timers and micros
Hey Nettron, hope my post didn't sound too snippy, you brought up a reasonable objection to using micros. There is a substantial amount of baggage associated with micro design, but it's quite possible to set yourself up on the cheap. Most come with free assemblers, and there are some fine freeware "C" compilers available (SDCC and GNUAVR are two that come to mind). And it may be a dirty word these days, but FORTH is a still a great fit for small micros, and has been ported to many.
For most simple projects, my preference would be to use a small flash-based micro like Atmels' TinyAVR or Microchips' PIC12F629. Both families have a large following, and plenty of info is freely available on the Web. An inexpensive in-circuit programmer can be made with your PCs' parallel port.
I need to update my micro setup (still using 8051's
, have dabbled with PICs and AVR) and this is the path I'm taking. I think the baggage that comes along with micros is worth the trouble. Instead of stocking dozens of different TTL and CMOS parts, a single (and easily re-programmable, for those of us who make mistakes :shock: ) generic chip can often suffice.
I also check the surplus outlets often, sometimes you come across amazing bargains. I use the DK catalog for quick price checks and it's a good barometer of which parts are popular (<$) and which are going obsolete. Prices are ridiculous for some parts, others are quite reasonable ($1 for the PICF629/qty 25 is pretty good).
As far as timing accuracy, 32Khz makes a good tradeoff between accuracy (they're watch Xtals after all), low cost (ditto) and low power consumption. Micros aren't any more or less accurate than TTL/CMOS dividers, both use a similar chain of divider stages. You may be thinking of software timing loops vs. using a micro's internal timer to trigger an interrupt. It's the second method I was talking about. Most micros these days contain at least one hardware timer, sometimes with an optional pre-scaler on the front end.
The oscillator is the key to timing accuracy, and one of the better schemes I've seen implemented on some of the RTC chips is to bring out the 1-second tap. Then you can trim the oscillator capacitor to get a more or less "exact" 1 second period. You can do the same thing with TTL/CMOS or micro's.
Of course, not everyone using this board has ready access to an inexpensive micro setup, and TTL/CMOS will be around for a long time to come.
- CAL
(This post probably belongs in the PIC micro section, but then it just feels like preaching to the choir)