8.1.1 Device Power-On

The AMC80 undergoes a power-on-reset condition when power is first applied to the device, or when the Configuration Register INITIALIZATION bit (address 00h, bit 7) is set high; this bit automatically clears after being set. The AMC80 can also be forced to a reset condition by taking the NTEST_IN/RESET_IN pin low for at least 50 ns.

To start the AMC80 monitoring functions (temperature, analog inputs, and fan speeds), write to the Configuration Register with a '0' to INT_Clear (bit 3) and a '1' to Start (bit 0). The AMC80 then performs continuous monitoring of all temperature, analog inputs, and fan speeds. The sequence of items that are monitored (except for the temperature reading) corresponds to locations in the Value RAM, respectively:

1. Temperature
2. CH0
3. CH1
4. CH2
5. CH3
6. CH4
7. CH5
8. CH6
9. Fan 1
10. Fan 2

The conversion results are available in the Value RAM (addresses 20h to 29h). Conversions can be read at any time and provide the result of the last conversion. A typical sequence of events after AMC80 power-on consists of these actions:

1. Set alarm limits
2. Set interrupt masks
3. Start the AMC80 monitoring process

8.1.2 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.

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.

8.1.3 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:

Equation 1.

where

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.