NXP Semiconductors UM11227 - Optional Battery Condition Check

NXP Semiconductors
UM11227
NTM88 family of tire pressure monitor sensors
UM11227 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2020. All rights reserved.
User manual Rev. 6 — 24 April 2020
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Once the sensitivity ΔA
MAX-MIN
has been calculated, the acceleration A
y
can be
calculated by the re-using the A
RATE-MIN
, 1 value of the offset step and the returned
A
yCODE
value with the following transfer function:
(8)
The pressure, and optional X or Z-axis accelerometer also share the same signal path in
the Transducer interface and all the sensors share the same ADC. Therefore, only one of
the sensors can be accessed at a given moment.
10.26.5 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 58
(any of the PTA[3:0] can be used for this purpose).
Port pin driven highPort pin driven low
aaa-032035
NTM88
V
BATT
+
I
DD
R
LOAD
R
BATT
V
DD
V
SS
PTA0
NTM88
V
BATT
+
I
DD
+ I
LOAD
I
LOAD
R
LOAD
R
BATT
V
DD
V
SS
PTA0
Figure 58. Battery check circuit
The battery voltage can first be checked using the method given in Section 10.26.3
"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: