Count=

1.35

10

´

´

6

RPM

Divisor

AMC80

www.ti.com

SBOS559

– MAY 2011

ANALOG INPUTS

In the default state, the 10-bit ADC has a 2.5-mV LSB, yielding a 2.56-V full-scale input range. The input range

can also be programmed with several values up to the V+ full-scale input range with a 6-mV LSB. These settings

are programmed by bits 3 to 5 in the ADC Control Register.

In most applications, the analog inputs are often connected to power supplies. The voltage inputs should be

attenuated with external resistors to any desired value within the input range.

CAUTION

Care should be taken not to exceed V+ on the device input pins at any time.

In select applications where inputs to be monitored are differential in nature, analog inputs (CH0 to CH5) can be

configured as up to three differential pairs (inputs 0-1, 2-3, and 4-5) by setting bits 0, 2, and 4 in the Input Mode

Register. If needed, the input pair polarity can be changed by setting bits 1, 3, and 5 in the Input Mode Register.

FAN INPUTS

Inputs are provided on the AMC80 for signals from fans equipped with tachometer outputs. Signal conditioning in

the AMC80 accommodates the slow rise and fall times typical of fan tachometer outputs. The maximum input

signal range is 0 V to 5.5 V. In the event that these inputs are supplied from fan outputs that exceed 0 V to 5.5 V,

either resistive division or diode clamping must be included to keep inputs within an acceptable range.

The Fan Inputs gate an internal 22.5-kHz oscillator for one period of the Fan signal into an 8-bit counter

(maximum count = 255). The default divisor is set to 2 (choices are 1, 2, 4, and 8) providing a nominal count of

153 for a 4400 RPM fan with two pulses per revolution. Typical practice is to consider 70% of normal RPM a fan

failure, at which point the count will be 219. The fan count can be determined as shown in Equation 1:

Where:

RPM = fan speed

Divisor = fan 1 or fan 2 divisor set through the Fan_Divisor/RST_OUT/OS Register (address 05h)

(1)

FAN1 and FAN2 inputs can also be programmed to be level-sensitive interrupt inputs. Fans that provide only one

pulse per revolution require a divisor that is set twice as high as fans that provide two pulses, thus maintaining a

nominal fan count of 153. Therefore, using Equation 1, the divisor should be set to 4 for a fan that provides one

pulse per revolution with a nominal RPM of 4400.

Copyright

© 2011, Texas Instruments Incorporated

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