PIC16CR58A Microchip Technology, PIC16CR58A Datasheet - Page 127

• Manufacturer Part Number: PIC16CR58A
• Description: ROM-Based 8-Bit CMOS Microcontroller Series
• Manufacturer: Microchip Technology

13.2.3
Either a prepackaged oscillator can be used or a simple
oscillator circuit with TTL gates can be built. Prepack-
aged 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 series
resonance, or one with parallel resonance.
Figure 13-6 shows implementation of a parallel reso-
nant 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 par-
allel oscillator requires. The 4.7 k
the negative feedback for stability. The 10 k
ometer biases the 74AS04 in the linear region. This
could be used for external oscillator designs.
FIGURE 13-6: EXTERNAL PARALLEL
Figure 13-7 shows a series resonant oscillator circuit.
This circuit is also designed to use the fundamental fre-
quency of the crystal. The inverter performs a 180-
degree phase shift in a series resonant oscillator cir-
cuit. The 330 k
back to bias the inverters in their linear region.
FIGURE 13-7: EXTERNAL SERIES
1997 Microchip Technology Inc.
74AS04
330 k
10k
EXTERNAL CRYSTAL OSCILLATOR
CIRCUIT
+5V
20 pF
10k
0.1 F
XTAL
74AS04
XTAL
4.7k
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
resistors provide the negative feed-
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
74AS04
20 pF
330 k
10k
74AS04
74AS04
To Other
Devices
resistor provides
To Other
Devices
CLKIN
PIC16CXX
PIC16CXX
CLKIN
potenti-
13.2.4
For timing insensitive applications the RC device option
offers additional cost savings. The RC oscillator fre-
quency is a function of the supply voltage, the resistor
(Rext) 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 param-
eter variation. Furthermore, the difference in lead frame
capacitance between package types will also affect the
oscillation frequency, especially for low Cext values.
The user also needs to take into account variation due
to tolerance of external R and C components used.
Figure 13-8 shows how the RC combination is con-
nected to the PIC16CXX. For Rext values below
2.2 k , the oscillator operation may become unstable
or stop completely. For very high Rext values (e.g.
1 M ), the oscillator becomes sensitive to noise,
humidity and leakage. Thus, we recommend keeping
Rext between 3 k and 100 k .
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 pack-
age lead frame capacitance.
See characterization data for desired device for RC fre-
quency variation from part to part due to normal pro-
cess variation. The variation is larger for larger R (since
leakage current variation will affect RC frequency more
for large R) and for smaller C (since variation of input
capacitance will affect RC frequency more).
See characterization data for desired device for varia-
tion of oscillator frequency due to V
Cext values as well as frequency variation due to oper-
ating temperature for given R, C, and V
The oscillator frequency, divided by 4, is available on
the OSC2/CLKOUT pin, and can be used for test pur-
poses or to synchronize other logic (see Figure 3-5 for
waveform).
FIGURE 13-8: RC OSCILLATOR MODE
Cext
Rext
V
SS
V
DD
RC OSCILLATOR
Fosc/4
OSC1
OSC2/CLKOUT
PIC16C6X
DS30234D-page 127
DD
DD
for given Rext/
PIC16CXX
values.
Internal
clock