Rainbow Electronics LM93 - Manual (Page 56)

16.0 Registers
(Continued)
16.6.11 Register 6E–75h Fan Tachometer Readings
Register
Address
Read/
Write
Register
Name
Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Default
Value
6Eh R
Fan Tach
1
LSB
TACH[5:0] RES N/D
6Fh R
Fan Tach
1
MSB
TACH[13:6] N/D
70h R
Fan Tach
2
LSB
TACH2[5:0] RES N/D
71h R
Fan Tach
2
MSB
TACH2[13:6] N/D
72h R
Fan Tach
3
LSB
TACH3[5:0] RES N/D
73h R
Fan Tach
3
MSB
TACH3[13:6] N/D
74h R
Fan Tach
4
LSB
TACH4[5:0] RES N/D
75h R
Fan Tach
4
MSB
TACH4[13:6] N/D
The 14-bit fan tach readings indicate the number of 22.5 kHz clock periods that occurred during two full periods of the tachometer
input signal. Most fans produce two tachometer pulses per full revolution. These registers must be updated at least once every
second.
The fan tachometer reading registers must always return an accurate fan tachometer measurement, even when a fan is disabled
or non-functional. 3FFFh indicates that the fan is stalled, not spinning fast enough to measure, or the tachometer input is not
connected to a valid signal.
If the pulses per revolution of the fan is known, the RPM can be calculated with the following equation:
RPM= 22500 cycles/sec * 60 sec/min * 2 pulses / COUNT cycles / PULSES_PER_REV
where:
PULSES_PER_REV = the number of pulses that the fan produces per revolution
COUNT = The 14-bit value read from the tach register
LM93
www.national.com 56