Intel Pentium 4 Processor In the 423-pin Package Thermal Design Guidelines

Pentium® 4 Processor in the 423-pin Package Thermal Design Guidelines

A new on-die thermal management feature on the Pentium® 4 processor called Thermal Monitor, resolves these

issues so that thermocouples are no longer needed. By using a highly accurate on-die temperature sensing circuit and

a fast acting temperature control circuit (~50ns) the processor can rapidly initiate thermal management control. As a

result, large guard bands are unnecessary and the system performance impact is minimized if not eliminated.

8.2 Thermal Monitor Implementation

On the Pentium 4 processor, a new thermal management feature called Thermal Monitor is integrated into the

processor silicon. Thermal Monitor includes a highly accurate on-die temperature sensing circuit, a signal which

indicates the processor is too hot (PROCHOT#), registers to determine the processor thermal status and a thermal

control circuit which can reduce the processor temperature by modulating the processor clocks. The processor

temperature is determined through an analog thermal sensor circuit comprised of a diode; a factory calibrated

reference current source, and a current comparator (see Figure 10). A voltage applied across the diode will induce a

current flow that varies with temperature. By comparing this current with the reference current, the processor

temperature can be determined. The reference current source corresponds to the diode current when at the maximum

permissible processor operating temperature. Each processor is individually calibrated during manufacturing to

eliminate any potential manufacturing variations. Once configured, the processor temperature at which the

PROCHOT# signal is asserted (trip point) is not re-configurable.

Figure 10. Thermal Sense Circuit

The PROCHOT# signal is available both internally to the processor as well as externally. External indication of the

processor temperature status is provided through the bus signal PROCHOT#. When the processor temperature is

equal to or above the trip point, PROCHOT# is asserted. When the processor temperature is below the trip point,

PROCHOT# is deasserted. Assertion of the PROCHOT# signal is independent of any register settings within the

processor and will be asserted any time the processor die temperature is equal to or exceeds the trip point. The point

where the thermal control circuit goes active will be set to approximately the same temperature (case temperature) at

which the processor is tested. This value is specified in the processor datasheet.

The Thermal Monitor’s thermal control circuit, when active, lowers the processor temperature by modulating the

internal processor clocks. The thermal control circuit will turn the processor clocks off and then back on with a 50%

duty cycle of approximately 4µs in length for an 1.5 GHz processor (~2 µs on, ~2µs off). Refer to Figure 11 for an

illustration. Cycle times are processor speed dependent and will decrease linearly as processor core frequencies

increase.

An ACPI register, performance counter register, and model specific register (MSR) support are available to monitor

and control the Thermal Monitor feature. Details regarding the use of these registers are described in the IA32 Intel

Architecture Software Developer Manuals.

Temperature sensing

diode

Current comparator

Reference current

source

PROCHOT#