PIC12F508ESLASHMS datasheet(46/110 Pages) MICROCHIP

46 / 110 page

PIC12F508/509/16F505

DS41236E-page 46

Although the oscillator will operate with no external

capacitor (CEXT = 0 pF), we recommend using values

above 20 pF for noise and stability reasons. With no or

small external capacitance, the oscillation frequency

can vary dramatically due to changes in external

capacitances, such as PCB trace capacitance or

package lead frame capacitance.

Section 10.0 “Electrical Characteristics” shows RC

frequency variation from part-to-part due to normal

process variation. The variation is larger for larger val-

ues of R (since leakage current variation will affect RC

frequency more for large R) and for smaller values of C

(since variation of input capacitance will affect RC

frequency more).

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 7-5:

EXTERNAL RC

OSCILLATOR MODE

7.2.5

INTERNAL 4 MHz RC OSCILLATOR

The internal RC oscillator provides a fixed 4 MHz

(nominal) system clock at VDD = 5V and 25°C, (see

Section 10.0

“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

calibration value for the internal RC oscillator. This

location is always uncode 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

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.

For the PIC12F508/509/16F505 devices, only bits

<7:1> of OSCCAL are implemented. Bits CAL6-CAL0

are used for calibration. Adjusting CAL6-CAL0 from

‘0000000’ to ‘1111111’ changes the clock speed. See

VDD

REXT

CEXT

VSS

OSC1

Internal

clock

PIC16F505

FOSC/4

OSC2/CLKOUT

PIC12F508

PIC12F509

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.

Note:

The 0 bit of OSCCAL is unimplemented

and should be written as ‘0’ when

modifying OSCCAL for compatibility with

future devices.

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