mc9s08qg8 Freescale Semiconductor, Inc, mc9s08qg8 Datasheet - Page 136

• Manufacturer Part Number: mc9s08qg8
• Description: Hcs08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Analog-to-Digital Converter (S08ADC10V1)
9.6.1.1
The ADC module has analog power and ground supplies (V
separate pins on some devices. On other devices, V
and on others, both V
are separate pads for the analog supplies which are bonded to the same pin as the corresponding digital
supply so that some degree of isolation between the supplies is maintained.
When available on a separate pin, both V
as their corresponding MCU digital supply (V
noise immunity and bypass capacitors placed as near as possible to the package.
In cases where separate power supplies are used for analog and digital power, the ground connection
between these supplies must be at the V
supplies if possible. The V
9.6.1.2
In addition to the analog supplies, the ADC module has connections for two reference voltage inputs. The
high reference is V
reference is V
When available on a separate pin, V
driven by an external source that is between the minimum V
must never exceed V
voltage potential as V
immunity and bypass capacitors placed as near as possible to the package.
AC current in the form of current spikes required to supply charge to the capacitor array at each successive
approximation step is drawn through the V
current demand is a 0.1 µF capacitor with good high frequency characteristics. This capacitor is connected
between V
path is not recommended because the current will cause a voltage drop which could result in conversion
errors. Inductance in this path must be minimum (parasitic only).
9.6.1.3
The external analog inputs are typically shared with digital I/O pins on MCU devices. The pin I/O control
is disabled by setting the appropriate control bit in one of the pin control registers. Conversions can be
performed on inputs without the associated pin control register bit set. It is recommended that the pin
control register bit always be set when using a pin as an analog input. This avoids problems with contention
because the output buffer will be in its high impedance state and the pullup is disabled. Also, the input
buffer draws DC current when its input is not at either V
all pins used as analog inputs should be done to achieve lowest operating current.
Empirical data shows that capacitors on the analog inputs improve performance in the presence of noise
or when the source impedance is high. Use of 0.01 µF capacitors with good high-frequency characteristics
is sufficient. These capacitors are not necessary in all cases, but when used they must be placed as near as
possible to the package pins and be referenced to V
136
REFH
Analog Supply Pins
Analog Reference Pins
Analog Input Pins
REFL
and V
, which may be shared on the same pin as V
REFH
DDAD
SSAD
SSAD
REFL
, which may be shared on the same pin as V
). When available on a separate pin, V
SSAD
. Both V
and V
and must be placed as near as possible to the package pins. Resistance in the
MC9S08QG8 and MC9S08QG4 Data Sheet, Rev. 1.01
pin makes a good single point ground location.
DDAD
REFH
REFH
SSAD
are shared with the MCU digital supply pins. In these cases, there
DDAD
and V
may be connected to the same potential as V
REFH
pin. This should be the only ground connection between these
DD
and V
REFL
and V
and V
SSAD
SSA
SSAD
must be routed carefully for maximum noise
REFL
.
DD
SS
is shared on the same pin as the MCU digital V
must be connected to the same voltage potential
) and must be routed carefully for maximum
loop. The best external component to meet this
or V
DDAD
DDAD
SSAD
SS
REFL
. Setting the pin control register bits for
and V
spec and the V
DDAD
on some devices.
must be connected to the same
SSAD
on some devices. The low
) which are available as
DDAD
Freescale Semiconductor
DDAD
potential (V
, or may be
REFH
SS
,