MC68HC705JJ7 Datasheet, PDF (18/164 Pages) Freescale Semiconductor, Inc – Microcontrollers

General Description
1.6.2 Ceramic Resonator Oscillator
In cost-sensitive applications, a ceramic resonator can be used in place of the crystal. The circuit in
Figure 1-3 (a) can be used for a ceramic resonator. The resonator manufacturer’s recommendations
should be followed, as the resonator parameters determine the external component values required for
maximum stability and reliable starting. The load capacitance values used in the oscillator circuit design
should include all stray capacitances. The ceramic resonator and components should be mounted as
close as possible to the pins for startup stabilization and to minimize output distortion. An internal startup
resistor of approximately 2 MΩ can be provided between OSC1 and OSC2 for the ceramic resonator type
oscillator by use of the OSCRES bit in the MOR.
1.6.3 RC Oscillator
The lowest cost oscillator is the RC oscillator configuration where a resistor is connected between the two
oscillator pins as shown in Figure 1-3 (b).
The selection of the RC oscillator configuration is done by product part number selection as described in
Chapter 17 Ordering Information.
NOTE
Do not use the internal startup resistor between OSC1 and OSC2 for the
RC-type oscillator.
1.6.4 External Clock
An external clock from another CMOS-compatible device can be connected to the OSC1 input, with the
OSC2 input not connected, as shown in Figure 1-3 (c). This oscillator can be selected via software. This
configuration is possible regardless of whether the crystal/ceramic resonator or RC oscillator
configuration is used.
NOTE
Do not use the internal startup resistor between OSC1 and OSC2 for the
external clock.
1.6.5 Internal Low-Power Oscillator
An internal low-power oscillator (LPO) is provided which is the default oscillator out of reset. When
operating from this internal LPO, the other oscillator can be powered down by software to further conserve
power.
The selection of the LPO configuration is done by product part number selection as described in
Chapter 17 Ordering Information.
1.7 RESET Pin
The RESET pin can be used as an input to reset the MCU to a known startup state by pulling it to the low
state. It also functions as an output to indicate that an internal COP watchdog, illegal address, or
low-voltage reset has occurred. The RESET pin contains a pullup device to allow the pin to be left
disconnected without an external pullup resistor. The RESET pin also contains a steering diode that,
when the power is removed, will discharge to V_DD any charge left on an external capacitor connected
between the RESET pin and V_SS. The RESET pin also contains an internal Schmitt trigger to improve its
noise immunity as an input.
MC68HC705JJ7 • MC68HC705JP7 Advance Information Data Sheet, Rev. 4.1
18
Freescale Semiconductor