Datasheet

PIC12F519
DS41319B-page 40 © 2008 Microchip Technology Inc.
8.2.3 EXTERNAL CRYSTAL OSCILLATOR
CIRCUIT
Either a prepackaged oscillator or a simple oscillator
circuit with TTL gates can be used as an external
crystal oscillator circuit. Prepackaged oscillators provide
a wide operating range and better stability. A
well-designed crystal oscillator will provide good
performance with TTL gates. Two types of crystal
oscillator circuits can be used: one with parallel
resonance, or one with series resonance.
Figure 8-3 shows implementation of a parallel resonant
oscillator circuit. The circuit is designed to use the
fundamental frequency of the crystal. The 74AS04
inverter performs the 180-degree phase shift that a
parallel oscillator requires. The 4.7 kΩ resistor provides
the negative feedback for stability. The 10 kΩ
potentiometers bias the 74AS04 in the linear region.
This circuit could be used for external oscillator designs.
FIGURE 8-3: EXTERNAL PARALLEL
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
Figure 8-4 shows a series resonant oscillator circuit.
This circuit is also designed to use the fundamental
frequency of the crystal. The inverter performs a
180-degree phase shift in a series resonant oscillator
circuit. The 330Ω resistors provide the negative
feedback to bias the inverters in their linear region.
FIGURE 8-4: EXTERNAL SERIES
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
8.2.4 EXTERNAL RC OSCILLATOR
For timing insensitive applications, the RC circuit option
offers additional cost savings. The RC oscillator
frequency is a function of the supply voltage, the
resistor (R
EXT) and capacitor (CEXT) values, and the
operating temperature. In addition to this, the oscillator
frequency will vary from unit-to-unit due to normal
process parameter variation. Furthermore, the
difference in lead frame capacitance between package
types will also affect the oscillation frequency, especially
for low C
EXT values. The user also needs to take into
account variation due to tolerance of external R and C
components used.
Figure 8-5 shows how the R/C combination is
connected to the PIC12F519 device. For R
EXT values
below 3.0 kΩ, the oscillator operation may become
unstable, or stop completely. For very high R
EXT values
(e.g., 1 MΩ), the oscillator becomes sensitive to noise,
humidity and leakage. It is recommended keeping REXT
between 5.0 kΩ and 100 kΩ.
Although the oscillator will operate with no external
capacitor (CEXT = 0 pF), it is recommended 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. See Figure 11-1
and Figure 11-2.
FIGURE 8-5: EXTERNAL RC
OSCILLATOR MODE
8.2.5 INTERNAL 4/8 MHz RC
OSCILLATOR
The internal RC oscillator provides a fixed 4/8 MHz
(nominal) system clock at V
DD = 3.5V and 25°C, (see
Section 11.0 “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 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
20 pF
+5V
20 pF
10k
4.7k
10k
74AS04
XTAL
10k
74AS04
PIC12F519
CLKIN
To O t h e r
Devices
330
74AS04
74AS04
CLKIN
To Other
Devices
XTAL
330
74AS04
0.1 mF
PIC12F519
VDD
REXT
CEXT
VSS
OSC1
Internal
clock
PIC16F519
N