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'[PIC]: PIC12XXX internal oscillator accuracy'
2002\06\04@050858 by Tsvetan Usunov

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face
Hello,

I thought this may be interesting to all of you who intend to use internal
oscillator when design with PIC12xxx.
Customer of ours designed short distance IR communication units where one
master talks to many salaves.
The slaves are made with PIC12CE519 and Internal oscillator is used, OSCAL
constant is loaded in OSCAL register as needed.
The first run of 12000 pcs have no *any* problems at all. When we run nect
6000 pcs, 500 slaves of them didn't establish communication at all or even
worse communicate from time to time (we guess due to fluctoation in the
power supply and environmental temperature).
Upon investigation we found that the only reason for this may be the
internal oscillator inaccuracy (all times were designed to meet the graphs
and oscillator tolerances in Microchip datasheets). Almost 100% of these
"faulty" PICs were with same production code "885" i.e. made on same line
and packaged in same Microchip factory.
As we purchased the PICs I sent inquiry to Microchip with requset to
evaluate these PICs and replace them if they doesn't meet the parameters
written in the datasheets, but got the following reply:

>Mr. Usunov,
>
>regarding the RC calibration, at chapter 14 of the datasheet is written:
>
>...The graphs and tables provided in this section are for design guidance
and are not tested. In some graphs or tables
>the data presented are outside specified operating range (e.g., outside
specified VDD range). This is for information
>only and devices will operate properly only within the specified range.
>The data presented in this section is a statistical summary of data
collected on units from different lots over a period of
>time. "Typical" represents the mean of the distribution while "max" or
"min" represents (mean + 3s) and (mean - 3s)
>respectively, where s is standard deviation.
>
>So, we do not test this parameter, and we give only a "statistical summary"
of data collected for design guidance.

to my question why OSCAL ("calibration") constant/register exist if they
don't test the internal frequency parameter I never got any reply

so take care

Best regards
Tsvetan
---
PCB prototypes for $26 at http://run.to/pcb (http://www.olimex.com/pcb)
Development boards for PIC, AVR and MSP430  (http://www.olimex.com/dev)

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2002\06\04@073557 by Olin Lathrop

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> Upon investigation we found that the only reason for this may be the
> internal oscillator inaccuracy

Can the oscillators be brought into tolerance by programming different OSCAL
values yourself, or is the available range totally to one side of the
specified frequency?


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(978) 742-9014, http://www.embedinc.com

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2002\06\05@032603 by Tsvetan Usunov

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>> Upon investigation we found that the only reason for this may be the
>> internal oscillator inaccuracy
>
>Can the oscillators be brought into tolerance by programming different
OSCAL
>values yourself, or is the available range totally to one side of the
>specified frequency?

according to datasheets:
Adjusting CAL5-0 from 000000 to 111111 yields a higher clock speed.
Note that bits 1 and 0 of OSCCAL are unimplemented and should be written as
0 when modifying OSCCAL for compatibility with future devices.

I guess OSCCAL is just register with set of resistors connected to it's
outputs, which connetcs in parallel to the main internal resistor for the
oscillator when '1' is written in the correspondign bit. When you write '1'
total resistor value goes lower and increase the frequency.

Tsvetan
---
PCB prototypes for $26 at http://run.to/pcb (http://www.olimex.com/pcb)
Development boards for PIC, AVR and MSP430  (http://www.olimex.com/dev)

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2002\06\05@072128 by Olin Lathrop

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> >> Upon investigation we found that the only reason for this may be the
> >> internal oscillator inaccuracy
> >
> >Can the oscillators be brought into tolerance by programming different
> OSCAL
> >values yourself, or is the available range totally to one side of the
> >specified frequency?
>
> according to datasheets:
> Adjusting CAL5-0 from 000000 to 111111 yields a higher clock speed.
> Note that bits 1 and 0 of OSCCAL are unimplemented and should be written
as
> 0 when modifying OSCCAL for compatibility with future devices.

Yes, I know how OSCCAL is supposed to work.  You said you got a bunch of bad
chips, and I was wondering whether the problem was that the factory
programmed OSCCAL value was incorrect or that the chips couldn't be made to
oscillate at the correct frequency regardless of the OSCCAL value.

> I guess OSCCAL is just register with set of resistors connected to it's
> outputs, which connetcs in parallel to the main internal resistor for the
> oscillator when '1' is written in the correspondign bit. When you write
'1'
> total resistor value goes lower and increase the frequency.

Possibly.

> Tsvetan
> ---
> PCB prototypes for $26 at http://run.to/pcb (http://www.olimex.com/pcb)
> Development boards for PIC, AVR and MSP430  (http://www.olimex.com/dev)

Do you guys do more than two layers boards yet?  That has kept me from
trying you out a few times now.


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(978) 742-9014, http://www.embedinc.com

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2002\06\06@024533 by Tsvetan Usunov

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>Yes, I know how OSCCAL is supposed to work.  You said you got a bunch of
bad
>chips, and I was wondering whether the problem was that the factory
>programmed OSCCAL value was incorrect or that the chips couldn't be made to
>oscillate at the correct frequency regardless of the OSCCAL value.

There is no way to find this out as the PICs are programmed (no space for
additional code to play with OSCCAL) and with codeprotect set.
We don't know if the PICs are OK until we program them and test for
communication.
I was told by our Microchip rep that Microchip have their way to check
OSCCAL out even the PIC is programmed and protected, as few years ago they
have similar problem with another customer with faulty PIC12C509 which have
been replaced and no question asked, but I guess now Microchip gave up on
the oscillator accuracy and put those "we don't test" message on datasheets
to prevent any future claims from customers.

>Do you guys do more than two layers boards yet?  That has kept me from
>trying you out a few times now.

The technology we use doesn't allow us to make the 4L PCB tooling in 3-5
days,
so our multilayer PCB service is useful for small runs rather for
prototypes:
4L PCB setup (including E-test) is $250.00
prototypes cost $20/sq.dm and go down to $3-$4/sq.dm in volume (includes
double soldermask and component print)

Best regards
Tsvetan
---
PCB prototypes for $26 at http://run.to/pcb (http://www.olimex.com/pcb)
Development boards for PIC, AVR and MSP430  (http://www.olimex.com/dev)

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