NXP Semiconductors
FXTH87E
FXTH87E, Family of Tire Pressure Monitor Sensors
FXTH87ERM
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© NXP B.V. 2019. All rights reserved.
Reference manual
Rev. 5.0 — 4 February 2019
102 / 183
12.5 Optional battery condition check
The condition of the battery can be periodically checked to determine the battery’s
internal impedance, R
BATT
, which is a function of both temperature and the remaining
battery capacity. This can be performed by user supplied software routine and an
external load resistor, R
LOAD
, connected from the PTA0 pin to V
SS
as shown in
Figure 21
(any of the PTA[3:0] can be used for this purpose).
Port pin driven high
Port pin driven low
aaa-028015
FXTH87E
VBATT
+
IDD
RLOAD
RBATT
VDD
VSS
PTA0
FXTH87E
VBATT
+
IDD + ILOAD
ILOAD
RLOAD
RBATT
VDD
VSS
PTA0
Figure 21. Battery check circuit
The battery voltage can first be checked using the method given in
Section 12.3 "Voltage
measurements"
with the selected PTA0 pin set as an output and driven low and then high
to determine V
DD
where only I
DD
flows or when I
DD
plus I
LOAD
flows. The resulting battery
impedance can then be calculated as:
(9)
If it is assumed that I
DD0
and I
DD1
are not appreciably different at the small change in
V
DD
, then the resulting battery impedance can be approximated as:
(10)
where:
•
V
DD0
is the voltage determined with the external load resistor connected to V
SS
•
V
DD1
is the voltage determined with the external load resistor connected to V
DD
•
R
LOAD
is the resistance of the external load resistance in ohms
•
R
BATT
is the implied battery impedance in ohms
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