Intel Xeon Processor and Intel E7500/E7501Chipset Compatible Platform Design Guide

Intel
®
Xeon™ Processor and Intel
®
E7500/E7501 Chipset Compatible Platform Design Guide 97
Memory Interface Routing Guidelines
6.7.3 DDRVREF and ODTCOMP
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, command, and control signals. VREF of the
receiving device must track changes in VTERM to maximize DDR interface margin. However,
VTERM and VREF cannot be the same power plane due to the sensitivity of the DRAM VREF
buffers to the termination plane noise. If a voltage regulator is used, it must reference VTERM
(See Figure 6-14). If a local resistor divider is used, VREF and VTERM 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 Figure 6-15). Decouple VREF locally at the divider and
DIMMs/MCH using one 0.1 uF capacitor per VREF pin.
On E7500 chipset platforms that implement a resistor divider network to obtain DDRVREF, the
recommended resistor values for the MCH divider are 35.7 Ω pull-up to DDR VDD and a 37.4 Ω
pull-down to ground. These values are chosen to assure ½ VDD at DDRVREF at the MCH. This
change is to compensate for an electrical loading of the Intel E7500 chipset MCH. Failure to assure
the accurate reference voltage can cause erratic behavior on the memory bus. For boards that are
already in production, ensure that the values of DDRVREF at the DIMMs and at the MCH are
within 1% of each other. This does not apply to the E7501 chipset MCH. Continue to use equal
resistor values to derive DDRVREF at the DIMMs and for the E7501 chipset MCH.
NOTE: The resistor values only apply to supply VREF to the MCH when an E7500 chipset MCH is populated.
Use equal resistor values at the DIMMs and for the E7501 chipset MCH.
Figure 6-14. DDRVREF Voltage Regulator
Figure 6-15. DDRVREF Voltage Divider
DDR VDD
(2.5 V)
DDR VTERM
(1.25 V)
Vin
Vout
DDR VREF
(1.25 V)
Ref
DDR VREF
Voltage
Regulator
0.1 µF
Vin Vout
DDR VTERM
Voltage
Regulator
DDR VTERM
(1.25 V)
Vin
Vout
DDR VREF
(1.25 V)
DDR VDD
(2.5 V)
E7500:
35.7 Ω 1%
DDR VTERM
Voltage
Regulator
0.01 µF
E7500:
37.4 Ω 1%