Crystal

[leonel]

Just one stupid question...
Why we have to use sometimes one crystal and sometimes not? Why we can´t use always RC circuit?
Using RC circuit can I have accurate timing?

[Luciano]

Microcontroller Clock - RC Oscillator, Crystal or Resonator?
http://www.maxim-ic.com/appnotes.cfm/appnote_number/2154

Luciano

[Melanie]

>> "Using RC circuit can I have accurate timing?"

Not accurate (or stable) enough.

If that was the case, employment welfare lines would be full of Crystal manufacturers looking for a new career.

[leonel]

When you say "not accurate enough", for example in 10 minutes how much it could be the diference? 5 seconds or minutes?

[Melanie]

That is over-simplistic and impossible to determine.

It could be less than a Second, or it could be a lot more than ten Seconds.

1. It will vary from day-to-day with temperature, humidity and the type of Components you've used. And if you are going to precisely adjust your Resistor with a variable such as a POT or Trimmer, then your component cost is already more than a crystal (and that's not even factoring-in any labour).
2. The ONLY thing you can guarantee with a Capacitor is that it is NOT the value that is printed on it's case. This is TRUE! Try measuring the value of any Capacitor that you have.
3. I can show you two Resistors... one you'll buy four for a cent, the other costs eight dollars... but they're both the same value and the same wattage... but they will behave completely differently.
4. I can show you two Capacitors... one you'll buy for two cents, the other costs almost ten dollars... but they're both the same value and the same voltage... but they will behave differently too.

If you are considering RC oscillator for time-keeping, then let me know what your finished product is - I'll need to know in order to avoid it.

[NavMicroSystems]

Could someone help me to understand this discussion?

If I have a (in terms of timing) non critical application
and there is a PIC with INTRC-OSC running fast enough for what the program needs to do,
fine, this would save some board space.

If I need an external clock source anyway,I would not even think about RC.

BTW

for a prototype on a breadboard you could go even cheaper,
the capacitance of the breadboard in conjunction with a few drops of your favourite drink (as a resistor) at the right place on the board would also give a clock source.

But with this design don't ask us why your serial Comms are not working @ 115kbps

[leonel]

Yes, I can explain you this discussion...
One RC system it´s cheaper than a Crystal.
My system it´s not critical, I don´t need such an accurate timing, but I need accurate enough... For example in 9m50s I don´t mind having 9m47, but I do mind having 9m40s.
My doubt was if with one crystal I woul have 9m50s and with an RC 9m47.
If it was the case I would prefer an RC system, but I already see it isn´t.
Regards
Leonel Monteiro

[Melanie]

Then you might as well chose a PIC that has an internal oscillator and save yourself the bother of an external RC.

[leonel]

I already made my decision... I will chose a Crystal. As you said is much more accurate than the other one´s.
Thank you, and I´m sorry to NavMicroSystems, but today I´m with bad humor :)