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Semiconductor Group
80
On-Chip Peripheral Components
The external reference voltage supply need only be applied when the A/D converter is used,
otherwise the pins
V
AREF
and
V
AGND
may be left unconnected. The reference voltage supply has to
meet some requirements concerning the level of
V
AGND
and
V
AREF
and the output impedance of the
supply voltage (see also "A/D Converter Characteristics" in the data sheet).
– The voltage
V
AREF
must meet the following specification:
V
AREF
=
V
CC
±
5 %
– The voltage
V
AGND
must meet a similar specification:
V
AGND
=
V
SS
±
0.2 V
– The differential output impedance of the analog reference supply voltage should be less than
1 k
Ω
lf the above mentioned operating conditions are not met the accuracy of the converter may be
decreased.
Furthermore, the analog input voltage
V
AINPUT
must not exceed the range from (
V
AGND
– 0.2 V) to
(
V
AREF
+ 0.2 V). Otherwise, a static input current might result at the corresponding analog input
which will also affect the accuracy of the other input channels.
7.4.3
A/D Converter Timing
A conversion is started by writing into special function register DAPR. A write-to-DAPR will start a
new conversion even if a conversion is currently in progress. The conversion begins with the next
machine cycle and the busy flag BSY will be set.
The conversion procedure is divided into three parts:
Load time (
t
L
):
During this time the analog input capacitance
C
I
(see data sheet) must be loaded to the analog input
voltage level. The external analog source needs to be strong enough to source the current to load
the analog input capacitance during the load time. This causes some restrictions for the impedance
of the analog source. For a typical application the value of the impedance should be less than
approx. 5 k
Ω
.
Sample time (
t
S
):
During this time the internal capacitor array is connected to the selected analog input channel. The
sample time includes the load time which is described above. After the load time has passed the
selected analog input must be held constant for the rest of the sample time. Otherwise the internal
calibration of the comparator circuitry could be affected which might result in a reduced accuracy of
the converter. However, in typical applications a voltage change of approx. 200 - 300 mV at the
inputs during this time has no effect.
*
Summary of Contents for SAB 80515 Series
Page 9: ...Semiconductor Group 9 Introduction Figure 1 2 Block Diagram ...
Page 12: ...Semiconductor Group 12 Fundamental Structure Figure 2 1 Detailed Block Diagram ...
Page 18: ...Semiconductor Group 18 Central Processing Unit Figure 3 1 Fetch Execute Sequence ...
Page 73: ...Semiconductor Group 73 On Chip Peripheral Components Figure 7 25 A D Converter Block Diagram ...
Page 83: ...Semiconductor Group 83 On Chip Peripheral Components Figure 7 33 a Timer 2 Block Diagram ...
Page 215: ...Device Specifications Semiconductor Group 215 ...
Page 217: ...Device Specifications Semiconductor Group 217 Pin Configuration P LCC 68 ...
Page 219: ...Device Specifications Semiconductor Group 219 Logic Symbol ...
Page 226: ...Device Specifications Semiconductor Group 226 Figure 1 Block Diagram ...
Page 229: ...Device Specifications Semiconductor Group 229 Figure 2 Memory Address Spaces ...
Page 239: ...Device Specifications Semiconductor Group 239 Figure 4 Block Diagram of the A D Converter ...
Page 241: ...Device Specifications Semiconductor Group 241 Figure 5 Interrupt Request Sources ...
Page 242: ...Device Specifications Semiconductor Group 242 Figure 6 Interrupt Priority Level Structure ...