mc9s08qd4 Freescale Semiconductor, Inc, mc9s08qd4 Datasheet - Page 118

• Manufacturer Part Number: mc9s08qd4
• Description: Hcs08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Analog-to-Digital Converter (S08ADC10V1)
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
8.6.1.2
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).
8.6.1.3
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 must 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
For proper conversion, the input voltage must fall between V
exceeds V
(full scale 8-bit representation). If the input is equal to or less than V
to $000. Input voltages between V
brief current associated with V
3.5 cycles of the ADCK source when ADLSMP is low, or 23.5 cycles when ADLSMP is high.
For minimal loss of accuracy due to current injection, pins adjacent to the analog input pins must not be
transitioning during conversions.
118
REFH
REFH
Analog Reference Pins
Analog Input Pins
REFL
, the converter circuit converts the signal to $3FF (full scale 10-bit representation) or $FF
and V
, which may be shared on the same pin as V
REFH
DDAD
SSAD
REFL
, which may be shared on the same pin as V
SSAD
). When available on a separate pin, V
. Both V
and must be placed as near as possible to the package pins. Resistance in the
REFL
pin makes a good single point ground location.
MC9S08QD4 Series MCU Data Sheet, Rev. 3
REFH
REFH
REFH
when the sampling capacitor is charging. The input is sampled for
SSAD
and V
and V
may be connected to the same potential as V
REFH
pin. This must be the only ground connection between these
REFL
REFL
and V
SSA
are straight-line linear conversions. There will be a
must be routed carefully for maximum noise
REFL
DD
.
or V
loop. The best external component to meet this
DDAD
REFH
SSAD
SS
REFL
. Setting the pin control register bits for
spec and the V
and V
REFL
DDAD
on some devices.
must be connected to the same
, the converter circuit converts it
REFL
on some devices. The low
. If the input is equal to or
DDAD
Freescale Semiconductor
DDAD
potential (V
, or may be
REFH