Jinx,

Holy cow! Thanks a lot this is everything I wanted to know and more. I
didnt know any of that about the crystals. I have a couple questions
related to that.

1. Does the code syntax change depending on the crystal speed? it seems
like if it didnt why wouldnt you just always use the fastest one, the
price differences seem minimal.

2. Are there any internet resources that explain more about a hexadecimal
value crystal vs. a decimal value crystal? I have a problem understand how
a crystal that emits frequencies would have anything to do with binary or
decimal, I would like to know more about that.

3. Ceramic Resonators, I imagine when you talk about accuracy you are
referring to the fact that a clock cycle varies in the time it execute? so
basically this only matters when you need accuracy within milliseconds and
consistantly correct?

BTW, I have been considering if I should get into the PICs or the AVRs,
and from my initial emails it appears that I get a lot more support from
the PIC users than the AVR. I wonder why if the AVR's are faster and
'better' overall why the support base is so skewed towards PIC users...

- Matt


On Wed, 25 Sep 2002, Jinx wrote:

> > I am new to this. I was wondering for this beginner circuit:
> >
> > http://www.piclist.com/techref/piclist/cheapic/bincnt.htm
> >
> > does the C = 33pf designate a Crystal? if not, if it is a capacitor then I
> > am confused by the drawing because it says 32.768 kHz but does not
> > show an icon for the crystal
>
> It does - the drawing could have been spread out a little more. A 32k
> crystal with two 33pF caps is shown. Really
>
> > Which leads me to a generic question. When designing a circuit how
> > do you choose which crystal to use? do all projects use the same
> > generally?
>
> "all" and "generally" don't go together. The first consideration is
> probably speed. What do you want the micro to do, and how fast
> does it need to do it. But crystal speed isn't always reflected in the
> price. Common fast crystals may be be cheaper than uncommon
> slow crystals. 32k is pretty cheap, so are 4MHz, 10MHz and 20MHz
> so you havve that to consider as well. Then there's accuracy. Can you
> use a ceramic resonator (often cheaper than a crystal and may have
> built-in capacitors) even though it's not as accurate ? Size isn't an
> issue in most circuits
>
> For "general" work, you'd probably go with 4, 10 or 20MHz, depending
> of course on the speed rating of the micro. It's like the choice of
> "general" resistors. Everyone reaches for 1k, 4k7 and 10k when you
> need "a" resistor
>
> Specialised operations call for more thought. For example, if you need
> RS232 data comms with the micro, then a hexadecimal value crystal,
> eg 3.6864MHz, is easier to work with than a decimal value (eg 4, 10,
> 20MHz) because of the binary nature of the micro. However, if you wanted
> time delays or pulses that were exact multiples of say 10 microseconds,
> then a decimal crystal is more suitable
>
> > Also, what other components need to be ordered (like 5v regulator, etc).
> > I am going to make an order to digikey and I dont want to get the stuff
> and
> > realize I have to place a whole other order becasue I forgot some stupid
> > support component
>
> Regulators - 7805 (1.5A), 78L05 (100mA), LP2950 (low power, good for
> using the micro on battery power)
>
> Small transformer or plug-pack (wall wart), eg 6V, 12V
>
> A few 470R, 1k, 10k, 100k resistors, 10n caps, 10uF caps, 18 pin sockets,
> LEDs, small transistors (eg BC548, 2N3904, BC337) for switching relays
> etc, push-button switches, DC connector (for wall wart), toggle switch
> (power
> on/off), battery clips, would cover the basics
>
> You can look around for discarded circuit boards and envulturate many
> many components off them. Doing this can save you particularly on hard-
> ware like switches, LEDs, LCDs, connectors, capacitors, hook-up wire,
> some exotic parts. You'd be surprised what ends up in rubbish bins. One
> thrown-out VCR would keep you in basic parts for a while. Get cheeky
> and ask at a repair shop, they'll more than likely let you rummage
>
> --
> http://www.piclist.com hint: PICList Posts must start with ONE topic:
> [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads
>
>
>

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