MSC8256 Six-Core Digital Signal Processor Data Sheet, Rev. 3
Electrical Characteristics
Freescale Semiconductor
28
SR[1–2]_TX) or a receiver input (SR[1–2]_RX and SR[1–2]_RX). Each signal swings between A volts and B volts where
A>B.
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.
Figure 4. Differential Voltage Definitions for Transmitter or Receiver
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.
Differential Swing, VID or VOD = A – B
A Volts
B Volts
Differential Peak Voltage, VDIFFp = |A – B|
Differential Peak-Peak Voltage, VDIFFpp = 2 × VDIFFp (not shown)
SR[1–2]_TX or
SR[1–2]_RX
SR[1–2]_TX or
SR[1–2]_RX
Vcm = (A + B)/2