LM63: Tachometer Mode

Hi Sam,

Below is my feedback to your inquires.

From LM63 datasheet show "Note: If the PWM Clock is 360 kHz, mode 00 is used regardless of the setting of these two bits." it's mean register "4A" bit 3 set to "0" the Tachometer Mode can't use "10" and "11" mode?

You are correct, when using 360kHz mode the Tachometer mode will function as mode 00.

What is the working principle of Smart-tach mode?

Smart-tach measurement accommodates for the corrupted tachometer pulses when using switching transistor drive.

If my fan PWM frequency is 25kHz, how can I do the Lookup table to control the fan speed?

Please see the application note on page 29 of the datasheet. With the internal clock set to 360 kHz, setting registers 4D to 7 the PWM frequency should be 25.71 kHz.

Thanks,

Hi Peter,

Below is my feedback.

1) what does it means about "Tach_Count_Multiple" and "Edges Used" on register 0x46 on pp25 of datasheet? how to apply these parameters to RPM calculation?

Please see "COMPUTING RPM OF THE FAN FROM THE TACH COUNT" on page 30 on how to calculate RPM.

2) what does "Step resolution %" means on pp29 of datasheet

The step resolution % is the Duty Cycle resolution percentage that is represented in register 0x4C.
If you are using 25.71kHz setting your Duty cycle resolution will be 7.14%. Therefore values in register 0x4C will be the following
0x01: ~7.14%
0x02: ~14.28%
0x03: ~21.42%
....
0x07: ~50%
...
0x0E: ~100%

3) What’s the definition and usage of the “Edges Used” item of register 0x4B bit [1..0]?

Do you mean in registers 0x46? I do not see this "Edges Used" for these bits. If so please see answer to question 1.

4) There are two Remote Temperature OFFSET registers 0x11 (MSB) and 0x12 (LSB). Does it mean that we could set the offset values (added or subtracted) to compensate for the different values measured by other sensors to generate the final Remote

Yes, negative offsets will be in 2's compliments form.

Diode Temperature reading in registers 0x01 and 0x10?

Yes

Thanks,

Hi Todd,

You are correct; these bits are used to represent the RPM and to indicate how the LM63 calculates the RPM. The "Edges Used" bit indicates how many edges the LM63 detected and used when calculating the RPM. The "Tach Count Multiple" bit indicates the multiplier the LM63 used to calculate the RPM.

The Tach mode set in 0x4A <1:0> determines when the Tach monitor will detect the edges of the tach input.

Mode 00: The Tach monitor is on regardless of its corresponding PWM.

Mode 01: The Tach monitor is on only when its corresponding PWM is high.

Modes 10 and 11: The Tach monitor is on only when its corresponding PWM is high. The PWM high time will be extended up to 50ms to allow enough time to calculate the fan speed.

For example, if the LM63 only detects 2 edges the LM63 multiplies this time period by a factor of 4. This results in the Bits [1:0] of Register 46hex to be set to 01 [Edges Used = 2, Tach_Count_Multiple = 4].

If only 3 edges are detected, then the LM63 multiplies this time period by a factor of 2. This results in the Bits [1:0] of Register 46hex to be set to 10 [Edges Used = 3, Tach_Count_Multiple = 2].

If 5 edges are detected, then the LM63 multiplies this time period by a factor of 1. This results in the Bits [1:0] of Register 46hex to be set to 11 [Edges Used = 5, Tach_Count_Multiple = 1].

Please see the below diagrams for further explanation

Hi Todd,

Besides, what does the "Smart-tach mode" mean for mode 10 or 11?

Please see above reply on Smart-tach mode.

"Smart-tach measurement accommodates for the corrupted tachometer pulses when using switching transistor drive"

What does "Use with direct PWM drive of fan power" mean for mode 10 or 11?

Direct PWM drive of fan power is the use case where the PWM directly involves turning the fan's power supply on and off typically by controlling a MOSFET. Please see Figure 3 for an example.

Is this rule applied for fans with 2 pulses/rev? Or it is applied to all fans?

This applies to all fans. Please see "COMPUTING RPM OF THE FAN FROM THE TACH COUNT" section of the datasheet to determine RPM if not using a 2-pulse/rev fan.

Since we set PWM_Clock_Select to 360 kHz, the Tach_Count_Multiple is fixed to 1 according to the datasheet. Does it mean bit 0 of register 0x46 will always be fixed to 1? Or we need to forcefully set this bit to 1 RPM calculation?

This will be fixed to 1 when PWM_Clock_Select is set to 360 kHz.

By the way, how to interpret the "Edges Used" bit when setting PWM_Clock_Select to 1?

No change in interpretation. When PWM_Clock_Select is set to 360 kHz the number edges used will be 5.

Thanks,

Hi Todd,

This is not the expected behavior of the device.  Device should change PWM with change of temperature.  What are your PWM settings (0x4A-0x5F)?  Also what is your Tcrit and Remote alerts values set to?

Thanks,

Hi Todd,

Glad to hear you have resolved the issue.  Yes, the PWM value will only change after dropping below the "Lookup Table Temperature Entry" minus the "Lookup Table Hysteresis" set in register 0x4F.  

Thanks,