Texas Instruments MSP430x1xx - Manual (Page 277)

Comparator_A in Applications
14-11
Comparator_A
14.4.2 Comparator_A Used to Measure Resistive Elements
The Comparator_A can be used to measure resistive elements. For example,
temperature can be converted into digital data via a thermistor, by comparing
the thermistor’s discharge time to that of a reference resistor. See Figure 14–7.
Figure 14–7. Temperature Measurement Systems
_
+
0
1
CAF
e.g.
Capture
Input of
Timer_A
CAOUT to
External Pin
Set
CAIFG
τ ∼ 2 µ s
0
1
0
1
0
1
0
1
CA0
CA1
0
1
VCC VCC
R(ref)
R(meas)
P2CA0
P2CA1
VCAREF
CARSEL
0
2
1
3
3 2 1 0
CAREF
0.5
×
VCC
0.25
×
VCC
CAON
C
1
0
VCC 0 V
1 0
CAON
VCC 0 V
1 0
The resistive elements are compared using a capacitor charge-discharge
cycle, as shown in Figure 14–8. This is based on a ratiometric conversion
principle, as the ratio of two capacitor-discharge times is compared. Absolute
V
CC
and the actual capacitor value are not critical, as the ratiometric principle
cancels these values out. V
CC
and the capacitor value should simply remain
constant during the conversion.
N
meas
N
ref
+
–R
meas
C ln
V
ref
V
CC
–R
ref
C ln
V
ref
V
CC
N
meas
N
ref
+
R
meas
R
ref
R
meas
+ R
ref
N
meas
N
ref