synchronizing data collection on picaxe

[chipwich]

My beatbox made the Makezine blog! No doubt it will guide some new minds to experiment with these neat little chips as I've already received a handful of emails.

One email was from someone pointing out a similar beatbox concept located at <A href='http://extraversion.co.uk/beatbox/' Target=_Blank>External Web Link</a> which is very cool, even though it probably does cost more than $5 and probably doesn't fit in 256 bytes!

So this brings me back to a question regarding my implementation:

My device contains some small &quot;jitter&quot; on the sample timing. I believe it is on playback (and not record) since it is most noticable *after* the loop is played one time. How best to synchronize the playback and avoid timing errors? Is there a way to do this with an *internal* timer? On the 08M?

Thanks.

 

[chipwich]

One thought I had was to use PWMOUT to create a timer and loop this back through an input pin. Unfortunately, the slowest PWMOUT frequency still looks to be far too high for reasonable sampling on an input pin.

I could create a timing signal with another 08M, but prefer to keep it on one chip if possible. Do any picaxe chips have the capability for accurate timings without using an external timebase?

 

[xstamp]

I know its not the single chip solution you are looking for but how about feeding the pwmout pulses back to a PICAXE input via a ripple counter? For example, the 74HC4040 would give you upto 12-stages.

 

[xstamp]

Just to make things a bit clearer, for a PWMOUT of 4kHz the output from the 12-stage of the ripple counter would be about 1Hz but you also have the option of driving the PICAXE input from another output stage/frequency. Although a two chip solution it has the advantage of being directly adjustable from the standard PWMOUT command, rather than address an internal timer.

 

[Dippy]

Have you experimented with poking Timer2 prescaler? (12h bits 0&amp;1).
(I haven't understood the original question - but thought I'd throw this in).

And didn't Jeremy fiddle with his scalers for something like this? I don't know whether anything else would get mucked if care not taken.

 

[chipwich]

xstamp, The ripple counter is a good idea if I have to go to a second chip. With 12 stages, I can definitely find a useful clock signal. Unfortunately I don't have one in my very limited IC box. Though I do have spare 08M's, so that's probably what I'll end up doing.

Dippy, sorry if the question wasn't clear. I would like to acquire samples regularly (i.e. a datalogger), and then play them back with the same timing. Using the &quot;pause&quot; command for delay, however, I noticed that there is a small amount of timing inaccuracy in the process. This is presumably due to calculations which much be performed (i.e. data compression) and take variable lengths of time to compute.

Using a clock for timing rather than a software command is probably the way to solve this. I was wondering: can I implement this with a single chip or is it necessary to use 2 ICs?

Thanks.

BTW, the device is at <A href='http://www.medcosm.com/picaxe_beatbox.htm' Target=_Blank>External Web Link</a>
if it helps to see the current implementation.

 

[hippy]

It should be possible to poke the Timer/CCP SFR's in the 08M to generate an output toggle at a fixed rate of your own choosing.

I haven't tried it with an 08M but it worked okay on the 18X. Make sure Pin 2 is made an output.

http://homepage.ntlworld.com/the.happy.hippy/picaxe/timers.zip

An alternative approach might be to sample the timer when inputting then poke the timer when outputting and wait for a timeout to move on to the next sample.