Datasheet

+
−
CA0
CCI1B
Capture
Input
Of Timer_A
+
−
Rmeas
Rref
Px.x
Px.y
0.25xV
CC
D Q
IRQ, Interrupt Service Requested
Reset
V
CC
POR
SET_CAIFG
IRACC, Interrupt RequestAccepted
CAIE
CAIES
0
1
Comparator_A+ Operation www.ti.com
21.2.7 Comparator_A+ Interrupts
One interrupt flag and one interrupt vector are associated with the Comparator_A+ as shown in Figure 21-
5. The interrupt flag CAIFG is set on either the rising or falling edge of the comparator output, selected by
the CAIES bit. If both the CAIE and the GIE bits are set, then the CAIFG flag generates an interrupt
request. The CAIFG flag is automatically reset when the interrupt request is serviced or may be reset with
software.
Figure 21-5. Comparator_A+ Interrupt System
21.2.8 Comparator_A+ Used to Measure Resistive Elements
The Comparator_A+ can be optimized to precisely measure resistive elements using single slope analog-
to-digital conversion. For example, temperature can be converted into digital data using a thermistor, by
comparing the thermistor’s capacitor discharge time to that of a reference resistor as shown in Figure 21-
6. A reference resister Rref is compared to Rmeas.
Figure 21-6. Temperature Measurement System
The MSP430 resources used to calculate the temperature sensed by Rmeas are:
• Two digital I/O pins to charge and discharge the capacitor.
• I/O set to output high (V
CC
) to charge capacitor, reset to discharge.
• I/O switched to high-impedance input with CAPDx set when not in use.
• One output charges and discharges the capacitor via Rref.
• One output discharges capacitor via Rmeas.
• The + terminal is connected to the positive terminal of the capacitor.
• The - terminal is connected to a reference level, for example 0.25 x V
CC
.
• The output filter should be used to minimize switching noise.
• CAOUT used to gate Timer_A CCI1B, capturing capacitor discharge time.
More than one resistive element can be measured. Additional elements are connected to CA0 with
available I/O pins and switched to high impedance when not being measured.
528
Comparator_A+ SLAU144J–December 2004–Revised July 2013
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