Calculating the tachometer reg
this application note explains how the max6615/max6616 temperature monitors and fan-speed controllers calculate the tachometer register value. the relationship between tachometer resolution and fan speed is also discussed.
calculating the tachometer count to determine the value of the tachometer register for a given fan, you need to know several values: the internal clock frequency of the max6615/max6616 used for counting; the maximum rated rpm (revolutions per minute) of the fan; and the number of pulses that the fan issues per revolution.
the max6615/max6616 have an internal clock counter which runs at 3.9khz. every 67 seconds, the max6615/max6616 count the number of clock cycles that occur between two consecutive tachometer pulses, edge to edge, of the attached fan. the number of clock pulses is stored in the corresponding fan tachometer register.
when determining the tachometer value, first calculate the time between fan pulses. to do so, you must know the maximum rated fan speed (rpm) and the number of pulses issued per revolution for the chosen fan. the fan's rpm value is divided by 60 seconds per minute to get the rps (revolutions per second) value. that rps value is then multiplied by the number of pulses per revolution. the result is the number of pulses per second which the fan generates at maximum speed. if this value is inverted, then you determine the time between fan pulses.
as an example, if a 4000rpm fan produces two pulses per revolution, then the number of pulses per second would be 133.33. to determine the time between consecutive pulses, invert the value of 133.33 pulses per second and get 7.50ms/pulse.
now that you know the time between pulse edges, you can determine how many clock cycles occur between the pulse edges. since the max6615/max6616 use a 3.9khz clock for the tachometer counter, you can determine the value of the tachometer register by multiplying the time between pulses by the clock-counter frequency of 3.90khz.
continuing the 4000rpm fan example, take the time between pulses, i.e., 7.50ms/pulse, and multiply that by the clock-counter frequency of 3.90khz. the result is 29.25 clocks per pulse. consequently, a tachometer value of 29d (1dh) would be placed into the tachometer register for that fan. this will be the value for the maximum speed of that fan. if the fan were to reduce speed, there would be fewer revolutions per second; the value in the tachometer register would increase as the number of clock cycles per pulse would increase because of the greater time between pulses. thus, the higher the value in the tachometer register, the lower the fan's speed.
determining tachometer register resolution to determine the resolution of the tachometer measurements, use the above steps to create an equation to represent the fan rpm-to-tachometer-count relationship. this relationship can be simplified to the following equation: using this equation, you can input the possible values for the tachometer register (0 to 255 decimal) and determine the corresponding fan rpm value. figure 1 shows the relationship between fan rpm and tachometer register value.
figure 1. relationship between tachometer register value and fan speed.
tachometer measurement resolution is best when the fan rpm value is 4000rpm or less for a two-pulse-per-revolution fan. the maximum fan speed that the max6615/max6616 can measure for a two-pulse-per-revolution fan is 117krpm, as that corresponds to a tachometer register value of 01h. this extended range is very useful when reading the tachometer register to determine if the attached fan is running.
盘点美国四大移动运营商的5G部署进展
首批“百城千屏”超高清视音频传播系统技术标准正式发布
SK海力士M16工厂或明年全面投产
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Calculating the tachometer reg
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calculating the tachometer count to determine the value of the tachometer register for a given fan, you need to know several values: the internal clock frequency of the max6615/max6616 used for counting; the maximum rated rpm (revolutions per minute) of the fan; and the number of pulses that the fan issues per revolution.
the max6615/max6616 have an internal clock counter which runs at 3.9khz. every 67 seconds, the max6615/max6616 count the number of clock cycles that occur between two consecutive tachometer pulses, edge to edge, of the attached fan. the number of clock pulses is stored in the corresponding fan tachometer register.
when determining the tachometer value, first calculate the time between fan pulses. to do so, you must know the maximum rated fan speed (rpm) and the number of pulses issued per revolution for the chosen fan. the fan's rpm value is divided by 60 seconds per minute to get the rps (revolutions per second) value. that rps value is then multiplied by the number of pulses per revolution. the result is the number of pulses per second which the fan generates at maximum speed. if this value is inverted, then you determine the time between fan pulses.
as an example, if a 4000rpm fan produces two pulses per revolution, then the number of pulses per second would be 133.33. to determine the time between consecutive pulses, invert the value of 133.33 pulses per second and get 7.50ms/pulse.
now that you know the time between pulse edges, you can determine how many clock cycles occur between the pulse edges. since the max6615/max6616 use a 3.9khz clock for the tachometer counter, you can determine the value of the tachometer register by multiplying the time between pulses by the clock-counter frequency of 3.90khz.
continuing the 4000rpm fan example, take the time between pulses, i.e., 7.50ms/pulse, and multiply that by the clock-counter frequency of 3.90khz. the result is 29.25 clocks per pulse. consequently, a tachometer value of 29d (1dh) would be placed into the tachometer register for that fan. this will be the value for the maximum speed of that fan. if the fan were to reduce speed, there would be fewer revolutions per second; the value in the tachometer register would increase as the number of clock cycles per pulse would increase because of the greater time between pulses. thus, the higher the value in the tachometer register, the lower the fan's speed.
determining tachometer register resolution to determine the resolution of the tachometer measurements, use the above steps to create an equation to represent the fan rpm-to-tachometer-count relationship. this relationship can be simplified to the following equation: using this equation, you can input the possible values for the tachometer register (0 to 255 decimal) and determine the corresponding fan rpm value. figure 1 shows the relationship between fan rpm and tachometer register value.
figure 1. relationship between tachometer register value and fan speed.
tachometer measurement resolution is best when the fan rpm value is 4000rpm or less for a two-pulse-per-revolution fan. the maximum fan speed that the max6615/max6616 can measure for a two-pulse-per-revolution fan is 117krpm, as that corresponds to a tachometer register value of 01h. this extended range is very useful when reading the tachometer register to determine if the attached fan is running.
盘点美国四大移动运营商的5G部署进展
首批“百城千屏”超高清视音频传播系统技术标准正式发布
SK海力士M16工厂或明年全面投产
电力监控系统助力变电站实现配电系统智能化管理
医疗物联网设备相关案例
Calculating the tachometer reg
具有锐截止特性的有源高通滤波器电路
Amkor收购扇型晶圆级半导体封装厂商NANIUM
博通(Beken)的无线音频SoC发展蓝图
如何解读运放数据规格书
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丰田在汽车设计中正在采用VR和AR工具来改变自己的理念
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如何提高稳压电源性能，有何方法？
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三款新iphone获工信部入网许可 其中两款内存都达4GB
磁性随机存取存储器（MRAM)制造工艺
LiDAR传感器的1D、2D 或 3D变型
基于FPGA的交流电机驱动器的电流控制器2
2021年我国的扫地机器人市场规模将达到224亿元左右