MPC8313VRGDDB datasheet(37/100 Pages) FREESCALE

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MPC8313E PowerQUICC™ II Pro Processor Hardware Specifications, Rev. 2.1

Freescale Semiconductor

High-Speed Serial Interfaces (HSSI)

4. Differential peak voltage, VDIFFp

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

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

5. 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 of 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|.

6. Differential waveform

The differential waveform is constructed by subtracting the inverting signal (TXn, for example)

from the non-inverting signal (TXn, 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 31 as an example for differential waveform.

7. Common mode voltage, Vcm

The common mode voltage is equal to one 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 =

(VTXn +VTXn)/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. Sometimes, it may be even different between the

receiver input and driver output circuits within the same component. It’s also referred as the DC

offset in some occasion.

Figure 22. Differential Voltage Definitions for Transmitter or Receiver

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 2.5 and 2.0 V. 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 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.

A Volts

B Volts

n or RXn

Vcm = (A + B)/2

Differential Swing, VID or VOD = A – B

Differential Peak Voltage, VDIFFp = |A – B|

Differential Peak-Peak Voltage, VDIFFpp = 2*VDIFFp (not shown)

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