Smvref Design Recommendations; Table 77. Termination Voltage And Current Specifications - Intel 855PM Design Manual

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Table 77. Termination Voltage and Current Specifications

Purpose
Definition
Voltage Nominal (V)
Tolerance (+/-%)
Tolerance (+/-V)
Max Absolute Spec Value (V)
Min Absolute Spec Value (V)
Max Relative Spec (calculated
from measured "VCCSM"
value)
Min Relative Spec (calculated
from measured "VCCSM"
value)
Absolute Maximum Current
Requirements (A)
11.5.3.1.

SMVREF Design Recommendations

There are two SMVREF pins on the Intel 855PM MCH that are used to set the reference voltage level
for the DDR system memory signals (SMVREF). The reference voltage must be supplied to both
SMVREF pins. The voltage level that needs to be supplied to these pins must be equal to VCCSM/2.
Note in Figure 144 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 MCH and memory. This is the
"VREF" signals to the DDR memory devices and the "SMVREF" signals (SMVREF[1:0]) to the MCH.
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[1:0] has
the tightest tolerance in the memory system of ± 2%. Using common 1% resistors consumes 1% of this
2% tolerance. This means SMVREF must now be controlled to a 1% tolerance (i.e. be able to divide
VCCSM/2 within 1%). 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.
®
Intel 855PM Chipset Platform Design Guide
Name
Termination Supply Voltage, Static
Vtt = ("Vref") ± 0.040 V
(Measured Vref) + 0.040 V
(Measured Vref) - 0.040 V
Platform Power Delivery Guidelines
"VTT"
(((2.5 V ± 8%)/2) ± 0.050 V) ±
1.250
3.2%
0.040
(((2.5 V + 8%)/2) + 0.050 V) +
1.440
(((2.5 V - 8%)/2) – 0.050 V) –
1.060
I (max)
TT
2.400
Description
0.040V
0.040V
0.040V
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