Platform Power Delivery Guidelines
R
Intel
®
855PM Chipset Platform Design Guide
259
Table 77.
Termination Voltage and Current Specifications
Name
"VTT"
Description
Purpose
Termination Supply Voltage, Static
Definition
Vtt = ("Vref") ± 0.040 V
(((2.5 V ± 8%)/2) ± 0.050 V) ±
0.040V
Voltage Nominal (V)
1.250
Tolerance (+/-%)
3.2%
Tolerance (+/-V)
0.040
Max Absolute Spec Value (V)
1.440
(((2.5 V + 8%)/2) + 0.050 V) +
0.040V
Min Absolute Spec Value (V)
1.060
(((2.5 V - 8%)/2) – 0.050 V) –
0.040V
Max Relative Spec (calculated
from measured "VCCSM"
value)
(Measured Vref) + 0.040 V
Min Relative Spec (calculated
from measured "VCCSM"
value)
(Measured Vref) - 0.040 V
I
TT
(max)
Absolute Maximum Current
Requirements (A)
2.400
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.
Содержание 855PM
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