Virtex-4 RocketIO MGT User Guide www.xilinx.com 139UG076 (v4.1) November 2, 2008RChapter 4PMA Analog Design ConsiderationsSerial I/O DescriptionThe RocketIO Multi-Gigabit Transceiver (MGT) transmits and receives serial differentialsignals, using a nominal termination supply voltage of 1.5 VDC. A serial differential pairconsists of a true (VP ) and a complement (V N) set of signals. The voltage differencerepresents the transferred data. Thus: V P – V N = V DATA. Differential switching isperformed at the crossing of the two complementary signals so that no separate referencelevel is needed. A graphical representation of this concept is shown in Figure 4-1.Differential TransmitterThe RocketIO transceiver is implemented in Current Mode Logic (CML). A CMLtransmitter output consists of transistors configured as shown in Figure 4-1. The CML usesa positive supply and offers easy interface requirements. In this configuration, both legs ofthe driver, V P and V N , sink current, with one leg always sinking more current than itscomplement. The CML output consists of a differential pair with 50Ω source resistors. Thesignal swing is created by switching the current in a common-source differential pair.The differential transmitter specifications are shown in the RocketIO DC Specifications andthe RocketIO Transmitter Switching Characteristics in the Virtex-4 Data Sheet.Output Swing and EmphasisThe output swing and emphasis levels of the MGTs are fully programmable. Each iscontrolled via attributes at configuration, but can be modified via the DynamicReconfiguration Port programming bus (Appendix C, “Dynamic Reconfiguration Port”).Figure 4-1: Differential AmplifierCML Output DriverU035_06_091903VPVNVP VN- = V DATA
