MPC8379E datasheet(72/117 Pages) FREESCALE

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MPC8379E PowerQUICC II Pro Processor Hardware Specifications, Rev. 8

Freescale Semiconductor

greater than 200 mV. This requirement is the same for both external DC-coupled or

AC-coupled connection.

— For external DC-coupled connection, as described in Section 20.2.1, “SerDes Reference

Clock Receiver Characteristics,” the maximum average current requirements sets the

requirement for average voltage (common mode voltage) to be between 100 mV and 400 mV.

Figure 50 shows the SerDes reference clock input requirement for DC-coupled connection

scheme.

— For external AC-coupled connection, there is no common mode voltage requirement for the

clock driver. Since the external AC-coupling capacitor blocks the DC level, the clock driver

and the SerDes reference clock receiver operate in different command mode voltages. The

SerDes reference clock receiver in this connection scheme has its common mode voltage set to

SGND_SRDSn. Each signal wire of the differential inputs is allowed to swing below and above

the command mode voltage (SGND_SRDSn). Figure 51 shows the SerDes reference clock

input requirement for AC-coupled connection scheme.

•

Single-ended Mode

— The reference clock can also be single-ended. The SD _REF_CLK input amplitude

(single-ended swing) must be between 400 mV and 800 mVp-p (from Vmin to Vmax) with

SDn_REF_CLK either left unconnected or tied to ground.

—The SDn_REF_CLK input average voltage must be between 200 mV and 400 mV. Figure 52

shows the SerDes reference clock input requirement for single-ended signaling mode.

— To meet the input amplitude requirement, the reference clock inputs might need to be DC or

AC-coupled externally. For the best noise performance, the reference of the clock could be DC

or AC-coupled into the unused phase (SDn_REF_CLK) through the same source impedance as

the clock input (SDn_REF_CLK) in use.

Figure 50. Differential Reference Clock Input DC Requirements (External DC-Coupled)

n_REF_CLK

Vmax < 800 mV

Vmin > 0 V

100 mV < Vcm < 400 mV

200 mV < Input Amplitude or Differential Peak < 800 mV

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