12.5.3.1. Smvref Layout And Routing Recommendations; Table 90. Intel Gmch System Memory Voltage And Current Requirements; Table 91. Termination Voltage And Current Requirements - Intel 852GME Design Manual

Platform Power Delivery Guidelines

Table 90. Intel GMCH System Memory Voltage and Current Requirements

Parameter
GMCH DDR Supply
Voltage (I/O), Static
GMCH DDR Supply
Current, Static
Intel GMCH
Reference Supply
Voltage, Static
Intel GMCH
Reference Supply
Current, Static
SMRCOMP
Termination Supply
Voltage, Static
SMRCOMP
Termination Supply
Current, Static

Table 91. Termination Voltage and Current Requirements

Parameter
Termination Supply
Voltage, Static
Termination Supply
Current, Static

12.5.3.1. SMVREF Layout and Routing Recommendations

There is one SMVREF pin on the 852GME / 852PM GMCH that is used to set the reference voltage
level for the DDR system memory signals (SMVREF_0). The reference voltage must be supplied to the
SMVREF pin. The voltage level that needs to be supplied to this pin must be equal to VCCSM/2. Note
in Figure 102 that although SMVREF is generated from the 2.5-V supply, a buffer is used as well. A
buffer has also been used to provide this reference to the system for the GMCH and memory. This is the
"VREF" signals to the DDR memory devices and the "SMVREF" signals (SMVREF_0) to the GMCH.
The reference design utilizes this buffer to provide the necessary current to these devices, which a
simple voltage divider is not capable of providing. The reference voltage supplied to SMVREF has the
tightest tolerance in the memory system of ± 2%. A simple resistor divider is not a voltage regulator and
is most definitely not a current source. Any current drawn across the resistor divider used to generate
this 1.25-V reference will cause a voltage drop across the top resistor, which distorts or biases this
reference to a lower voltage. The clarification below summarizes SMVREF.
220
Symbol Unit
VCCSM Volts, V
Ivccsm Amperes, A
SMVREF Volts, V
Isdref Amperes, A
Vtt Volts, V
Ittrc Amperes, A
Symbol Unit
Vtt Volts, V
Itt Amperes, A
® ®
Intel 852GME, Intel 852GMV and Intel
Definition Minimum
VCCSM 2.375
VCCSM/2 + VCCSM/2 –
2% 2%
SMVref + 0.04 SMVref – 0.04
V
Minimum Nominal
SMVref – 0.04V SMVref
®
852PM Chipset Platforms Design Guide
R
Nominal Maximum
2.5 2.625
2.8
VCCSM/2 VCCSM/2
+ 2%
0.01
SMVref SMVref +
0.04
0.025
Maximum
SMVref + 0.04V
2.4