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I have observed that when the AMC6821 THERMn input (pin 7) is pulled low, its PWM-OUT (pin 1) is low at power-on. When the AMC6821 THERMn input (pin 7) is pulled low, the THERM-IN bit is set.
AMC6821’s Configuration Register 1 (its description is found on page 36 of data sheet) Bit 3 (PWMINV) sets the polarity of PWM-OUT (pin 1). The power-on default = 0. When PMINV = 0, the PWM-OUT (pin 1) goes low for 100% duty cycle.
Configuration Register 2 (its description is found on page 37 of data sheet) Bit 0 (PWM-EN) enables the PWM-OUT pin (pin 1). Its power-on default = 1. Setting this bit to "1" enables the PWM-OUT (pin 1).
AMC6821's Status Register 2 (its description is found on page 40 of the data sheet) states Bit 7 is the THERM-IN bit. When this THERMn input is pulled low, THERM-In = 1. This bit is cleared when reading this register. This register will be written to '1,' if the THERMn pin remains pulled-low.
In my application, the AMC6821’s PWM-OUT (pin 1) drives a Fan's PWM input. At power-on, I want the AMC6821 PWM-OUT (pin 1) to be 0 volts. This means, that when the fan’s PWM input is 0 volts, the fan blades will not spin.
Question: If THERMn pin (pin 7) is permanently held low, am I still able to modulate the AMC6821’s PWM-OUT (pin 1) via the device's serial interface at 25kHz and vary its duty cycle, after power-on?
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The THERMn pin is a high priority hardware interrupt that should be high if not used. It alerts the AMC6821 that there is an over temperature condition forcing PWM-OUT to 100% duty cycle. Since it is for emergency cases, it will overwrite any duty cycle in the duty cycle register.
It can be held low during and after power up, but it must be set high afterwards to inform the AMC6821 that an over temperature condition isn't present and PWM-OUT can be released form 100% duty cycle. Other customers have done this before to keep the fan at 100% duty cycle while configuring the registers for the first time.
Precision Data ConvertersApplications Team
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