Datasheet

 1996 Microchip Technology Inc.
Preliminary
DS40122B-page 77
PIC14000
10.2.2 CRYSTAL OSCILLATOR/CERAMIC
RESONATOR
In HS mode, a crystal or ceramic resonator is con-
nected to the OSC1 and OSC2 pins to establish oscil-
lation. A parallel cut crystal is required. Use of a series
cut crystal may give a frequency outside of the crystal
manufacturer’s specifications. When in HS mode, the
device can have an external clock source to drive the
OSC1 pin.
FIGURE 10-3: CRYSTAL/CERAMIC
RESONATOR OPERATION
(HS OSC CONFIGURATION)
FIGURE 10-4: EXTERNAL CLOCK INPUT
OPERATION (HS OSC
CONFIGURATION)
TABLE 10-1: CERAMIC RESONATORS
Mode Freq C1 C2
HS 4 MHz
8 MHz
16 MHz
15 - 68 pF
10 - 68 pF
10 - 22 pF
15 - 68 pF
10 - 68 pF
10 - 22 pF
Note: Recommended values of C1 and C2 are identical to
the ranges tested table.
Higher capacitance increases the stability of
oscillator but also increases the start-up time.
These values are for design guidance only. Since
each resonator has its own characteristics, the user
should consult the resonator manufacturer for
appropriate values of external components.
Resonators Used:
4 MHz Murata Erie CSA4.00MG +/-.5%
8 MHz Murata Erie CSA8.00MT +/-.5%
16 MHz Murata Erie CSA16.00MX +/-.5%
All resonators used did not have built-in capacitors.
See Table 10-1 and Table 10-2 for recommended
values of C1 and C2.
C1
C2
XTAL
OSC2
Note1
OSC1
RF
SLEEP
To internal
logic
PIC14000
RS
SLPCON<3>
Note 1: A series resistor may be required for AT
strip cut crystals.
OSC1
Clock from
ext. system
OSC2Open
PIC14000
TABLE 10-2: CAPACITOR SELECTION
FOR CRYSTAL OSCILLATOR
10.2.3 EXTERNAL CRYSTAL OSCILLATOR
CIRCUIT
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 10-5 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 k Ω resistor provides
the negative feedback for stability. The 10 k Ω
potentiometer biases the 74AS04 in the linear region.
This could be used for external oscillator designs.
FIGURE 10-5: EXTERNAL PARALLEL
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
Mode Freq C1 C2
HS 4 MHz
8 MHz
20 MHz
15 - 33 pF
15 - 47 pF
15 - 47 pF
15 - 33 pF
15 - 47 pF
15 - 47 pF
Note: Higher capacitance increases the stability of
oscillator but also increases the start-up time.
These values are for design guidance only. Rs may
be required in HS mode to avoid overdriving
crystals with low drive level specification. Since
each crystal has its own characteristics, the user
should consult the crystal manufacturer for
appropriate values of external components.
For V
DD
> 4.5V, C1 = C2 ≈ 30pf is recommended.
20pF
+5V
20pF
10k
4.7k
10k
74AS04
XTAL
10k
74AS04
OSC1
To Other
Devices