2.3.5 External Crystal Oscillator Circuit for Device Clock
Sometimes more than one device needs to be clocked from a single crystal. Since Microchip
does not recommend connecting other logic to the PICmicro’s internal oscillator circuit, an exter-
nal crystal oscillator circuit is recommended. Each device will then have an external clock source,
and the number of devices that can be driven will depend on the buffer drive capability. This circuit
is also useful when more than one device (PICmicro) needs to operate synchronously to each
other.
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 cir-
cuits can be used; one with series resonance, or one with parallel resonance.
Figure 2-4 shows implementation of an external 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 kΩresistor provides the
negative feedback for stability. The 10 kΩ potentiometer biases the 74AS04 in the linear region
Figure 2-5 shows an external 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 330 kΩ resistors provide the negative feedback to bias the
inverters in their linear region.
When the device is clocked from an external clock source (as in Figure 2-4 or Figure 2-5) then
the microcontroller’s oscillator must be configured for LP, XT or HS mode ( Figure 2-3).