Re: PIC + DDS = Frequency Synthesizer? [LZ87010]

From: jim granville (no.spam_at_designtools.co.nz)
Date: 01/03/04 

Date: Sat, 03 Jan 2004 14:05:52 +1300

Paul Keinanen wrote:
> 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.
>
<snip>
> 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.
>
<snip>
> 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.

  The Sharp LZ87010 uC has two DACs, with 128 Byte WaveTable RAMS, so it
would seem to be well suited to this.

  Not sure how real this is: There is data, and also an errata,
(which is promising) but no sign it has hit 'critical-mass'
- has anyone designed in the LZ87010 yet ?

-jg