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Comparing two slow reference clock frequencies

Discussion in 'Electronic Design' started by arant, Aug 14, 2006.

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  1. arant

    arant Guest

    Hi Everyone,

    Does anyone have any idea of how to compare two reference clock
    frequencies using digital logic only .

    The problem is something like this .There are two slow ref clocks Khz
    range CLKA (RC oscillator clock) CLKB (crystal oscillator clock) and a
    fast sys_clk (pll clock) Mhz range

    If freq CLKA > freq CLKB then
    op1 = 1
    elsif freq CLKA < freq CLKB
    op2 = 1
    op3 = 1
    end if

    One solution that I can think of is running two parallel counters on
    the sys_clk for one period of the CLKA (count a) and CLKB 9ount b) and
    comparing the count values

    if count a = count b then
    op3 = 1
    elsif count a > count b then
    op2 = 1 -- freq CLKB > freq CLKA
    else -- freq CLKB < freq CLKA
    op1 = 1
    end if

    Is there a better way of doing this instead of using teo parallely
    running counters
    An optimum solution in terms of area will be highly appreciated

    Eagerly awaiting some solutions
  2. Luhan

    Luhan Guest

    How about just using the phase detector in a PLL?

  3. How quickly does this comparator need to respond? One cycle? One

    The simplest way I can think of is to get an up-down counter chip. Set
    it up so it counts up clock1 for a while, then counts down clock2 for
    the same while. If it underflows, then clock2 > clock1, otherwise
    clock1 > clock2.
  4. Most phase detectors are only phase but not frequency responsive. Look at
    the functional block diagram of a AD9901 and cover it in whatever logic
    family that you may have available.

    Ulrich Bangert
  5. arant

    arant Guest

    Thanks guys for your suggetsions but the problem is the solution has to
    be manged in the digital (sic) domain only i.e no phase comparators et
    The requirement is for an SoC clock manager that generates an
    interrupt/event to the CPU based on the clock comparison operation.
  6. arant

    arant Guest

    How quickly does this comparator need to respond? One cycle? One
    Well there is no stringent response time requirement for the comparator
    but the minimum possible is the best
    I think using two parallel counters instead of one would reduce the
    response time latency but at the cost of area ...
  7. vasile

    vasile Guest

    I don't know what microcontroller/DSP/microprocessor system are you
    using, but usually this is done with a compare/capture feature. PIC
    series has such a module
    which is able to compare an unknown period with a timer period and
    switch a flag as required.

  8. PeteS

    PeteS Guest

    Phase detection can be done purely in the digital domain - see this
    datasheet for an example of a standalone device:


  9. Ken Smith

    Ken Smith Guest

    Can't you use a digital phase comparitor? Its only a few flip-flops.
  10. arant

    arant Guest

    Can't you use a digital phase comparitor? Its only a few flip-flops.

    I would if I knew what it was as far as I know there is a XOR gate or a
    JK f/f that act as phase comparators in the digital domain
  11. Ken Smith

    Ken Smith Guest

    Simplest type:

    If the signal is a square wave, you can just apply the signal to the "D"
    input of a flip-flop and clock it with the reference. This way you latch
    the state of the input at the edge of the reference and thus whether or
    not it has gone high before the reference. This type only works for
    signals of nearly equal frequencies. If one is a multiple of the other,
    it will tend to lock you to the harmonic.

    The double flip-flop type is call a "three state phase detector".

    This is the basic idea of that type:

    ASCII art:
    -------! \
    ! !AND>----BOTH
    ! ---! /
    -------- ! !
    True--!D Q!----+----------! \
    Signal -----!> ! ! !AND>---- Early
    ! R Q!O--- ! ---! /
    -------- ! ! !
    ! ! ! !
    BOTH ----- ! ------! \
    -------- ! ! !AND>---Late
    True--!D Q!--------+- ! ------! /
    Ref -----!> ! !
    ! R Q!O----------

    Depending on the phase, you get a pulse on "early" or "late" for each
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