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ADT7490ARQZ-R7 Datasheet(PDF) 31 Page - ON Semiconductor |
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ADT7490ARQZ-R7 Datasheet(HTML) 31 Page - ON Semiconductor |
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31 / 75 page  ADT7490 http://onsemi.com 31 Because the actual fan TACH period is being measured, falling below a fan TACH limit by 1 sets the appropriate status bit and can be used to generate an SMBALERT. Fan TACH Limit Registers The fan TACH limit registers are 16−bit values consisting of two bytes. Register 0x54, TACH1 Minimum Low Byte = 0xFF default Register 0x55, TACH1 Minimum High Byte = 0xFF default Register 0x56, TACH2 Minimum Low Byte = 0xFF default Register 0x57, TACH2 Minimum High Byte = 0xFF default Register 0x58, TACH3 Minimum Low Byte = 0xFF default Register 0x59, TACH3 Minimum High Byte = 0xFF default Register 0x5A, TACH4 Minimum Low Byte = 0xFF default Register 0x5B, TACH4 Minimum High Byte = 0xFF default Fan Speed Measurement Rate The fan TACH readings are normally updated once every second. When set, the FAST bit (Bit 3) of Configuration Register 3 (0x78), updates the fan TACH readings every 250 ms. DC Bits If any of the fans are not being driven by a PWM channel but are powered directly from 5.0 V or 12 V, their associated dc bit in Configuration Register 3 should be set. This allows TACH readings to be taken on a continuous basis for fans connected directly to a dc source. For 4−wire fans, once high frequency mode is enabled, the dc bits do not need to be set because this is automatically done internally. Calculating Fan Speed Assuming a fan with a two pulses per revolution, and with the ADT7490 programmed to measure two pulses per revolution, fan speed is calculated by Fan Speed (RPM) = (90,000 x 60)/Fan TACH Reading where Fan TACH Reading is the 16−bit fan tachometer reading. Example TACH1 High Byte (Register 0x29) = 0x17 TACH1 Low Byte (Register 0x28) = 0xFF What is Fan 1 speed in RPM? Fan 1 TACH Reading = 0x17FF = 6143 (decimal) RPM = (f x 60)/Fan 1 TACH Reading RPM = (90000 x 60)/6143 Fan Speed = 879 RPM Fan Pulses per Revolution Different fan models can output either one, two, three, or four TACH pulses per revolution. Once the number of fan TACH pulses has been determined, it can be programmed into the TACH pulses per revolution register (0x7B) for each fan. Alternatively, this register can be used to determine the number or pulses per revolution output by a given fan. By plotting fan speed measurements at 100% speed with different pulses per revolution setting, the smoothest graph with the lowest ripple determines the correct pulses per revolution value. TACH Pulses per Revolution Register Bits [1:0], FAN1 default = 2 pulses per revolution Bits [3:2], FAN2 default = 2 pulses per revolution Bits [5:4], FAN3 default = 2 pulses per revolution Bits [7:6], FAN4 default = 2 pulses per revolution 00 = 1 pulse per revolution 01 = 2 pulses per revolution 10 = 3 pulses per revolution 11 = 4 pulses per revolution Fan Spin−Up The ADT7490 has a unique fan spin−up function. It spins the fan at 100% PWM duty cycle until two TACH pulses are detected on the TACH input. When two TACH pulses have been detected, the PWM duty cycle goes to the expected running value, for example, 33%. The advantage of this is that fans have different spin−up characteristics and take different times to overcome inertia. The ADT7490 runs the fans just fast enough to overcome inertia and is quieter on spin−up than fans programmed to spin up for a given spin−up time. Fan Startup Timeout To prevent the generation of false interrupts as a fan spins up, because the fan is below running speed, the ADT7490 includes a fan startup timeout function. During this time, the ADT7490 looks for two TACH pulses. If two TACH pulses are not detected, an interrupt is generated. Fan startup timeout can be disabled by setting Bit 3 (FSPDIS) of Configuration Register 7 (0x11). PWM1, PWM2, PWM3 Configuration (Register 0x5C, Register 0x5D, Register 0x5E) Bits [2:0] SPIN, startup timeout for PWM1 = 0x5C, PWM2 = 0x5D, and PWM3 = 0x5E. 000 = No startup timeout 001 = 100 ms 010 = 250 ms default 011 = 400 ms 100 = 667 ms 101 = 1 sec 110 = 2 sec 111 = 4 sec Disabling Fan Startup Timeout Although a fan startup makes fan spin−ups much quieter than fixed−time spin−ups, the option exists to use fixed spin−up times. Setting Bit 3 (FSPDIS) to 1 in Configuration Register 7 (Register 0x11) disables the spin−up for two TACH pulses. Instead, the fan spins up for the fixed time as selected in Register 0x5C to Register 0x5E. |
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