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CPU Operating Voltages

One trend that is clear to anybody that has been following processor design is that the operating voltages have gotten lower and lower. The benefits of lower voltage are threefold. The most obvious is that with lower voltage comes lower overall power consumption. By consuming less power, the system will be less expensive to run, but more importantly for portable or mobile systems, it will run much longer on existing battery technology. The emphasis on battery operation has driven many of the advances in lowering processor voltage, because this has a great effect on battery life.

The second major benefit is that with less voltage and therefore less power consumption, there will be less heat produced. Processors that run cooler can be packed into systems more tightly and will last longer. The third major benefit is that a processor running cooler on less power can be made to run faster. Lowering the voltage has been one of the key factors in allowing the clock rates of processors to go higher and higher.

Until the release of the mobile Pentium and both desktop and mobile Pentium MMX, most processors used a single voltage level to power both the core as well as run the input/output circuits. Originally, most processors ran both the core and I/O circuits at 5 volts, which was later was reduced to 3.5 or 3.3 volts to lower power consumption. When a single voltage is used for both the internal processor core power as well as the external processor bus and I/O signals, the processor is said to have a single or unified power plane design.

When originally designing a version of the Pentium processor for mobile or portable computers, Intel came up with a scheme to dramatically reduce the power consumption while still remaining compatible with the existing 3.3v chipsets, bus logic, memory, and other components. The result was a dual-plane or split-plane power design where the processor core ran off of a lower voltage while the I/O circuits remained at 3.3v. This was originally called Voltage Reduction Technology (VRT) and first debuted in the Mobile Pentium processors released in 1996. Later, this dual-plane power design also appeared in desktop processors such as the Pentium MMX, which used 2.8v to power the core and 3.3v for the I/O circuits. Now most recent processors, whether for mobile or desktop use, feature a dual-plane power design. Some of the more recent Mobile Pentium II processors run on as little as 1.6v for the core while still maintaining compatibility with 3.3v components for I/O.

Knowing the processor voltage requirements is not a big issue with Socket 8, Socket 370, Socket A, Pentium Pro (Socket 8), or Pentium II (Slot 1 or Slot 2) processors, because these sockets and slots have special voltage ID (VID) pins that the processor uses to signal to the motherboard the exact voltage requirements. This allows the voltage regulators built in to the motherboard to be automatically set to the correct voltage levels by merely installing the processor.

Unfortunately, this automatic voltage setting feature is not available on Socket 7 and earlier motherboard and processor designs. This means you must normally set jumpers or otherwise configure the motherboard according to the voltage requirements of the processor you are installing. Pentium (Socket 4, 5, or 7) processors have run on a number of voltages, but the latest MMX versions are all 2.8v, except for mobile Pentium processors, which are as low as 1.8v. Table 3.11 lists the voltage settings used by Intel Pentium (non-MMX) processors that use a single power plane. This means that both the CPU core and the I/O pins run at the same voltage.

Table 3.14 shows voltages used by Socket 7 processors.

Table 3.14 Socket 7 Single- and Dual-Plane Processor Voltages

Voltage Setting

Processor

Core Voltage

I/O Voltage

Voltage Planes

VRE (3.5v)

Intel Pentium

3.5v

3.5v

Single

STD (3.3v)

Intel Pentium

3.3v

3.3v

Single

MMX (2.8v)

Intel MMX Pentium

2.8v

3.3v

Dual

VRE (3.5v)

AMD K5

3.5v

3.5v

Single

3.2v

AMD-K6

3.2v

3.3v

Dual

2.9v

AMD-K6

2.9v

3.3v

Dual

2.4v

AMD-K6-2/K6-3

2.4v

3.3v

Dual

2.2v

AMD-K6/K6-2

2.2v

3.3v

Dual

VRE (3.5v)

Cyrix 6x86

3.5v

3.5v

Single

2.9v

Cyrix 6x86MX/M-II

2.9v

3.3v

Dual

MMX (2.8v)

Cyrix 6x86L

2.8v

3.3v

Dual

2.45v

Cyrix 6x86LV

2.45v

3.3v

Dual


Normally, the acceptable range is plus or minus five percent from the nominal intended setting.

Most Socket 7 and later Pentium motherboards supply several voltages (such as 2.5v, 2.7v, 2.8v, and 2.9v) for compatibility with future devices. A voltage regulator built into the motherboard converts the power supply voltage into the different levels required by the processor core. Check the documentation for your motherboard and processor to find the appropriate settings.

The Pentium Pro and Pentium II processors automatically determine their voltage settings by controlling the motherboard-based voltage regulator through built-in voltage ID (VID) pins. Those are explained in more detail later in this chapter.

See "Pentium Pro Processors."

See "Pentium II Processors."

Note that on the STD or VRE settings, the core and I/O voltages are the same; these are single plane voltage settings. Any time a different voltage other than STD or VRE is set, the motherboard defaults to a dual-plane voltage setting where the core voltage can be specifically set, while the I/O voltage remains constant at 3.3v no matter what.

Socket 5 was only designed to supply STD or VRE settings, so any processor that can work at those settings can work in Socket 5 as well as Socket 7. Older Socket 4 designs can only supply 5v, plus they have a completely different pinout (fewer pins overall), so it is not possible to use a processor designed for Socket 7 or Socket 5 in Socket 4.

Most Socket 7 and later Pentium motherboards supply several voltages (such as 2.2v, 2.4v, 2.5v, 2.7v, 2.8v, and 2.9v as well as the older STD or VRE settings) for compatibility with many processors. A voltage regulator built into the motherboard converts the power supply voltage into the different levels required by the processor core. Check the documentation for your motherboard and processor to find the appropriate settings.

The Pentium Pro, Celeron, and Pentium II/III processors automatically determine their voltage settings by controlling the motherboard-based voltage regulator. That's done through built-in voltage ID (VID) pins.

For hotrodding purposes, many newer motherboards for these processors have override settings that allow for manual voltage adjustment if desired. Many people have found that when attempting to overclock a processor, it often helps to increase the voltage by a tenth of a volt or so. Of course this increases the heat output of the processor and must be accounted for with adequate heat sinking.

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