The operating temperature of an integrated circuit (IC) affects both the life and the speed performance of the IC. Hence, manufacturers specify IC speed performance in terms of operating speed at a particular temperature and voltage.
IC operating temperatures are typically estimated by measuring package temperature. This method is imprecise for a number of reasons. First, different regions of an IC can dissipate substantially different levels of power, resulting in substantially different localized temperature differences; methods that rely on package temperature to estimate IC operating temperature are not capable of resolving these localized differences. Second, the thermal impedance .theta..sub.jc between the IC and the case is difficult to estimate. It is therefore necessary to factor in a margin of error to take into account the possibility of significant localized temperature differences that are not identified when measuring package temperature.
Several conventional temperature-sensing methods enable IC designers to identify relatively hot regions ("hot spots"), thereby allowing designers either to more accurately assess the speed performance of a given IC or to eliminate hot spots by redesigning the IC so that power-intensive circuitry is distributed more evenly.
According to one method, one or more dedicated temperature-sensing diodes are incorporated into the design of the IC. According to the method, a constant-current source supplies a diode while the voltage across the diode is measured. The temperature of the diode, and therefore the temperature of the region in which the diode is located, is then calculated using well-known relationships between current, voltage, and temperature exhibited by diodes. Unfortunately, such temperature sensing diodes and their requisite connections must be distributed over a given IC if the temperatures of the various regions are to be measured. Moreover, the constant current source and the voltage measurement must be very accurate if temperature is to be accurately determined.
According to a second method for identifying hot spots, a substrate containing an IC is coated with temperature-sensitive materials that change color with changes in temperature. Hot spots are located by viewing the warm IC and associating the resulting colors with temperature. This method is not always practical because the IC package must open to expose the substrate.
In light of the foregoing, there exists a need for a noninvasive method for measuring the temperatures of localized regions of an IC. More particularly, there exists a need for a method of assessing the speed performance of various discrete regions on programmable logic devices without using dedicated circuitry.