MC68HC05L28 Datasheet, PDF (30/156 Pages) Motorola, Inc – Flexible general-purpose microcomputer

2.2.5.1 Crystal
2
The circuit shown in Figure 2-3(a) is recommended when using a crystal . Figure 2-3(d) lists the
recommended capacitance and feedback resistance values. The internal oscillator is designed to
interface with an AT-cut parallel-resonant quartz crystal resonator in the frequency range specified
for f_OSC (see Section 12.4). Use of an external CMOS oscillator is recommended when crystals
outside the specified ranges are to be used. The crystal and associated components should be
mounted as close as possible to the input pins to minimize output distortion and start-up
stabilization time. The manufacturer of the particular crystal being considered should be consulted
for specific information.
2.2.5.2 External clock
An external clock should be applied to the OSC1 input, with the OSC2 pin left unconnected, as
shown in Figure 2-3(c). The t_OXOV specification (see Section 12.4) does not apply when using an
external clock input. The equivalent specification of the external clock source should be used in
lieu of t_OXOV.
2.2.6 RESET
This active low input pin is used to reset the MCU. Applying a logic zero to this pin forces the device
to a known start-up state. This input has an internal Schmitt trigger to improve noise immunity.
The MCU has its own internal power-on reset (POR) circuit. If, however, the user requires an
additional POR, then an RC network can be connected to this pin as shown in Figure 2-4. The user
must ensure that the RC time constant of this network is greater than the the oscillator stabilization
period. A time constant of at least 100 ms is recommended.
V_DD
R
C
V_SS
MC68HC05L28
RESET
Figure 2-4 RC connection for external POR
MOTOROLA
2-8
MODES OF OPERATION AND PIN DESCRIPTIONS
TPG
MC68HC05L28