Freescale Semiconductor MC68HC908MR32 - Manual (Page 51)

I/O Registers
MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1
Freescale Semiconductor 51
3.6.5 ADC Voltage In (ADVIN)
ADVIN is the input voltage signal from one of the 10 ADC channels to the ADC module.
3.6.6 ADC External Connections
This section describes the ADC external connections: V
REFH
and V
REFL
, ANx, and grounding.
3.6.6.1 V
REFH
and V
REFL
Both ac and dc current are drawn through the V
REFH
and V
REFL
loop. The AC current is in the form of
current spikes required to supply charge to the capacitor array at each successive approximation step.
The current flows through the internal resistor string. The best external component to meet both these
current demands is a capacitor in the 0.01
µF to 1 µF range with good high frequency characteristics. This
capacitor is connected between V
REFH
and V
REFL
and must be placed as close as possible to the
package pins. Resistance in the path is not recommended because the dc current will cause a voltage
drop which could result in conversion errors.
3.6.6.2 ANx
Empirical data shows that capacitors from the analog inputs to V
REFL
improve ADC performance. 0.01-
µF
and 0.1-
µF capacitors with good high-frequency characteristics are sufficient. These capacitors must be
placed as close as possible to the package pins.
3.6.6.3 Grounding
In cases where separate power supplies are used for analog and digital power, the ground connection
between these supplies should be at the V
SSAD
pin. This should be the only ground connection between
these supplies if possible. The V
SSA
pin makes a good single point ground location. Connect the V
REFL
pin to the same potential as V
SSAD
at the single point ground location.
3.7 I/O Registers
These I/O registers control and monitor operation of the ADC:
• ADC status and control register, ADSCR
• ADC data registers, ADRH and ARDL
• ADC clock register, ADCLK