MPC8548ECPXATHC datasheet(67/151 Pages) FREESCALE

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MPC8548E PowerQUICC III Integrated Processor Hardware Specifications, Rev. 9

Freescale Semiconductor

High-Speed Serial Interfaces (HSSI)

— The SD_REF_CLK and SD_REF_CLK are internally AC-coupled differential inputs as shown

in Figure 39. Each differential clock input (SD_REF_CLK or SD_REF_CLK) has a 50-



termination to SGND_SRDSn (xcorevss) followed by on-chip AC-coupling.

— The external reference clock driver must be able to drive this termination.

— The SerDes reference clock input can be either differential or single-ended. See the differential

mode and single-ended mode description below for further detailed requirements.

•

The maximum average current requirement that also determines the common mode voltage range:

— When the SerDes reference clock differential inputs are DC coupled externally with the clock

driver chip, the maximum average current allowed for each input pin is 8 mA. In this case, the

exact common mode input voltage is not critical as long as it is within the range allowed by the

maximum average current of 8 mA (see the following bullet for more detail), since the input is

AC-coupled on-chip.

— This current limitation sets the maximum common mode input voltage to be less than 0.4 V

(0.4 V/50 = 8 mA) while the minimum common mode input level is 0.1 V above

SGND_SRDSn (xcorevss). For example, a clock with a 50/50 duty cycle can be produced by

a clock driver with output driven by its current source from 0 to 16 mA (0–0.8 V), such that

each phase of the differential input has a single-ended swing from 0 V to 800 mV with the

common mode voltage at 400 mV.

— If the device driving the SD_REF_CLK and SD_REF_CLK inputs cannot drive 50

 to

SGND_SRDSn (xcorevss) DC, or it exceeds the maximum input current limitations, then it

must be AC-coupled off-chip.

•

The input amplitude requirement:

— This requirement is described in detail in the following sections.

Figure 39. Receiver of SerDes Reference Clocks

16.2.2

DC Level Requirement for SerDes Reference Clocks

The DC level requirement for the SerDes reference clock inputs is different depending on the signaling

mode used to connect the clock driver chip and SerDes reference clock inputs as described below:

•

Differential mode

Input

Amp



SD_REF_CLK

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