Re: PIC + DDS = Frequency Synthesizer?

From: Paul Keinanen (keinanen_at_sci.fi)
Date: 01/02/04

  • Next message: Terry: "Re: Newbie assistance for Atmel at89c2051 project" 
    Date: Fri, 02 Jan 2004 10:18:04 +0200

    On 1 Jan 2004 16:15:14 -0800, russ@sotainstruments.com (RussT) wrote:

    >What I am trying to do is make a frequency synthesizer that will
    >output 1Hz to say 40kHz in 1Hz resolution.

    In order to generate a sine signal at the maximum frequency f, you
    need to update the DAC at least twice during each cycle, however, in
    order to be able to filter out the aliases, the update frequency has
    to be at least 2.5f to 3f. Thus, in this case the update rate has to
    be at least 100 kHz, thus the software calculation loop cycle time
    must be _exactly_ 10 us, thus, if the processor runs at 2 MIPS, there
    are 20 instructions available to update the phase accumulator, do the
    sine(x), lookup and control the DAC.

    However, if the frequency step needs to be exactly 1.0 Hz, then the
    phase accumulator has to have at least 17 bits and be clocked at
    131.072 kHz and a new value must be outputed every 7.62939453125 us,
    thus with a 2 MIPS processor speed 15.258... instructions are
    available. In practice you would have to tune the CPU crystal
    frequency in order to have an integer number of instructions for each
    software loop cycle.

    >I need to input via a
    >keypad, and have an LCD display show me the inputted frequency.

    If no high frequency timer interrupts or a free running hardware
    counters are available, then the software calculation loop must
    contains exactly the same amount of CPU clock cycles during each
    iteration. This means no conditional branching (other than constant
    loop counting) or if a conditional loop is used, in cases the branch
    is _not_ taken, some dummy instructions must be included that
    consumes_exactly_ the same number of clock cycles as the other path
    when the conditional branch is taken.

    How you are going to handle the keyboard and display during the
    available 15 instructions in addition to the DDS is a mystery. You can
    not simply handle these every 100th main loop iteration, since this
    would upset the timing.
     
    >I know
    >virtually zero about assembly language!

    Then I suggest that you should learn it before going any further with
    this project.

    >I do not mind using 2 x
    >PICs for the interface and control.

    It might be doable if the phase accumulator increment is transferred
    through parallel ports from one controller to the other and the
    parallel ports can be read with a single instruction (to avoid race
    conditions, if the user interface is currently changing the value).

    I would really suggest that you should look for a faster processor,
    preferably with a high frequency timer interrupt (in which case the
    DAC can be written during the interrupt service routine without too
    much jitter) or at least contain a free running high resolution
    counter (and use a busy wait to wait for a specific value to be
    reached). By using these kinds of tricks, the actual calculation time
    does not have to be _exactly_ the cycle time, but now it can (and
    must) be shorter than the DAC update interval.
       
    Paul

  • Next message: Terry: "Re: Newbie assistance for Atmel at89c2051 project"

    Relevant Pages

    • Re: Floating point in interrupt handler
      ... arithmetic and a lookup table to create a sine wave on one of the DAC ... to use floating point instructions to create more advanced control ... At the start of my loop function I save the ... currently have successfully uses floating point instructions in the kernel. ...
      (freebsd-hackers)
    • Re: WaitForSingleObject() will not deadlock
      ... I'd like to see the EXACT SEQUENCE OF INSTRUCTIONS issued in the locking sequence, ... My issue about the 2 CPU clock cycles is that once the lock is set, ... cycle detection using non-recursive mutex: ...
      (microsoft.public.vc.mfc)
    • rotate revisited
      ... Suppose we have an array A containing n equi-spaced elements. ... The cost of this method (excluding loop costs) is: ... Each cycle is of length n/d. ... I will use the term "small slide" for a data move that is less ...
      (comp.programming)
    • Re: Semi-multithreaded application in VB6
      ... > Those libraries control a motion system and wait most of their time ... The cycle takes about 30 s to ... like your code is generating a series of instructions to the external machine. ... then go back to the send-each-instruction loop. ...
      (comp.lang.basic.visual.misc)
    • Re: picking a new random seed
      ... avoid reusing the same random number sequence, I wrote a loop to run ... The real-time cycle counter on modern X86 processors provide a ... should change often (every machine cycle!) enough to keep Charles ... one of the more modern RNG algorithms if cycle size, ...
      (comp.lang.apl)