mcf51ac256a Freescale Semiconductor, Inc, mcf51ac256a Datasheet - Page 190

• Manufacturer Part Number: mcf51ac256a
• Description: Mcf51ac Flexis
• Manufacturer: Freescale Semiconductor, Inc

Analog-to-Digital Converter (S08ADC12V1)
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.
If separate power supplies are used for analog and digital power, the ground connection between these
supplies must be at the V
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 between the minimum V
exceed V
as V
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 causes a voltage drop that 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 is in its high impedance state and the pullup is disabled. Also, the input buffer
draws DC current when its input is not at V
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
For proper conversion, the input voltage must fall between V
exceeds V
(full scale 10-bit representation) or 0xFF (full scale 8-bit representation). If the input is equal to or less
than V
straight-line linear conversions. There is a brief current associated with V
9-24
SSA
REFL
. V
DDA
REFH
REFH
REFH
, the converter circuit converts it to 0x000. Input voltages between V
Analog Reference Pins
Analog Input Pins
). When available on a separate pin, V
REFL
, the converter circuit converts the signal to 0xFFF (full scale 12-bit representation), 0x3FF
SSA
and V
and V
, which may be shared on the same pin as V
REFH
MCF51AC256 ColdFire Integrated Microcontroller Reference Manual, Rev. 5
pin makes a good single point ground location.
REFL
REFL
, which may be shared on the same pin as V
SSA
must be routed carefully for maximum noise immunity and bypass capacitors
and must be placed as near as possible to the package pins. Resistance in the
pin. This should be the only ground connection between these supplies if
REFH
DDA
may be connected to the same potential as V
REFH
DD
and V
DD
or V
and V
and V
SS
REFL
SSA
SSA
DDA
. Setting the pin control register bits for all pins used
REFL
.
SS
must be connected to the same voltage potential
must be connected to the same voltage potential
spec and the V
) and must be routed carefully for maximum
loop. The best external component to meet this
REFH
SSA
and V
on some devices.
DDA
DDA
REFL
on some devices. The low
REFL
potential (V
. If the input is equal to or
when the sampling
REFH
Freescale Semiconductor
and V
DDA
REFH
, or may be
REFL
must never
are