PIC16F526 Datasheet, PDF (47/122 Pages) Microchip Technology – 14-Pin, 8-Bit Flash Microcontroller

Also, see the Electrical Specifications section for
variation of oscillator frequency due to VDD for given
REXT/CEXT values, as well as frequency variation due
to operating temperature for given R, C and VDD
values.
FIGURE 8-5:
VDD
EXTERNAL RC
OSCILLATOR MODE
REXT
OSC1
Internal
clock
CEXT
VSS
FOSC/4
N
PIC16F526
OSC2/CLKOUT
PIC16F526
8.2.5
INTERNAL 4/8 MHz RC
OSCILLATOR
The internal RC oscillator provides a fixed 4/8 MHz
(nominal) system clock at VDD = 5V and 25°C, (see
Section 14.0 “Electrical Characteristics” for
information on variation over voltage and temperature).
In addition, a calibration instruction is programmed into
the last address of memory, which contains the calibra-
tion value for the internal RC oscillator. This location is
always non-code protected, regardless of the code-
protect settings. This value is programmed as a MOVLW
XX instruction where XX is the calibration value, and is
placed at the Reset vector. This will load the W register
with the calibration value upon Reset and the PC will
then roll over to the users program at address 0x000.
The user then has the option of writing the value to the
OSCCAL Register (05h) or ignoring it.
OSCCAL, when written to with the calibration value, will
“trim” the internal oscillator to remove process variation
from the oscillator frequency.
Note:
Erasing the device will also erase the pre-
programmed internal calibration value for
the internal oscillator. The calibration
value must be read prior to erasing the
part so it can be reprogrammed correctly
later.
For the PIC16F526 device, only bits 7:1 of OSCCAL
are used for calibration. See Register 4-3 for more
information.
Note:
The bit 0 of the OSCCAL register is
unimplemented and should be written as
‘0’ when modifying OSCCAL for
compatibility with future devices.
 2010 Microchip Technology Inc.
DS41326D-page 47