MSC8152SVT1000B datasheet(29/68 Pages) FREESCALE

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Electrical Characteristics

MSC8152 Dual-Core Digital Signal Processor Data Sheet, Rev. 1

Freescale Semiconductor

Using this waveform, the definitions are listed in Table 10. To simplify the illustration, the definitions assume that the SerDes

transmitter and receiver operate in a fully symmetrical differential signaling environment.

To illustrate these definitions using real values, consider the example of a current mode logic (CML) transmitter that has a

common mode voltage of 2.25 V and outputs, TD and TD. If these outputs have a swing from 2.0 V to 2.5 V, the peak-to-peak

voltage swing of each signal (TD or TD) is 500 mV p-p, which is referred to as the single-ended swing for each signal. Because

the differential signaling environment is fully symmetrical in this example, the transmitter output differential swing (VOD) has

the same amplitude as each signal 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.

Table 10. Differential Signal Definitions

Term

Definition

Single-Ended Swing

The transmitter output signals and the receiver input signals SR[1–2]_TX, SR[1–2]_TX, SR[1–2]_RX

and SR[1–2]_RX each have a peak-to-peak swing of A – B volts. This is also referred to as each

signal wire’s single-ended swing.

Differential Output Voltage, VOD (or

Differential Output Swing):

The differential output voltage (or swing) of the transmitter, VOD, is defined as the difference of the

two complimentary output voltages: VSR[1–2]_TX – VSR[1–2]_TX. The VOD value can be either positive

or negative.

Differential Input Voltage, VID (or

Differential Input Swing)

The differential input voltage (or swing) of the receiver, VID, is defined as the difference of the two

complimentary input voltages: VSR[1–2]_RX – VSR[1–2]_RX. The VID value can be either positive or

negative.

Differential Peak Voltage, VDIFFp

The peak value of the differential transmitter output signal or the differential receiver input signal is

defined as the differential peak voltage, VDIFFp = |A – B| volts.

Differential Peak-to-Peak, VDIFFp-p

Since the differential output signal of the transmitter and the differential input signal of the receiver

each range from A – B to –(A – B) volts, the peak-to-peak value of the differential transmitter output

signal or the differential receiver input signal is defined as differential peak-to-peak voltage,

VDIFFp-p =2 × VDIFFp = 2 × |(A – B)| volts, which is twice the differential swing in amplitude, or twice

of the differential peak. For example, the output differential peak-peak voltage can also be calculated

as VTX-DIFFp-p = 2 × |VOD|.

Differential Waveform

The differential waveform is constructed by subtracting the inverting signal (SR[1–2]_TX, for

example) from the non-inverting signal (SR[1–2]_TX, for example) within a differential pair. There is

only one signal trace curve in a differential waveform. The voltage represented in the differential

waveform is not referenced to ground. Refer to Figure 16 as an example for differential waveform.

Common Mode Voltage, Vcm

The common mode voltage is equal to half of the sum of the voltages between each conductor of a

balanced interchange circuit and ground. In this example, for SerDes output,

Vcm_out =(VSR[1–2]_TX +VSR[1–2]_TX) ÷ 2 = (A + B) ÷ 2, which is the arithmetic mean of the two

complimentary output voltages within a differential pair. In a system, the common mode voltage may

often differ from one component’s output to the other’s input. It may be different between the receiver

input and driver output circuits within the same component. It is also referred to as the DC offset on

some occasions.

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