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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