Semiconductor Group
6-62
On-Chip Peripheral Components
C515
6.4.2.4
Programmable Reference Voltages of the A/D Converter (DAPR Register)
The C515 has two pins to which a reference voltage range for the on-chip A/D converter is applied
(pin
V
AREF for the upper voltage and pin VAGND for the lower voltage). In contrast to conventional A/
D converters it is now possible to use not only these externally applied reference voltages for the
conversion but also internally generated reference voltages which are derived from the externally
applied ones. For this purpose a resistor ladder provides 16 equidistant voltage levels between
V
AREF and VAGND. These steps can individually be assigned as upper and lower reference voltage for
the converter itself. These internally generated reference voltages are called
V
lntAREF and VlntAGND.
The internal reference voltage programming can be thought of as a programmable "D/A converter"
which provides the voltages
V
IntARFF and VIntAGND for the A/D converter itself.
Special Function Register DAPR (Address DAH) )
Reset Value : 00H
Any write-access to DAPR starts conversion.
The SFR DAPR is provided for programming the internal reference voltages
V
IntAREF and VlntAGND. For
this purpose the internal reference voltages can be programmed in steps of 1/16 of the external
reference voltages (
V
AREF – VAGND) by four bits each in register DAPR. Bits 0 to 3 specify VlntAGND,
while bits 4 to 7 specify
V
IntAREF. A minimum of 1 V difference is required between the internal
reference voltages
V
lntARF and VIntAGND for proper operation of the A/D converter. This means, for
example, in the case where
V
AREF is 5 V and VAGND is 0 V, there must be at least four steps difference
between the internal reference voltages
V
IntAREF and VIntAGND.
The values of
V
IntAGND and VIntAREF are given by the formulas:
Bit
Function
DAPR.7-.4
Programming of
V
IntAREF
These bits are used for adjustment of the internal
V
IntAREF voltage.
DAPR.3-.0
Programming of
V
IntAGND
These bits are used for adjustment of the internal
V
IntAGND voltage.
76543210
.5
.4
.3
.2
.1
.0
DAH
DAPR
Bit No.
MSB
LSB
.6
.7
V
IntAGND = VAGND +
DAPR.3-.0
16
(
V
AREF – VAGND)
with DAPR.3-.0 < CH;
V
IntAREF = VAGND +
DAPR.7-.4
16
(
V
AREF – VAGND)
with DAPR.7-.4 > 3H;