MC68HC705JJ7 Datasheet, PDF (17/164 Pages) Freescale Semiconductor, Inc – Microcontrollers

VDD and VSS Pins
1.5 V_DD and V_SS Pins
Power is supplied to the MCU through V_DD and V_SS. V_DD is the positive supply, and V_SS is ground. The
MCU operates from a single power supply.
Very fast signal transitions occur on the MCU pins. The short rise and fall times place very high
short-duration current demands on the power supply. To prevent noise problems, special care should be
taken to provide good power supply bypassing at the MCU by using bypass capacitors with good
high-frequency characteristics that are positioned as close to the MCU as possible.
1.6 OSC1 and OSC2 Pins
The OSC1 and OSC2 pins are the connections for the external pin oscillator (EPO). The OSC1 and OSC2
pins can accept these sets of components:
• A crystal as shown in Figure 1-3 (a)
• A ceramic resonator as shown in Figure 1-3 (a)
• An external resistor as shown in Figure 1-3 (b)
• An external clock signal as shown in Figure 1-3 (c)
The selection of the crystal/ceramic resonator or RC oscillator configuration is done by product part
number selection as described in Chapter 17 Ordering Information.
The frequency, f_OSC, of the EPO or external clock source is divided by two to produce the internal
operating frequency, f_OP.
MCU
2 MΩ
OSC1
OSC2
MCU
OSC1
R
OSC2
MCU
OSC1
OSC2
UNCONNECTED
EXTERNAL CLOCK
(a) Crystal or
Ceramic Resonator
Connections
(b) RC Oscillator
Connections
(c) External Clock
Source Connection
Figure 1-3. EPO Oscillator Connections
1.6.1 Crystal Oscillator
The circuit in Figure 1-3 (a) shows a typical oscillator circuit for an AT-cut, parallel resonant crystal. The
crystal manufacturer’s recommendations should be followed, as the crystal parameters determine the
external component values required to provide maximum stability and reliable startup. The load
capacitance values used in the oscillator circuit design should include all stray capacitances. The crystal
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 crystal type oscillator by use of the OSCRES bit in the MOR.
MC68HC705JJ7 • MC68HC705JP7 Advance Information Data Sheet, Rev. 4.1
Freescale Semiconductor
17