1.5.1. Leveling Circuitry
For DDR2 at frequencies below 240 MHz, you should use a tree-style layout. For frequencies above 240 MHz, you can choose either a leveled or balanced-T or Y topology, as the leveled PHY calibrates to either implementation. Regardless of protocol, for devices without a levelling block—such as Arria II GZ, Arria V, and Cyclone V—a balanced-T PCB topology for address/command/clock must be used because fly-by topology is not supported.
For details about leveling delay chains, consult the memory interfaces hardware section of the device handbook for your FPGA.
The following figure shows the write datapath for a leveling interface. The full-rate PLL output clock phy_write_clk goes to a leveling delay chain block which generates all other periphery clocks that are needed. The data signals that generate DQ and DQS signals pass to an output phase alignment block. The output phase alignment block feeds an output buffer which creates a pair of pseudo differential clocks that connect to the memory. In full-rate designs, only the SDR-DDR portion of the path is used; in half-rate mode, the HDR-SDR circuitry is also required. The use of DDIO_OUT in both the output strobe and output data generation paths ensures that their timing characteristics are as similar as possible. The <variation_name>_pin_assignments.tcl script automatically specifies the logic option that associates all data pins to the output strobe pin. The Quartus Prime Fitter treats the pins as a DQS/DQ pin group.
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