Texas Instruments MSP430x1xx - Manual (Page 295)

ADC12 Description and Operation
15-5
ADC12
Caution! ADC12 Turnon Time
When the ADC12 is turned on with the ADC12ON bit, the turnon
time noted in the data sheet (t
ADC12ON
) must be observed before a
conversion is started. Otherwise, the results will be false.
15.2.2 Reference
The ADC12 A/D converter contains a built-in reference with two selectable
reference-voltage levels (1.5 V and 2.5 V). Either of these reference voltages
may be applied to V
R+
of the A/D core and also may be available externally on
pin V
REF+
(check device data sheet for availability of V
REF+
pin). Additionally,
an external reference may be supplied for V
R+
through pin Ve
REF+
(check data
sheet for availability of Ve
REF+
pin).
The reference-voltage level for V
R–
can be selected to be AV
SS
or may be sup-
plied externally through the V
REF–
/Ve
REF–
pin (check device data sheet for
V
REF–
/Ve
REF–
pin). If the V
REF–
/Ve
REF–
pin is not available, then V
R
– is con-
nected to AV
SS
.
Configuration of the reference voltage(s) is done with the Sref bits (bits 4, 5,
and 6) in the ADC12MCTLx registers. Up to six combinations of positive and
negative reference voltages are supported as described in Table 15–1.
If only external references are used, the internal-reference generator can be
turned off with the REFON bit to conserve power.
Table 15–1.Reference Voltage Configurations
Sref Voltage at V
R+
Voltage at V
R–
0 AV
CC
AV
SS
1 V
REF+
(internal) AVSS
2,3 Ve
REF+
(external) AV
SS
4 AV
CC
V
REF–
/ Ve
REF–
(internal or external)
5 V
REF+
(internal) V
REF–
/ Ve
REF–
(internal or external)
6,7 Ve
REF+
(external) V
REF–
/ Ve
REF–
(internal or external)
The voltage levels V
R+
and V
R–
establish the upper and lower limits of the ana-
log inputs to produce a full-scale and zero-scale reading, respectively. The val-
ues of V
R+
, V
R–
, and the analog input should not exceed the positive supply
or be lower than AV
SS
, consistent with the absolute maximum ratings specified
in the device data sheet. The digital output is full scale when the input signal
is equal to or higher than V
R+
, and zero when the input signal is equal to or
lower than V
R–
.