
MPC8548E PowerQUICC III Integrated Processor Hardware Specifications, Rev. 9
66
Freescale Semiconductor
High-Speed Serial Interfaces (HSSI)
of a balanced interchange circuit and ground. In this example, for SerDes output, Vcm_out =
VSD_TX + VSD_TX = (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 is also referred to as the DC
offset.
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 2.5 and 2.0 V. Using these values, the peak-to-peak voltage swing of each signal (TD or
TD) is 500 mVp-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 mVp-p.
16.2
SerDes Reference Clocks
The SerDes reference clock inputs are applied to an internal PLL whose output creates the clock used by
the corresponding SerDes lanes. The SerDes reference clocks inputs are SD_REF_CLK and
SD_REF_CLK for PCI Express and serial RapidIO.
The following sections describe the SerDes reference clock requirements and some application
information.
16.2.1
SerDes Reference Clock Receiver Characteristics
Figure 39 shows a receiver reference diagram of the SerDes reference clocks.
The supply voltage requirements for XVDD_SRDS2 are specified in Table 1 and Table 2. SerDes Reference clock receiver reference circuit structure:
Differential Swing, VID or VOD = A – B
A Volts
B Volts
SD_TX or
SD_RX
SD_TX or
SD_RX
Differential Peak Voltage, VDIFFp = |A – B|
Differential Peak-Peak Voltage, VDIFFpp = 2*VDIFFp (not shown)
Vcm = (A + B)/2