Ddr Reference Voltage; Ddr Vref Voltage Regulator; Ddr Vref Voltage Divider - Intel Xeon Design Manual

Memory Interface Routing Guidelines
6.9

DDR Reference Voltage

The DDR system memory reference voltage (VREF) is used by the DRAM devices and the MCH
to determine the logic level being driven on the data, strobe, and control signals. VREF of the
receiving device must track changes in VTT to maximize DDR interface margin. If a voltage
regulator is used, it must reference VTT (See
and VTT must have a common source voltage between them (i.e., both VREF and VTT are derived
from the same voltage plane), and 1% resistors should be used (See
locally at the divider and DIMMs/MCH using one 0.1uF capacitor per VREF pin.
Figure 6-16. DDR VREF Voltage Regulator
Figure 6-17. DDR VREF Voltage Divider
80
Figure
DDR VTT
(1.25V)
DDR VDD
(2.5V)
DDR VDD
(2.5V)
DDR VREF
1%
(1.25V)
1%
0.1 µF
6-16). If a local resistor divider is used, VREF
Figure
Ref
Voltage
DDR VREF
Regulator
(1.25V)
Vin
Vout
0.1 µF
Vin
DDR VTT
Voltage
(1.25V)
Regulator
Vout
6-17). Decouple VREF
Design Guide