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Remote Temperature Sensor Failure Detection

PWM Output

+5V

PWM-Out

PWM

Control

AMC6821

PWM-EN

PWMINV=1fordrivingtheNMOS.

+5V

PWM-Out

PWM

Control

AMC6821

PWM-EN

PWMINV=0(default)fordrivingthePMOS.

PWMINV=0

PWM WAVEFORM SETTING

RPMreducesasthe

duty cycledecreases.

Period

OFF

AMC6821

SBAS386C – MAY 2006 – REVISED JULY 2007

The remote temperature sensor failure detection determines whether the remote sensor diode has an

open-circuit condition, a short-circuit to ground, or a short-circuit (IN+) to (IN–) condition. This fault detection is

based on the analog input voltage and is not checked until the first monitoring cycle is completed after power-on.

Reading the fault sensor returns a value of –128

°C (0x80). Since the power-on default value of the temperature

data registers is 0x80 (–128

°C), a reading of 0x80 from the temperature data register immediately after power-on

does not indicate a diode fault condition. The remote temperature sensor failure is only checked after the first

monitoring cycle has been completed after power-on or reset.

When a remote sensor failure occurs, the remote sensor failure bit (RTF in the Status Register) is set to '1', the

OVR pin is forced low, and if the interrupt is enabled (RTFIE = 1), the RTF interrupt is generated through the

SMBALERT pin. Once this interrupt is generated, the RTF bit remains '1' and the OVR pin stays low until a

power-on reset or software reset is issued, whether or not the failure condition persists.

The PWM-Out pin is an open-drain output. When PWM-EN of Configuration Register 2 is cleared ('0'), the

PWM-Out pin is disabled and goes into a high-impedance status. When PWM-EN is set ('1'), the PWM-Out pin is

enabled to drive the fan. When enabled, the status of the PWM-Out pin is determined by the PWM duty cycle

and phase bits (PWMINV of Configuration Register 1). When PWMINV = 0 (default), the PWM-Out pin goes low

for 100% duty cycle (suitable for driving the fan using a PMOS FET). Setting PWMINV to '1' makes the PWM-Out

pin go high (with an external pull-up resistor) for a 100% duty cycle. This setting is used to drive an NMOS-power

FET.

Figure 15. PWM Output

PWM frequency and duty cycle are programmable. The value of the DCY Register defines the duty cycle: it has

8-bit resolution, 1LSB corresponding to 1/255 (0.392%). Writing 0x00 sets the duty cycle to 0%; writing 0xFF sets

the duty cycle to 100%.

PWM frequency has two ranges: the high range is from 1kHz to 40kHz, and the low range is from 10Hz to 94Hz.

The PWM-MODE pin status determines which range is selected. When the PWM-MODE pin is tied to ground,

the high range is selected. Otherwise, the low range is selected. Bits [PWM2:PWM0] in the Fan Characteristics

µs,

corresponding to a 3.2MHz clock. The default value after power-on is 30Hz when the low range is selected, or

25kHz when the high range is selected.

Figure 16. PWM Waveform (PWMINV = 1)

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