Intel Pentium 4 Processor with 512-KB L2 Cache on 0.13 Micron Process Thermal Design Guidelines

Thermal Specifications

34 Intel

Pentium

4 Processor Thermal Design Guide

above so that external thermocouples are no longer needed. By using an accurate on-die

temperature sensing circuit and a fast acting temperature control circuit, the processor can rapidly

initiate thermal management control. As a result, added thermal margins can be significantly

reduced and the resulting system performance impact can be minimized if not eliminated.

3.4.2 Thermal Monitor Implementation

On the Pentium 4 processor with 512-KB L2 cache on 0.13 micron process, the Thermal Monitor

is integrated into the processor silicon. The Thermal Monitor includes a highly accurate on-die

temperature sensing circuit, a signal (PROCHOT#) that indicates the processor has reached its

maximum operating temperature, registers to determine status, and a thermal control circuit that

can reduce processor temperature by controlling the duty cycle of the processor clocks. The

processor temperature is determined through an analog thermal sensor circuit comprised of a

temperature sensing diode, a factory calibrated reference current source, and a current comparator

(See Figure 16). 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 16. Thermal Sensor Circuit

PROCHOT#

Temperature sensing diode

Reference current source

Current comparator

The PROCHOT# signal is available 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 reaches 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. It is asserted any

time the processor die temperature reaches the trip point. The point where the thermal control

circuit goes active is set to the same temperature at which the processor is tested.

The Thermal Monitor’s thermal control circuit (TCC), when active, lowers the processor

temperature by reducing the duty cycle of the internal processor clocks. The thermal control

circuit portion of the Thermal Monitor must be enabled by the system BIOS for the

processor to be operating within specifications. When active, the TCC turns the processor

clocks off and then back on with a predetermined duty cycle. The actual duty cycle varies from

one product to another. Refer to Figure 17 for an illustration. Cycle times are processor speed

dependent and decrease as processor core frequencies increase.