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LM96000: Tachometer reading

oren zvuluni
Intellectual 325 points
Part Number: LM96000

Hi,

i have LM96000 device on my linux board.

I am working with Tachometer 2 and i succeed to read the value in the register.

I have few questions about the value.

1. according to  the Datasheet the tachometers registers are 16 bit and the two first bits (bits 0 and 1)in the list significant byte are indicating the tachometer accuracy data:

so, the tachometer value is the 16 bites? it's just the other 14 bits? if it's the other 14 bites, am i need to shift left the data  two places?

2. how can i calculate the round per minute of the fan according to the tachometer? (what is the meaning of "full fan revolution" ?)

Thanks,

Oren

  • 0
    TI__Expert 5695 points
    Hi Oren,
    Our support team for these products are on vacations due to the end of year holidays. We will send a reply in the first week of January 2018.
    Sorry for the inconvenience.

    Regards,
    Jose
  • 0
    TI__Expert 7845 points
    Oren,

    I will need to look into this and get back to you within 24 hours.

    -Kelvin
  • 0 in reply to Kelvin Le THS
    Intellectual 325 points

    Kelvin,

    It's O.K, thanks A lot!

    Oren

  • 0 in reply to oren zvuluni
    TI__Expert 7845 points
    1. Bit2 to Bit15 are the count value between tach pulses. yes you will need to shift it.
    2. RPM will depends on your fan tach pulse rate per revolution. Most fans produce two
    tachometer pulses per full revolution according to the datasheet. Fan Tachometer Reading registers contain the number of 11.111 µs periods (90 kHz) between full fan revolutions. Basically you count the number of clock cycles between the two tach pulses to calculate the period of one fan revolution.
    Ex: If your count is 100 cycles then the RPM = 1 revolution/(100cycles*11.111us/cycle*1min/60s)= 54000.5 revolution/min

    "Full revolution" = when the fan has turned 360 degrees in circle.

    -Kelvin
  • 0 in reply to Kelvin Le THS
    Intellectual 325 points
    Kelvin.

    When my Fan 2 working in full speed, i get 345 in Tachometer 2 --> RPM= 15653 revolutions/min ( i shifted right the value two times according to your's answer above).
    I think it's does not make sense that my fan is making 15653 revolutions in 1 minute, and in seconds it's sounds more crazy - 260.8 revolutions in 1 second.

    maybe we are missing something here?

    Thanks,

    Oren
  • 0 in reply to oren zvuluni
    TI__Expert 7845 points

    Oren,

    I might be wrong on my interpretation of the datasheet.  If you don't double shift the 345 TACH value it would be 345x4=1380 (I'm ball parking here).  The RPM = 90000*60/1380 = 3913 RPM.

    This number seems more reasonable.  Do you know what's the full speed RPM spec of your fan?  That would be a good sanity check. 

    -Kelvin

  • 0 in reply to Kelvin Le THS
    TI__Expert 7845 points

    Oren,

    I went over the datasheet again. You will need to use the 16-bit value to determine the RPM. Sorry about the confusion. The DS specified a 16-bit max value in the EC table for the Fan Full-Scale Count on pg 6. Furthermore, on pg 15 the datasheet mentioned that 0xFFFF indicates the fan is not spinning. Based on these statements, we must assume that we need to use the entire 16-bit value to determine the RPM.

    RPM = [90kcycle/s*60s/min/]/[TACH count]

    -Kelvin

  • 0 in reply to Kelvin Le THS
    Intellectual 325 points

    Kelvin,


    Thank you so much.
    The full speed RPM spec of my fan is 3900 - now it's make sense.

    Oren