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MEC1322 Datasheet(PDF) 305 Page - Microchip Technology |
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MEC1322 Datasheet(HTML) 305 Page - Microchip Technology |
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305 / 456 page  2014 - 2015 Microchip Technology Inc. DS00001719D-page 305 MEC1322 26.9.11 TACH TARGET REGISTER The TACH Target Registers hold the target tachometer value that is maintained for the RPM based Fan Control Algo- rithm. If the algorithm is enabled, setting the TACH Target Register High Byte to FFh will disable the fan driver (or set the PWM duty cycle to 0%). Setting the TACH Target to any other value (from a setting of FFh) will cause the algorithm to invoke the Spin Up Routine after which it will function normally. These registers only apply if the Fan Speed Control Algorithm is used. 26.9.12 TACH READING REGISTER The TACH Reading Registers’ contents describe the current tachometer reading for the fan. By default, the data rep- resents the fan speed as the number of 32.768kHz clock periods that occur for a single revolution of the fan. The Equation below shows the detailed conversion from tachometer measurement (COUNT) to RPM. where: - Poles = number of poles of the fan (typically 2) -fTACH = the frequency of the tachometer measurement clock - n = number of edges measured (typically 5 for a 2 pole fan) - m = the multiplier defined by the RANGE bits - COUNT = TACH Reading Register value (in decimal) The following equation shows the simplified translation of the TACH Reading Register count to RPM assuming a 2-pole fan, measuring 5 edges, with a frequency of 32.768kHz. Offset 0Ah Bits Description Type Default Reset Event 15:3 FAN_DRIVE_FAIL_BAND[12:0] The number of Tach counts used by the Fan Drive Fail detection cir- cuitry RES 000000000 0000b VCC1_R ESET 2:0 RESERVED R/W 000b - Offset 0Ch Bits Description Type Default Reset Event 15:3 TACH_TARGET[12:0] The target tachometer value. RES 1111111111 111b VCC1_R ESET 2:0 RESERVED R/W 000b - RPM 1 Poles -------------- n 1 – () COUNT 1 m ---- × -------------------------------- × fTACH × 60 × = RPM 3932160 m × COUNT ------------------------------- = |
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