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PICList Thread
'12C509- calibration value question'
1999\06\22@145248 by Kennedy, William L

If the cal value has been erased, can it be re-programmed?  Can this value
be seen by reading the chip?


1999\06\22@154445 by Clyde Smith-Stubbs

On Tue, Jun 22, 1999 at 02:51:58PM -0400, Kennedy, William L wrote:
> If the cal value has been erased, can it be re-programmed?

If you know what it is, yes.

> Can this value
> be seen by reading the chip?

Yes, that's usually how you get to find out what it is (hopefully you did
it *before* erasure!)

I have seen some code somewhere to re-calibrate a chip.

Clyde Smith-Stubbs               |            HI-TECH Software
Email:          |          Phone            Fax
WWW:    | USA: (408) 490 2885  (408) 490 2885
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HI-TECH C: compiling the real world.

1999\06\22@192748 by Jerry Merrill

At 01:51 PM 6/22/99 , you wrote:
>If the cal value has been erased, can it be re-programmed?  Can this value
>be seen by reading the chip?

You can read the value BEFORE erasing the chip.  In fact, you MUST read it
so that you can restore this location before each programming of the device.

Most software will allow you to read this location.  PicWriter also lets
you store it to a file for later retrieval (a minor point with single
calibration value chips like this...more of an issue with the 14000 which
has dozens of calibration locations).

If you erased the device before recording the calibration value, you have
two choices:

1. Use a mid-range calibration value if accuracy is not at all important or
if you are not using the internal oscillation modes.

2. Use an iterative approach to zero-in on the correct value if accuracy is
not TOO important.

If accuracy IS IMPORTANT, you would not be using the internal OSC so the
calibration value is irrelevant, mid range would be fine.

The calibration value is actually an instruction 'movlw  #xx', where 'xx'
is determined empirically.  0x80 is presumably the middle of the range.

With a proper calibration value, the internal oscillator can vary +- 20%.
With the wrong value it could vary by ?%  (some have said +100% - 50%????).

Jerry Merrill
FAX: (972) 494-5814
VOICE:(972) 272-9392
PO Box 462101
Garland,  TX  75046-2101

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