MPC8315E PowerQUICC II Pro Processor Hardware Specifications, Rev. 0
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Freescale Semiconductor
High-Speed Serial Interfaces (HSSI)
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
Because 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 47 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 38. 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 2.5V 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
Differential Swing, VID or VOD = A - B
A Volts
B Volts
TXn or RXn
Differential Peak Voltage, VDIFFp = |A - B|
Differential Peak-Peak Voltage, VDIFFpp = 2*VDIFFp (not shown)
Vcm = (A + B) / 2