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Using the oscillator calibration factor on devices with internal RC oscillators
When Microchip ships devices with an internal oscillator, they store an
oscillator calibration factor in the last location of code space. You can
use this data to calibrate the internal oscillator to exactly 4MHz.
To calibrate manually, you first have to read the data from the new
device. This must be done before you erase the device, because erasure
will destroy the stored data.
To read the calibration factor, you'll need some method of reading the code
space in the device. A device programmer like our EPIC™ Programmer is the
best thing to use. The rest of this document assumes that you have an EPIC
programmer.
Insert the new device into the socket on the programmer and select the device
part number from the drop down list on the toolbar. Click the read button
on the toolbar or select Read from the program menu.
To view the code space as read from the device, select Code from the view
menu. Scroll down to the very last location in the code space window to
find the calibration factor. The value will be shown as a 4-digit hexadecimal
number. You only need the 2 rightmost digits.
| In your source code, you will need to write the calibration factor
value to the OSCCAL register. A typical PICBASIC PRO statement is:
OSCCAL = $C0
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Using DEFINEs to automate
If you are programming new parts that have not been erased, you can
automate this process with a PICBASIC PRO define. The compiler will insert
code at the beginning of your program that reads the calibration factor and
writes it to the OSCCAL register. This method will only work with new,
never-erased devices.
| For devices with 1K of code space:
DEFINE OSCCAL_1K 1
For devices with 2K of code space:
DEFINE OSCCAL_2K 1
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Add one of these 2 DEFINEs
near the beginning of the PICBASIC PRO program to perform the setting of OSCCAL.
PICBASIC PRO will automatically load the OSCCAL value for
the PIC12C5xx and 12CE51x devices , if it is available. It is unnecessary to use
the above DEFINEs
with these devices.
If a UV erasable device has been erased, the value cannot
be read from memory. If one of these DEFINEs
is used on an erased part, it will cause the program to loop endlessly.
Calibration is only necessary when you need precise timing from you PICmicro®,
as for serial communication.
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