Accurate Internal Oscilator

On Sun, 10 Feb 2008 09:03:18 -0800 (PST), linnix
<me@linnix.info-for.us> wrote:


>> While the AC mains have a very good long time accuracy in most
>> countries, since the generators are driven at a small overspeed at
>> night to catch up any cycles lost during high demand during the day, a
>> mains driven clock can be off by several seconds during the day
>> (frequency error up to 100 ppm or even more).
>
>The uC can adjust for this error if necessary, since it needs to
>sample the frequency anyway.  

Not very practical for the mains+RC oscillator case, more realistic in
case of mains+crystal oscillator case. 

You do not know when the mains is at the nominal frequency and even if
you measure the cycles for any 24 hour period,  this could still give
an error of a few seconds. Averaging such measurements over a period
of one week or one month will reduce the error significantly. Crystal
oscillators have some initial frequency error and some aging and these
can be compensated by the averaged mains reference after a few weeks.

However, the short term (minutes and hours) frequency stability for
both the mains and an RC oscillator is so bad, I very much doubt that
you could get reliable values even with long averaging, at least you
would have to use a temperature sensor to compensate for the RC
oscillator temperature sensitivity.
   
>Perhaps you can also adjust it with a
>light sensor.

You are going to have to use quite long averaging (years), if you
intend to use the light sensor to detect night and day and thus adjust
the internal oscillator :-).

If you intend to get the mains frequency from an AC powered lamp, this
may work if the sensor is mounted inside a fluorescent lamp using a
conventional ballast, however, ambient (solar) light and light from
other fluorescent lights driven from an other mains phase will degrade
the light variations significantly. 

A 15 W/230 V incandescent lamp may also produce significant light
variations during each mains cycle, however lamps with higher power or
lower operating voltage will have much higher thermal inertia,
recusing the light variations significantly during a mains cycle.

Most modern "energy-saver" lamps have a free running switched mode
power supply, so you can not determine the mains frequency from the
light output.

Paul
 

>royazriel wrote:
>
>> For many years we are using pic 8 bit micros for our product. We have
come
>> to a point we are considering a new solution. The pic internal
oscilator is
>> not accurate enough for our application because it is based on RC
>> technology.
>> We need an internal oscillator because we are becoming very tight in
>> phisical space(volume wise).
>> 
>> My Question is. can someone point 8bit mcu with Acuurate Intenal
>> Oscillator- less then 50ppm tolerance)
>
>I've never seen anything that accurate for an internal oscillator.
>
>Have you considered a miniature external crystal ? For example:
>
>http://www.ecsxtal.com/store/pdf/ecx-2236.pdf
>
We were not farmilure with that spacific manufacturer. do you know if they
make it also for lower frequencies like 4Mhz?

Thanks,

Roy

royazriel wrote:

>> Have you considered a miniature external crystal ? For example:
>>
>> http://www.ecsxtal.com/store/pdf/ecx-2236.pdf
>>
> We were not farmilure with that spacific manufacturer. do you know if they
> make it also for lower frequencies like 4Mhz?

No, they don't. The smaller crystals only come in higher frequencies.

http://www.ecsxtal.com/store/c-2-crystals.aspx

If you like the 16 MHz xtal solution, you may be able to switch to a 16 
MHz MCU. There are some available in packages down to 3x3 mm, depending 
on your requirements.

For instance:
http://www.silabs.com/public/documents/tpub_doc/dsheet/Microcontrollers/Small_Form_Factor/en/C8051F52x-F53x.pdf

On Feb 12, 11:46 pm, Arlet Ottens <usene...@c-scape.nl> wrote:
> royazriel wrote:
> >> Have you considered a miniature external crystal ? For example:
>
> >>http://www.ecsxtal.com/store/pdf/ecx-2236.pdf
>
> > We were not farmilure with that spacific manufacturer. do you know if they
> > make it also for lower frequencies like 4Mhz?
>
> No, they don't. The smaller crystals only come in higher frequencies.

Depends on where you get them from.

>
> http://www.ecsxtal.com/store/c-2-crystals.aspx
>
> If you like the 16 MHz xtal solution, you may be able to switch to a 16
> MHz MCU. There are some available in packages down to 3x3 mm, depending
> on your requirements.
>
> For instance:http://www.silabs.com/public/documents/tpub_doc/dsheet/Microcontrolle...

I got a couple of 8 MHz samples in 3x2.  They are close to $1 for
1000.  They are so small that I can't find them anymore, may be in one
of my pockets or the washer/dryer.

On Feb 10, 5:35=A0am, Vladimir Vassilevsky <antispam_bo...@hotmail.com>
wrote:
> royazriel wrote:
> > For many years we are using pic 8 bit micros for our product. We have co=
me
> > to a point we are considering a new solution. The pic internal oscilator=
 is
> > not accurate enough for our application because it is based on RC
> > technology.
> > We need an internal oscillator because we are becoming very tight in
> > phisical space(volume wise).
>
> > My Question is. can someone point 8bit mcu with Acuurate Intenal
> > Oscillator- less then 50ppm tolerance)
>
> The 50ppm tolerance requires a crystal or ceramic resonator. Among the
> other suggested solutions, you can use the AC power frequency as a
> reference. A microcontroller like MSP430 or HC908 can use the PLL to
> lock on the AC frequency directly. With the other microcontrollers, you
> may be able to correct either the internal oscillator itself or the
> critical frequencies derived from the oscillator by using the AC as the
> ref. clock.
>
Interesting info :):)

Karthik Balaguru