Reply by karthikbalaguru●February 17, 20082008-02-17
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
Reply by linnix●February 13, 20082008-02-13
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.
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.
Reply by Arlet Ottens●February 13, 20082008-02-13
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?
>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
Reply by Paul Keinanen●February 11, 20082008-02-11
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
Reply by linnix●February 10, 20082008-02-10
> > 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.
>
> 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. Perhaps you can also adjust it with a
light sensor.
However, when space is the constraint, AC is usually not available.
This is true in my case and probably true in the OP's case as well.
Reply by Spehro Pefhany●February 10, 20082008-02-10
On Sat, 09 Feb 2008 11:18:57 -0600, the renowned "royazriel"
<royaz@zahav.net.il> wrote:
>Hello,
>
>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)
>
>Thanks,
>
>Roy Azriel
The only practical way to get 50ppm absolute frequency tolerance over
temperature is with a crystal or an external oscillator that uses a
crystal. With the latter you'll get a guarantee, but it tends to use
more power. If absolute tolerance isn't important (say 5000ppm) and 15
or 25ppm/K typical tempco is okay then consider a small SMT ceramic
resonator.
Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
Reply by Paul Keinanen●February 10, 20082008-02-10
On Sat, 09 Feb 2008 18:35:57 -0600, Vladimir Vassilevsky
<antispam_bogus@hotmail.com> wrote:
>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.
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). Last time I looked at
http://www.fingrid.fi/portal/in_english/electricity_market/state_of_power_system/
the Finnish (and hence also Nordic network) frequency was 49.97 Hz and
the time error more than 23 s.
This may be an issue if the time is compared to an accurate time
source (NTP, GPS etc.) e.g. one system running of the mains and the
other from some NTP source.
However, if it is just needed that two (or more) devices keep the same
time, regardless of accuracy, the AC mains is a good clock source,
however, there may be a 7-15 ms time offset, if the devices are
powered from different phases in a 3-phase distribution system.
Some precaution is also needed to avoid false clock pulses due to all
kind of interference riding on the AC mains voltage. For devices
intended to work all over the world, some logic is needed to sense if
50 or 60 Hz mains is used.
Paul
Reply by linnix●February 10, 20082008-02-10
On Feb 9, 8:47 pm, Jim Granville <no.s...@designtools.maps.co.nz>
wrote:
> Arlet Ottens wrote:
> > 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)
>
> Over what temperature and voltage range ?
> Does this have any field-signals for Cal-Check, or do you also
> want this stable over the product life too ?
>
>
>
> > 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
>
> No one makes that precision, which is one part in 20,000.
> Trimmed RC osc are in the one part in 100 range, with trims steps to
> around one part in 250 - so the OP is chasing around 100x the
> start of the art.
>
> Single package precisions like this, may be possible in years to come,
> with the Silicon MEMS oscillators.
> This is likely to be Dual-Die solutions, but with a custom Osc that
> matches the needs of the MEMS block. See SiTime's web site - their
> special bridge is 0.8mm x 0.6mm x 0.15mm, so can fit into a
> molded package.
This might work for us. We will have three to four die on a hybrid
anyway. We will likely buy a life-time (project or my life, whichever
come first) supply of AVR169 die to pack with it.
Reply by Jim Granville●February 10, 20082008-02-10
Arlet Ottens wrote:
> 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)
Over what temperature and voltage range ?
Does this have any field-signals for Cal-Check, or do you also
want this stable over the product life too ?
>
>
> 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
No one makes that precision, which is one part in 20,000.
Trimmed RC osc are in the one part in 100 range, with trims steps to
around one part in 250 - so the OP is chasing around 100x the
start of the art.
Single package precisions like this, may be possible in years to come,
with the Silicon MEMS oscillators.
This is likely to be Dual-Die solutions, but with a custom Osc that
matches the needs of the MEMS block. See SiTime's web site - their
special bridge is 0.8mm x 0.6mm x 0.15mm, so can fit into a
molded package.
-jg