Re: [Amforth-devel] Thanks

[pito <pi...@vo...>]

Hi Leon,
the accuracy of an internal RC oscillator according to atmel is +/- 3%.
You may calibrate it (see atmega docs) to get better. There is also
a dependency on voltage and temperature, what can be another few percent
with RC. The recomended precision for 8N1 communication is +/- 4.5%
for a given baudrate. So you may get problems. It does not matter whether
you use 1,4,or 8MHz internal RC oscillator.
As a rule of thumb the RC oscilator stability is 10^-2, cheapo Xtals
10^-5, good Xtals 10^-6, thermostated (ovenised) 10^-7, rubidum source
10^-10, cesium 10^-13. Short time stability 10x better (e.g. for 10sec).
However, there is "aging" factor as well. With cheapo Xtals 100ppm per
annum. So you have the choice...
Pito

----- PŮVODNÍ ZPRÁVA -----
Od: "Leon Nathaniel Maurer" <leo...@gm...>
Komu: amf...@li...
Předmět: [Amforth-devel] Thanks
Datum: 8.7.2010 - 6:33:35

> It looks like I've got amforth working (photographic
> evidence:
> http://tinyurl.com/2d7cddx) -- thanks for your help!
> 
> To avoid future problems, I do have a couple related
> quick questions. The FAQ
> notes the default 1MHz clock (8 MHz internal RC clock
> divided by 8) can be
> unreliable and suggests: "Try to maximize the CPU frequency."
> What are the
> symptoms of this problem? Just not being able to get
> the serial communication
> to work? Does it help to use the internal clock but
> not divide by 8 (giving an
> 8MHz clock), or is inaccuracy with the internal clock
> part of the problem?
> 
> Thanks.
> -Leon
> 
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