
I/O Signals
MC68HC908GT16 MC68HC908GT8 Data Sheet, Rev. 3
Freescale Semiconductor
51
3.5.2 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 after an external interrupt. Allow one
conversion cycle to stabilize the analog circuitry.
3.6 I/O Signals
The ADC module has eight pins shared with port B, PTB7/AD7–PTB0/AD0.
3.6.1 ADC Analog Power Pin (V
DDA
)
The ADC analog portion uses V
DDA
as its power pin. Connect the V
DDA
pin to the same voltage potential
as V
DD
. External filtering may be necessary to ensure clean V
DDA
for good results.
NOTE
For maximum noise immunity, route V
DDA
carefully and place bypass
capacitors as close as possible to the package.
3.6.2 ADC Analog Ground Pin (V
SSA
)
The ADC analog portion uses V
SSA
as its ground pin. Connect the V
SSA
pin to the same voltage potential
as V
SS
.
NOTE
Route V
SSA
cleanly to avoid any offset errors.
3.6.3 ADC Voltage Reference High Pin (V
REFH
)
The ADC analog portion uses V
REFH
as its upper voltage reference pin. The V
REFH
pin must be connected
to the same voltage potential as V
DDA
. External filtering is often necessary to ensure a clean V
REFH
for
good results. Any noise present on this pin will be reflected and possibly magnified in A/D conversion
values.
NOTE
For maximum noise immunity, route V
REFH
carefully and place bypass
capacitors as close as possible to the package. Routing V
REFH
close and
parallel to V
REFL
may improve common mode noise rejection.
3.6.4 ADC Voltage Reference Low Pin (V
REFL
)
The ADC analog portion uses V
REFL
as its lower voltage reference pin. The V
REFL
pin must be connected
to the same voltage potential as V
SSA
. External filtering is often necessary to ensure a clean V
REFL
for
good results. Any noise present on this pin will be reflected and possibly magnified in A/D conversion
values.
NOTE
For maximum noise immunity, route V
REFL
carefully and, if not connected
to V
SS
, place bypass capacitors as close as possible to the package.
Routing V
REFH
close and parallel to V
REFL
may improve common mode
noise rejection.