V
C
V
or V
CC
REF0
V
REF1
Phase I:
Charge
Phase II:
Discharge
Phase III:
Charge
t
ref
Phase IV
Discharge
t
meas
t
R
meas
R
ref
Rref
Rmeas
+
-
VREF
Capture
Input Of a Timer
C0
Px.x
Px.y
COMP_E Operation
893
SLAU356I – March 2015 – Revised June 2019
Copyright © 2015–2019, Texas Instruments Incorporated
Comparator E Module (COMP_E)
Figure 23-6. Temperature Measurement System
The resources used to calculate the temperature sensed by Rmeas are:
•
Two digital I/O pins charge and discharge the capacitor.
•
I/O is set to output high (V
CC
) to charge capacitor, reset to discharge.
•
I/O is switched to high-impedance input with CEPDx set when not in use.
•
One output charges and discharges the capacitor through Rref.
•
One output discharges capacitor through Rmeas.
•
The + terminal is connected to the positive terminal of the capacitor.
•
The – terminal is connected to a reference level, for example 0.25 × V
CC
.
•
The output filter should be used to minimize switching noise.
•
CEOUT is used to gate a timer capturing capacitor discharge time.
More than one resistive element can be measured. Additional elements are connected to C0 with
available I/O pins and switched to high impedance when not being measured.
The thermistor measurement is based on a ratiometric conversion principle. The ratio of two capacitor
discharge times is calculated as shown in
Figure 23-7. Timing for Temperature Measurement Systems
The V
CC
voltage and the capacitor value should remain constant during the conversion but are not critical,
because they cancel in the ratio in
