Circuit for controlling rotation speed of computer fan

A circuit for controlling rotation speed of a computer fan, the control circuit comprising:

• a fan connector comprising a power pin for receiving power and for controlling a rotation speed of the computer fan if the computer fan is a 3-pin fan, a pulse-width modulation (PWM) control pin for controlling the rotation speed of the computer fan if the computer fan is a 4-pin fan, and a detection pin;
• a rotation speed detector configured for detecting the rotation speed of the computer fan, an input of the rotation speed detector connected to the detection pin of the fan connector;
• a basic input/output system (BIOS) chip comprising an input terminal connected to an output of the rotation speed detector, and configured to receive and store rotation speed signals output from the rotation speed detector;
• a super input/output (I/O) chip comprising a first terminal connected to an output terminal of the BIOS chip, and second and third terminals respectively outputting first and second PWM signals, the PWM control pin of the fan connector connected to the third terminal of the super I/O chip to receive the second PWM signals;
• and
• an integrated circuit having an input terminal connected to the second terminal of the super I/O chip to receive the first PWM signal, and an output terminal configured to output a corresponding analog voltage signal for the power pin of the fan connector;
• wherein the BIOS chip is further configured to compare two rotation speed signals corresponding to two different second PWM signals output to the PWM control pin of the fan connector, and send a result of the comparison to the super I/O chip;
• and
• the super I/O chip is configured to output either first PWM signals or second PWM signals as rotation speed control signals according to the result of the comparison.