The ADM1021A is a two-channel digital thermometer and under/ over temperature alarm, intended for use in personal computers and other systems requiring thermal monitoring and management. The device can measure the temperature of a microprocessor using a diode-connected PNP transistor, which may be provided on-chip in the case of the Pentium III or similar processors, or can be a low-cost discrete NPN/PNP device such as the 2N3904/2N3906. A novel measurement technique cancels out the absolute value of the transistor's base emitter voltage, so that no calibration is required. The second measurement channel measures the output of an on-chip temperature sensor, to monitor the temperature of the device and its environment. The ADM1021A communicates over a two-wire serial interface compatible with SMBus standards. Under and over temperature limits can be programmed into the devices over the serial bus, and an A LERT o utput signals when the on-chip or remote temperature is out of range. This output can be used as an interrupt, or as an SMBus alert.