MPC8640DEC datasheet(61/140 Pages) FREESCALE

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

Freescale Semiconductor

High-Speed Serial Interfaces (HSSI)

To illustrate these definitions using real values, consider the case of a CML (Current Mode Logic)

transmitter that has a common mode voltage of 2.25 V and each of its outputs, TD and TD, has a swing

that goes between 2.5V and 2.0V. Using these values, the peak-to-peak voltage swing of each signal (TD

or TD) is 500 mV p-p, which is referred as the single-ended swing for each signal. In this example, since

the differential signaling environment is fully symmetrical, the transmitter output’s differential swing

(VOD) has the same amplitude as each signal’s single-ended swing. The differential output signal ranges

between 500 mV and –500 mV, in other words, VOD is 500 mV in one phase and –500 mV in the other

phase. The peak differential voltage (VDIFFp) is 500 mV. The peak-to-peak differential voltage (VDIFFp-p)

is 1000 mV p-p.

13.2

SerDes Reference Clocks

The SerDes reference clock inputs are applied to an internal PLL whose output creates the clock used by

the corresponding SerDes lanes. The SerDes reference clocks inputs are SDn_REF_CLK and

SDn_REF_CLK for PCI Express and Serial RapidIO.

The following sections describe the SerDes reference clock requirements and some application

information.

13.2.1

SerDes Reference Clock Receiver Characteristics

Figure 39 shows a receiver reference diagram of the SerDes reference clocks.

•

The supply voltage requirements for XVDD_SRDSn are specified in Table 1 and Table 2.

•

SerDes Reference Clock Receiver Reference Circuit Structure

—The SDn_REF_CLK and SDn_REF_CLK are internally AC-coupled differential inputs as

shown in Figure 39. Each differential clock input (SDn_REF_CLK or SDn_REF_CLK) has a

50-

Ω termination to SGND 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. Refer to 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 8mA. 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 (refer to 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 0.4V

(0.4V/50 = 8mA) while the minimum common mode input level is 0.1V above SGND. 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 0mA to 16mA (0-0.8V), such that each phase of the differential input

has a single-ended swing from 0V to 800mV with the common mode voltage at 400mV.

— If the device driving the SDn_REF_CLK and SDn_REF_CLK inputs cannot drive 50 ohms to

SGND DC, or it exceeds the maximum input current limitations, then it must be AC-coupled

off-chip.

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