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'12C508 Oscillator Frequency Variation?'
1998\02\02@234618 by

If I use the internal oscillator on the 12C508 with the internal calibration
value, does anyone know how much to expect the oscillator frequency to deviate
from the expected 4MHz value?

What I would really like is a description of what the distribution of
frequency values looks like. Is it a standard distribution around a 4MHz mean?
If so, what is the frequency variance at 1, 2, and 3 standard deviations from
the mean? This would help me know how much to expect the frequency to vary
from part to part.

Thanks in advance for any info...

Mark Winters
>If I use the internal oscillator on the 12C508 with the internal calibration
>value, does anyone know how much to expect the oscillator frequency to
deviate
>from the expected 4MHz value?

I«ve read 5% somewhere, but I can«t find it in the 12C508 datasheet!?
The frequency is also teperature and voltage dependant.

PIC14000 also have internal 4MHz osc:
Internal Oscillator data for PIC14000 from datasheet DS40122B:
"Frequency Range fosc(in) min3.0 typ4.0 max5.0 MHz"
** That«s 4MHz +-25%  Ouch!! **
"Temperature Coefficient tc(in) typ -0.04 %/¡C From Tmin to Tmax"
"Supply Sensitivity ss(in) typ 0.8 %/V From VDDmin to VDDmax"
"Jitter jit(in) typ 100 ppm ±3 sigma from mean" - What is sigma?

In a program example from Mchip for the PIC14000 a dealy variable for
software RS232 is calcualted like this:
RS232DLY=23+0.09*Fosc-0.0009*TCFosc*(BTEMP-25)
BTEMP = temperature in one C resolution (calcualted earlier from internal
temp sensor)
Fosc and TCFosc (frequency at 25¡C) calibration data are required to run this
routine.
The pic14c000 is shipped without TCFosc data. (temp coefficient of internal
oscillator)
This program example used a fix TCFosc of 10h based on char data.
/Morgan
Morgan Olsson, MORGANS REGLERTEKNIK, Sweden
mrtiname.com, ph: +46 (0)414 70741; fax 70331
-
Mark,

All of the data you need is in the data sheet for the '5xx parts.  There
are a series of graphs that show the frequency tolerance given a
particular operating voltage, and another series of graphs that show
tolerance over temperature.  Pick your Vdd and T and run with it.  The
tolerance, of course, is due to process variations over a series of
wafers that were tested.  Across a given wafer, corner to corner
variations are typically (although they don't have to be) smaller than
the whole chart.

If you choose wisely, you'll find that you can operate in a +/- 3% band
- perfect for RS232.  I did - and my Fosc results were fine.  I
typically saw 4MHz + 0.5 - 1.0%.  The devices are tested and calibrated
for 25 C @5V, so that should help.  Use the calibration value they come
with - if you want to run faster or slower, you can change it, but
again, they come preset with the best value that targets them for 4MHz
at 25C, 5V.

As far as distribution, yes, process variations for most IC's follow
normal distribution curves, but as far as what you can expect to see
from any manufacture: it can easily be out to +/- 3 sigma over volume.

Chuck Mauro

> {Original Message removed}

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