MC68HC05L16 Datasheet, PDF (17/146 Pages) Freescale Semiconductor, Inc – Microcontrollers

Functional Pin Description
1.5.1 V_DD and V_SS
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. Bypassing requirements vary, depending on how heavily the MCU pins are loaded.
1.5.2 OSC1 and OSC2
The OSC1 and OSC2 pins are the connections for the 2-pin on-chip oscillator. The OSC1 and OSC2 pins
can accept:
• A crystal as shown in Figure 1-3(a)
• An external clock signal as shown in Figure 1-3(b)
The frequency, f_OSC, of the oscillator or external clock source is divided by 64 to produce the internal
operating frequency, f_OP, by default.
1.5.2.1 Crystal or Ceramic Resonator
The circuit in Figure 1-3(a) shows a typical 2-pin 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 feedback resistor of R_OF between OSC1 and OSC2 may
be selected as a mask option for MC68HC05L16. Typical R_OF resistor value is 5.5 MΩ.
MASK OPTIONS
MCU
R_OF
MCU
OSC1
OSC2
4 MHz (TYP)
OSC1
OSC2
C_O1
C_O2
UNCONNECTED
EXTERNAL CLOCK
(a) Crystal Connections
(b) External Clock
Source Connection
Figure 1-3. Oscillator Connections
MC68HC05L16 • MC68HC705L16 Data Sheet, Rev. 4.1
Freescale Semiconductor
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