1.8.3 VTT Power

VTT is memory bus termination voltage. To maintain noise margins, VTT must be equal to VDDQ/2, with an accuracy of ± 3%. VTT terminates command and address signals to VDDQ/2 using a parallel resistor (RT) tied to a low impedance source.

VTT is not used to terminate any DDR clock pairs. Rather, the xDDR_CLK and xDDR_CLK_N termination consists of a parallel 100-121 Ω resistor between the two lines.

• VTT islands require a 10 μF capacitor.
• Since each data line is connected to VTT with relatively low impedance, this supply must be extremely stable. Any noise on this supply directly affects the data lines.

• Sufficient bulk and bypass capacitance must be provided to keep this supply at VDDQ/2. VREF power must not be derived from VTT, but must be derived from VDDQ with a 1% or better resistor divider.