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'[PICLIST] PIC12CE519 int rc oscillator'
2000\11\22@063649 by Bardos Csaba

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Dear PICListers,

I am new to the 12ce family and I have a question regarding it
since the manual is a little bit confusing for me.
The manual says that the reset vector is 0x3ff for '519 and
this address contains a movlw xx instruction, where xx is a
factory trimmed calibration value for the internal rc osc.
I have to start at address 0x00 and if I want I can start with
movwf osccal to trim the internal rc osc. (It seems to me that
I need so I start the program with movwf osccal).
It is clear for OTP parts. When I want to use for
the development a JW - windowed - part I have to save this value
before I do anything with the chip. With the first burning I don't
have to anything since the chip contains the value. Since the first
program won't work usually I have to erase the part. It erases the
movlw xx instruction also. So when I want to use the calibration
value I have to start my program with movlw xx @ 0x3ff where
xx is the saved value. The manual says that this value should
never be overwritten. Or does this warning refer to the OTP parts only ?
So the statements above are true or not ?
The second thing: I want to use this chip for serial tx/rx with
internal rc osc. The baud rate is 1200bps. Since the manual doesn't
say too much how about the frequency change vs temperature ?
This gadget will work in a room so the temp. won't varies too much.

Thanks in advance,
Cs.

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2000\11\22@074940 by Bob Ammerman

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----- Original Message -----
From: Bardos Csaba <.....cbardosKILLspamspam@spam@MAIL.INEXT.HU>
To: <PICLISTspamKILLspamMITVMA.MIT.EDU>
Sent: Wednesday, November 22, 2000 6:26 AM
Subject: PIC12CE519 int rc oscillator


{Quote hidden}

For the JW you would typically add the following to your source code:

   org        0x3FF
   movlw   ?????

You would remove this when creating code for OTPs.

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2000\11\22@083104 by mike

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On Wed, 22 Nov 2000 12:26:33 +0100, you wrote:

>Dear PICListers,
>
>I am new to the 12ce family and I have a question regarding it
>since the manual is a little bit confusing for me.
>The manual says that the reset vector is 0x3ff for '519 and
>this address contains a movlw xx instruction, where xx is a
>factory trimmed calibration value for the internal rc osc.
>I have to start at address 0x00 and if I want I can start with
>movwf osccal to trim the internal rc osc. (It seems to me that
>I need so I start the program with movwf osccal).
>It is clear for OTP parts. When I want to use for
>the development a JW - windowed - part I have to save this value
>before I do anything with the chip. With the first burning I don't
>have to anything since the chip contains the value. Since the first
>program won't work usually I have to erase the part. It erases the
>movlw xx instruction also. So when I want to use the calibration
>value I have to start my program with movlw xx @ 0x3ff where
>xx is the saved value. The manual says that this value should
>never be overwritten. Or does this warning refer to the OTP parts only ?
>So the statements above are true or not ?
The 'correct' way to do it is to enter the cal data in the 'cal data'
window in Picstart Plus. If your programmer doesn't support this you
can use a MOVLW in your code, but remember to take it out for
production. (You don't need to worry about putting something at 3ff as it will
roll over to 0)


>The second thing: I want to use this chip for serial tx/rx with
>internal rc osc. The baud rate is 1200bps. Since the manual doesn't
>say too much how about the frequency change vs temperature ?
>This gadget will work in a room so the temp. won't varies too much.
The internal RC osc is not really quite accurate for async serial
comms - it may work for a 1-off but can't be guaranteed in production.

However what you can do is measure the speed of an incoming character
to calibrate your baudrate - just define which character the device
will see first (e.g. after resetting via DTR or a watchdog timeout) in
your protocol.
Use a byte value which gives a single, long pulse -  7F is a good
choice, as it will give a single pulse which is 8 bit-times long, so
it's easy to measure accurately and divide down to get the bit-time.

Use a variable bit-delay subroutine for your tx/rx code to do the
calibration - don't be tempted to fiddle with the OSCCAL reg as it's
the sort of thing Microchip have been known to change with die
revisions, and you don't know how much adjustability you will have on
any given part, or the size of the adjustment step.

This should be easy to do at 1200 baud, as the delay trim doesn't need
to be accurate down to the last microsecond to get within the +/-5%
accuracy you need for async comms.
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