NXP Semiconductors
FXTH87E
FXTH87E, Family of Tire Pressure Monitor Sensors
12.3.1 Internal band gap
An internal band gap voltage reference is provided to take measurements of the supply
voltage. The resulting transfer equation:
(5)
12.3.2 External voltages
Measurements of an external voltage on either the PTA0 or PTA1 pins can be made and
referenced to the internal band gap voltage. The resulting transfer equation:
(6)
where x = 0, 1 refers to PTA0 or PTA1.
12.4 Optional acceleration measurements
The acceleration measurement consists of an interface to an optional acceleration
sensing element. Control bits on the MCU operate the SMI to power up the g-Cell and
capture a voltage which is converted by the ADC10. The data from the ADC10 is then
pre-processed by a dynamic range firmware routine that will return the two values
necessary to calculate the acceleration, Ay, (y = X-axis or Z-axis, depending on selection)
in conjunction with values taken from the tables in the data sheet.
The first value from the firmware routine is the offset step identifier, STEP, with integer
values 0 to 15 (i.e. the 16 offset steps). The other value is the ADC10 data, AyCODE
,
with integer values 0 to 511. AyCODE values 1 through 510 are usable; values 0 and
511 indicate fault conditions. The X-axis acceleration is scaled for ~20g range within
each of the 16 offset steps, ~10g per step. The Z-axis acceleration is scaled for ~80g
range within each of the 16 offset steps, ~80g or ~60g. The steps are at ~40g or ~30g
increments, allowing for adequate overlaps. The product data sheet provides tables of
acceleration values resulting from characterizations.
Acceleration sensitivity, ΔAMAX-MIN, varies between each offset step, and should be
calculated by dividing the range of g’s for each offset step by the usable AyCODE range
(i.e. 509):
(7)
(8)
Once the sensitivity ΔAMAX-MIN has been calculated, the acceleration Ay can be
calculated by the re-using the ARATE-MIN, 1 value of the offset step and the returned
AyCODE value with the following transfer function:
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.
Note: The included accelerometers are designed with a self-test feature. Consult sales/
application support for information regarding the recommended use of the accelerometer
self-test features.
FXTH87ERM
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© NXP B.V. 2019. All rights reserved.
Reference manual
Rev. 5.0 — 4 February 2019
101 / 183
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