MSC8254_11 Datasheet, PDF (29/68 Pages) Freescale Semiconductor, Inc – Quad-Core Digital Signal Processor

Electrical Characteristics
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 signalling environment.
Table 10. Differential Signal Definitions
Term
Definition
Single-Ended Swing
Differential Output Voltage, V_OD (or
Differential Output Swing):
Differential Input Voltage, V_ID (or
Differential Input Swing)
Differential Peak Voltage, V_DIFFp
Differential Peak-to-Peak, V_DIFFp-p
Differential Waveform
Common Mode Voltage, V_cm
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.
The differential output voltage (or swing) of the transmitter, V_OD, is defined as the difference of the
two complimentary output voltages: V_{SR[1–2]_TX} – V_{SR[1–2]_TX.} The V_OD value can be either positive
or negative.
The differential input voltage (or swing) of the receiver, V_ID, is defined as the difference of the two
complimentary input voltages: V_{SR[1–2]_RX} – V_{SR[1–2]_RX.} The V_ID value can be either positive or
negative.
The peak value of the differential transmitter output signal or the differential receiver input signal is
defined as the differential peak voltage, V_DIFFp = |A – B| volts.
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,
V_DIFFp-p = 2 × V_DIFFp = 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 V_TX-DIFFp-p = 2 × |V_OD|.
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.
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,
V_{cm_out} = (V_{SR[1–2]_TX} + V_{SR[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.
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 (V_OD) has
the same amplitude as each signal single-ended swing. The differential output signal ranges between 500 mV and –500 mV. In
other words, V_OD is 500 mV in one phase and –500 mV in the other phase. The peak differential voltage (V_DIFFp) is 500 mV.
The peak-to-peak differential voltage (V_DIFFp-p) is 1000 mV p-p.
MSC8254 Quad-Core Digital Signal Processor Data Sheet, Rev. 6
Freescale Semiconductor
29