
Advance Information
MC68HC08AS20
—
Rev. 4.1
310
Freescale Semiconductor
19.6.2 Stop Mode
The ADC module is inactive after the execution of a STOP instruction.
Any pending conversion is aborted. ADC conversions resume when the
MCU exits stop mode. Allow one conversion cycle to stabilize the analog
circuitry before attempting a new ADC conversion after exiting stop
mode.
19.7 I/O Signals
The ADC module has 15 channels that are shared with I/O ports B and
D and one channel with an input-only port bit on port D. Refer to
21.7
ADC Characteristics
for voltages referenced in the next three
subsections.
19.7.1 ADC Analog Power Pin (V
DDA
/V
DDAREF
)/ADC Voltage Reference Pin (V
REFH
)
The ADC analog portion uses V
DDA
/V
DDAREF
as its power pin. Connect
the V
DDA
/V
DDAREF
pin to the same voltage potential as V
DD
. External
filtering may be necessary to ensure clean V
DDA
/V
DDAREF
for good
results.
V
REFH
is the high reference voltage for all analog-to-digital conversions.
Connect the V
REFH
pin to a voltage potential between 1.5 volts and
V
DDAREF
/V
DDA
depending on the desired upper conversion boundary.
NOTE:
Route V
DDA
/V
DDAREF
carefully for maximum noise immunity and place
bypass capacitors as close as possible to the package.
19.7.2 ADC Analog Ground Pin (V
SSA
)/ADC Voltage Reference Low Pin (V
REFL
)
The ADC analog portion uses V
SSA
as its ground pin. Connect the V
SSA
pin to the same voltage potential as V
SS
.
V
REFL
is the lower reference supply for the ADC. Connect the V
REFL
pin
to a voltage potential between V
SSA
and 0.5 volts depending on the
desired lower conversion boundary.