641 datasheet(13/106 Pages) INTEL

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Datasheet

Electrical Specifications

This chapter describes the electrical characteristics of the processor interfaces and

signals. DC electrical characteristics are provided.

2.1

Power and Ground Lands

The Pentium 4 processor has 226 VCC (power), 24 VTT and 273 VSS (ground) inputs

for on-chip power distribution. All power lands must be connected to V

CC, while all VSS

lands must be connected to a system ground plane. The processor VCC lands must be

supplied the voltage determined by the Voltage IDentification (VID) lands.

Twenty-four (24) signals are denoted as VTT, that provide termination for the front side

bus and power to the I/O buffers. A separate supply must be implemented for these

lands, that meets the VTT specifications outlined in Table 4.

2.2

Decoupling Guidelines

Due to its large number of transistors and high internal clock speeds, the processor is

capable of generating large current swings. This may cause voltages on power planes

to sag below their minimum specified values if bulk decoupling is not adequate. Larger

bulk storage (CBULK), such as electrolytic or aluminum-polymer capacitors, supply

current during longer lasting changes in current demand by the component, such as

coming out of an idle condition. Similarly, they act as a storage well for current when

entering an idle condition from a running condition. The motherboard must be designed

to ensure that the voltage provided to the processor remains within the specifications

listed in Table 4. Failure to do so can result in timing violations or reduced lifetime of

the component.

2.2.1

VCC Decoupling

VCC regulator solutions need to provide sufficient decoupling capacitance to satisfy the

processor voltage specifications. This includes bulk capacitance with low effective series

resistance (ESR) to keep the voltage rail within specifications during large swings in

load current. In addition, ceramic decoupling capacitors are required to filter high

frequency content generated by the front side bus and processor activity. Consult the

Voltage Regulator-Down (VRD) 10.1 Design Guide For Desktop and Transportable

LGA775 Socket for further information.

2.2.2

VTT Decoupling

Decoupling must be provided on the motherboard. Decoupling solutions must be sized

to meet the expected load. To insure compliance with the specifications, various factors

associated with the power delivery solution must be considered including regulator

type, power plane and trace sizing, and component placement. A conservative

decoupling solution would consist of a combination of low ESR bulk capacitors and high

frequency ceramic capacitors.

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