Prescaler question that no one can answer! :)

Discussion in 'General Electronics Chat' started by KLillie, Mar 5, 2015.

  1. KLillie

    Thread Starter Member

    May 31, 2014
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  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    If it gives you the division you want I don't see why not.
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    I would have preferred to use 74F161 which goes to 130MHz.
     
  4. KLillie

    Thread Starter Member

    May 31, 2014
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    Thanks MrC. The 74f161 datasheet says Fmax of 100MHz. What does that mean? Also I didn't see a minimum input frequency. My problem is my arduino frequency counter will only be able to read to about half my 16MHz clock speed without a prescaler. I've seen lots of prescalers that will go into the Ghz range but have Fmin of 100+MHz. This would leave me with a measuring gap between 8-100Mhz. If I use my internal scalers I should be able to get 30-50MHz reliable readings (from what I've read), so I would want a prescaler that could take the lead @ that range, right? Cheap is the key word. The problem is hunting components!
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    There is no minimum input frequency with TTL counters. They will clock down to zero Hz.

    SN74F161A is available from well known suppliers for under $1 each ($0.30 at Newark). Read the data sheets closely and you will see 120MHz quoted (TI and Fairchild).

    NXP N74F161A is quoted at 130MHz.
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Fmax is the maximum permitted input frequency, the A versions are upto 120Mhz speed.

    What frequency are you trying to measure, from DC to what?
     
  7. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the ecl 10,000 series will go to the ghz range. a little power hungry tho.
     
  8. KLillie

    Thread Starter Member

    May 31, 2014
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    Thanks guys. ideally I'd like to be able to measure any and all frequencies in the know universe. Practically, I'd be happy with FM frequencies. The SN74F161A sounds good and cheap! ;) Dodgydave- building oscillating circuits has been kind of my restart point in this hobby, so it's more AC. ANALOG BABY!!! :)
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    So, get two 74F161 4-bit binary counters and that will give you 8 bits. Interface the 8-bit output of the counters to an 8-bit port.
    This means that your MCU only has to accept clock frequencies up to 500kHz and your frequency counter is good to 128MHz.
     
  10. KLillie

    Thread Starter Member

    May 31, 2014
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    Nice! Thank you.
     
  11. RichardO

    Well-Known Member

    May 4, 2013
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    I have seen a trick for reading a counter that only uses a few pins on the MCU. First you select the external clock source and count the pulses. Then you switch the clock source to be from the MCU. The MCU clocks the counter until an overflow/carry occurs. External_counts = 2^Number_of counter_bits - Number_of_MCU_counts.

    edit: Fixed equation.
     
    Last edited: Mar 8, 2015
  12. KLillie

    Thread Starter Member

    May 31, 2014
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    Sounds a lot like these frequency counter codes I've been studying. Thanks for input!
     
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