Method and apparatus for accurate fan tachometer readings of PWM fans with different speeds

A system for generating test signal pulses used in determining speed of a fan in a computer system, wherein the fan is operable to generate a tachometer signal indicative of the current speed of the fan, the system comprising:

• a first circuit operable to receive a first coefficient value corresponding to a maximum frequency of the test signal pulses and further operable to provide a scaled base frequency signal based on the first coefficient value;
• a second circuit operable to receive the scaled base frequency signal and a second coefficient value and further operable to generate the test signal pulses based on the scaled base frequency signal and the second coefficient value;
• wherein the scaled base frequency is commensurate with the maximum frequency of the test signal pulses;
• wherein a current frequency of the test signal pulses is proportional to a current frequency of the tachometer signal;
• and
• wherein the test signal pulses operate to provide power to the fan.

A method for generating a test signal used in determining speed of a rotating device, wherein the rotating device is operable to generate a device signal indicative of the current speed of the rotating device, the method comprising:

• generating a base frequency signal;
• obtaining a scaled base frequency signal from the base frequency signal, wherein the scaled base frequency is the base frequency divided by a scale coefficient;
• obtaining the test signal from the scaled base frequency signal, wherein a current frequency of the test signal is the scaled base frequency divided by a fraction coefficient;
• wherein the scaled base frequency is commensurate with a maximum frequency of the test signal;
• wherein the current frequency of the test signal is proportional to a current frequency of the device signal;
• and
• wherein the test signal operates to provide power to the rotating device.

A method for generating a test signal used in determining speed of a fan in a computer system, wherein the fan is powered by a PWM signal, and wherein the fan is operable to generate a tachometer signal indicative of the current speed of the fan, the method comprising:

• generating a base frequency signal;
• obtaining a scaled base frequency signal from the base frequency signal, wherein the scaled base frequency is the base frequency divided by a scale coefficient;
• obtaining, the test signal from the scaled base frequency signal, wherein a current frequency of the test signal is the scaled base frequency divided by a fraction coefficient;
• wherein the scaled base frequency is commensurate with a maximum frequency of the test signal;
• wherein the current frequency of the test signal is proportional to a current frequency of the tachometer signal;
• and
• wherein at least one respective test signal pulse of the test signal is operable to power the fan when the respective test signal pulse is generated during a time when the PWM signal is not asserted, wherein the fan will remain powered for the duration of the respective test signal pulse.