AVR-Chat

--- In AVR-Chat@yahoogroups.com, Thomas Keller <tkeller1@n...> wrote:
>
> On Tue, 2005-11-15 at 22:03 -0800, Reid Smith wrote:
> >   The Pic chip requires a 4 mhz crystal in order for
> > it to function. I'm building a serial lcd driver and
> > want the cheapest chip I can find. It requires in
> > order for it to work correctly 6 out and 2 in with error
> > checking so any 18-20 pin should work fine. 
> 
>    AS may be.  Many of the AVR chips will run at 8 Mhz (which would be
> the equivalentof a 32 Mhz PIC) without a crystal, they cost less, and
> there are a number os AVRs specifically designed to drive LCD displays
> (some offer as many as 100 segments of drive).
> 
> Tom
> 

Many PIC's have an internal oscilator.  but AFAIK, none have an 8mhz
internal.

But, that brings up another question.

I was looking for a tachometer.  it seems that one needs a very
accurate timing base to compare things too.

one range would be less than about 6,000 RPM or 100hZ and the other
range would be less than 50,000 RPM.

As I understand, one can count to 'n' and while counting, also count
inputs from the sensor.   then calculate speed.

The other way is to count input pulses to reach 'x' and then see how
long it took and then calculate speed.

One works better for low speeds, the other for high speed.

but, both need to have an accurate refreance.

So, the question is that of crystal accuracy ?

is the interanl any better than an exteral oscilator ?
better than an external crystal ?

Dave