SNAS207B May 2004 – January 2024 LM64
PRODUCTION DATA
The Tach Count Registers 46HEX and 47HEX count the number of periods of the 90 kHz tachometer clock in the LM64 for the tachometer input from the fan assuming a 2 pulse per revolution fan tachometer, such as the fans supplied with the Pentium 4 boxed processors. The RPM of the fan can be computed from the Tach Count Registers 46HEX and 47HEX. This can best be shown through an example.
Example:
Given: the fan used has a tachometer output with 2 per revolution.
Let:
Register 46 (LSB) is BFHEX = Decimal (11 x 16) + 15 = 191 and
Register 47 (MSB) is 7HEX = Decimal (7 x 256) = 1792.
The total Tach Count, in decimal, is 191 + 1792 = 1983.
The RPM is computed using the formula
where
f = 1 for 2 pulses/rev fan tachometer output;
f = 2 for 1 pulse/rev fan tachometer output, and
f = 2 / 3 for 3 pulses/rev fan tachometer output
For our example