The EMC6D100 and EMC6D101 are environmental monitoring and control devices with automatic fan control capability for PCs and embedded systems. These ACPI compliant devices provide hardware monitoring for up to eight voltages and three thermal zones, measure the speed of up to four fans, and control the speed of multiple DC fans using three Pulse Width Modulator (PWM) outputs. Note that it is possible to control more than three fans by connecting two fans to one PWM output. The EMC6D100 and EMC6D101 hardware monitoring provide analog inputs for monitoring external voltages of +2.5V, +5V, +12V and Vccp. These devices have the capability to monitor their own internal VCC or VSB. The EMC6D100 has the added functionality of monitoring +3.3V, +1.5V and +1.8V. These devices include support for monitoring three thermal zones: two external and one internal. The external temperatures are measured via thermal diode inputs capable of monitoring remote devices. In addition, they are equipped with an ambient temperature sensor for measuring the internal temperature. Pulse Width Modulators (PWM) control the speed of the fans by varying the output duty cycle of the PWM. The speed of each fan is monitored by a Fan Tachometer input. The measured values are compared to values stored in Limit Registers to detect if a fan has stalled or seized. Fan speed may be under host software control or automatic control. In host control mode, the host software continuously monitors temperature and fan speed registers, makes decisions as to desired fan speed and sets the PWM''''s to drive the required fan speed. In auto "zone" mode, the logic continuously monitors the temperature and fan speeds and adjusts speeds without intervention from the host CPU. Fan speed is adjusted according to an algorithm using the temperature measured in the selected zone, the high and low limits set by the user, and the current fan speed. The EMC6D100 and EMC6D101 registers are accessible through the SMBus interface in both "standby mode" and "active mode".