Ddr Reference Voltage; Table 74. Absolute Vs. Relative Voltage Specification - Intel 855PM Design Manual

Platform Power Delivery Guidelines
Additionally, a small 2.5-V copper flood shape should be placed under the MCH to encompass and
increase the copper flooding to the back row of the 2.5-V MCH pins. This flood must not be placed
under any of the DDR signals. In order to maximize the copper flooding, these signals should be kept as
short as possible in order to reduce the amount of serpentining needed in this area on the bottom layer.
Also, a minimum of 12-mil isolation spacing should be maintained between the copper flooding and the
DDR signals. Finally, the six, MCH 2.5-V high frequency decoupling capacitors located on the top
signal layer should have their 2.5-V vias placed directly over and connected to a separate 2.5-V copper
finger.
11.5.3.

DDR Reference Voltage

Table 75 through Table 77 below have grouped the voltage and current specifications together for each
the Intel 855PM MCH, memory, and termination voltages. There are seven voltages/power rails
specified here for a DDR VR system. Although, there are only two unique voltage regulators for 2.5 V
and 1.25 V nominal, each specific power rail described here has a unique specification. Described below
are the memory components themselves first (the top three listed) and the MCH requirements (next row
of 3) and finally the termination voltage and current requirements.
For convenience, tolerances are given in both % and Volts though validation should be done using the
specification exactly as it is written. The voltage specs are clearly defined under "Specification
Definition". If this states a tolerance in terms of volts (e.g. VREF says ± 0.050 V) then that specific
voltage tolerance should be used, not a percentage of the measured value. Likewise, percentages should
be used where stated. If not stated then either way is fine.
Voltage specifications are defined as either "Absolute" or "Relative". These are described in Table 74.

Table 74. Absolute vs. Relative Voltage Specification

Type of Specification
Absolute Specification
Relative Specification
From the Table 75, it can be seen that only the 2.5-V supply is a fixed, absolute specification, whereas
all of the 1.25-V nominal supplies are relative to the 2.5-V supply directly or another 1.25-V supply
which is then relative to the 2.5 V supply. Due to these 1.25-V relative specifications, it becomes very
important that the 1.25 V supply can track the variations in the 2.5-V supply and respond according to
the 2.5-V rail variations. This can be implemented as shown in the block diagram in Figure 144 where
the 2.5-V output is divided in half and used to generate the 1.25 V reference into the 1.25-V VR
controller design. In this manner, the 1.25-V VR will respond proportionally to variations in the 2.5-V
supply, improving the voltage margin of the relative supply requirements and overall memory system
stability.
256
Description
This is a standard specification most commonly used. This means that
the voltage limits are based on a fixed nominal voltage and have a
symmetric ± tolerance added to determine the acceptable voltage
range. For example, a VDD specification does not depend on any
other voltage levels. It is simply 2.5 V ± 8%.
This is a specification whose nominal value is not fixed but is relative
to or is a function of another voltage. This means that the other voltage
must be measured to know what the nominal value is and then the
symmetrical ± tolerance added to that measured value. For example, a
VREF specification depends on the actual value of VDD to determine
VDD/2 and then tolerance ± 0.050 V from this calculated value.
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Intel 855PM Chipset Platform Design Guide
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