Datasheet

General Description
Technical Data MC68HC908JB8•MC68HC08JB8•MC68HC08JT8 — Rev. 2.3
34 General Description Freescale Semiconductor
1.5.1 Power Supply Pins (V
DD
, V
SS
)
V
DD
and V
SS
are the power supply and ground pins. The MCU operates
from a single power supply.
Fast signal transitions on MCU pins place high, short-duration current
demands on the power supply. To prevent noise problems, take special
care to provide power supply bypassing at the MCU as Figure 1-5
shows. Place the bypass capacitors as close to the MCU power pins as
possible. Use high-frequency-response ceramic capacitors for C
BYPASS
.
C
BULK
are optional bulk current bypass capacitors for use in applications
that require the port pins to source high current levels.
Figure 1-5. Power Supply Bypassing
1.5.2 Voltage Regulator Out (V
REG
)
V
REG
is the 3.3 V output of the on-chip voltage regulator. V
REG
is used
internally for the MCU operation and the USB data driver. It is also used
to supply the voltage for the external pullup resistor required on the
USB’s D– line. The V
REG
pin requires an external bulk capacitor 4.7µF
or larger and a 0.1 µF ceramic bypass capacitor as Figure 1-6 shows.
Place the bypass capacitors as close to the V
REG
pin as possible.
MCU
C
BULK
C
BYPASS
0.1 µF
+
NOTE: Values shown are typical values.
V
DD
V
DD
V
SS