PIC16F62X
14.2.3 EXTERNAL CRYSTAL OSCILLATOR
CIRCUIT
14.2.4 EXTERNAL CLOCK IN
For applications where a clock is already available else-
where, users may directly drive the PIC16F62X pro-
vided that this external clock source meets the AC/DC
timing requirements listed in Section 17.4. Figure 14-6
below shows how an external clock circuit should be
configured.
Either a prepackaged oscillator can be used or a simple
oscillator circuit with TTL gates can be built.
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 series resonance, or one with parallel
resonance.
FIGURE 14-6: EXTERNAL CLOCK INPUT
OPERATION (HS, XT OR LP
OSC CONFIGURATION)
Figure 14-4 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° 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 could be used for external oscillator designs.
Clock from
OSC1/RA7
ext. system
PIC16F62X
OSC2/RA6
RA6
14.2.5 ER OSCILLATOR
FIGURE 14-4: EXTERNAL PARALLEL
RESONANT CRYSTAL
For timing insensitive applications, the ER (External
Resistor) clock mode offers additional cost savings.
Only one external component, a resistor to VSS, is
needed to set the operating frequency of the internal
oscillator. The resistor draws a DC bias current which
controls the oscillation frequency. In addition to the
resistance value, the oscillator frequency will vary from
unit to unit, and as a function of supply voltage and tem-
perature. Since the controlling parameter is a DC cur-
rent and not a capacitance, the particular package type
and lead frame will not have a significant effect on the
resultant frequency.
OSCILLATOR CIRCUIT
+5V
To other
Devices
10k
74AS04
4.7k
PIC16F62X
CLKIN
74AS04
10k
XTAL
Figure 14-7 shows how the controlling resistor is con-
nected to the PIC16F62X. For Rext values below 38k,
the oscillator operation may become unstable, or stop
completely. For very high Rext values (e.g. 1M), the
oscillator becomes sensitive to noise, humidity and
leakage. Thus, we recommend keeping Rext between
38k and 1M.
10k
20 pF
20 pF
Figure 14-5 shows a series resonant oscillator circuit.
This circuit is also designed to use the fundamental
frequency of the crystal. The inverter performs a 180°
phase shift in a series resonant oscillator circuit. The
330 kΩ resistors provide the negative feedback to bias
the inverters in their linear region.
FIGURE 14-7: EXTERNAL RESISTOR
RA7/OSC1/CLKIN
FIGURE 14-5: EXTERNAL SERIES
RESONANT CRYSTAL
RA6/OSC2/CLKOUT
OSCILLATOR CIRCUIT
To other
Devices
330 kΩ
330 kΩ
PIC16F62X
The Electrical Specification section shows the relation-
ship between the resistance value and the operating
frequency as well as frequency variations due to oper-
ating temperature for given R and VDD values.
74AS04
74AS04
74AS04
CLKIN
0.1 µF
The ER oscillator mode has two options that control the
unused OSC2 pin. The first allows it to be used as a
general purpose I/O port. The other configures the pin
as an output providing the Fosc signal (internal clock
divided by 4) for test or external synchronization pur-
poses.
XTAL
DS40300B-page 98
Preliminary
1999 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
