used. Figure 45

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 240Ω 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 MPC8640D SerDes

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

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

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

Ω. Please

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

particular clock driver chip.

Figure 45. AC-Coupled Differential Connection with LVPECL Clock Driver (Reference Only)

n_REF_CLK

Clock Driver

100

Ω differential PWB trace

SerDes Refer.

CLK Receiver

Clock Driver

CLK_Out

LVPECL CLK

Driver Chip

MPC8640D

10nF

MPC8640DEC Datasheet(PDF) 66 Page - Freescale Semiconductor, Inc