Technical information

AN2317/D
MON08 Pods
Low-Cost Programming and Debugging Options for M68HC08 MCUs 11
Oscillator Circuits Figure 6 shows two ways to supply the external oscillator for normal monitor
mode operation. Figure 6a shows a canned oscillator while Figure 6b shows
a discrete inverter-based oscillator. The canned oscillator is perhaps the best
way to provide the external clock signal. It may cost a little more than the
inverter-based oscillator, but it has suitable drive strength to overdrive the
on-board crystal, and its small physical footprint (8-pin and 14-pin DIP versions
are available) makes it easy to implement in a custom MON08 interface.
The inverter-based oscillator in Figure 6b has been successfully used in
several circuits which required different oscillator frequencies. With a suitable
socket, the crystal can be changed from 4 MHz to 16 MHz. Care must be taken
with the layout of this circuit to avoid improper oscillation and noise. The
inverter should be a single-stage 74HCU04 to prevent unwanted phase shifts.
The unused inputs of the inverter package should be grounded to prevent
noise. The load capacitors were chosen for a parallel resonant crystal with a
20 pF load capacitance and assumes about 5 pF of stray capacitance due the
crystal socket and layout.
Figure 6a. Canned Oscillator Circuit
Figure 6b. Inverter-Based Oscillator Circuit
NC
NC
NC
V
SS
NC
NC
V
SS
V
CC
NC
NC
OUT
NC
NC
OUT
9.8304 MHz OSC
U1
1
2
3
4
5
6
7
14
13
12
11
10
9
8
V
DD
OSCOUT
U1A
1
2
OSCOUT
74HCU04
RF
5 M
Y1
9.8304 MHz
CL1
22 pF
CL2
47 pF
Freescale Semiconductor, I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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