NXP Semiconductors
FXTH87E
FXTH87E, Family of Tire Pressure Monitor Sensors
If the module generates a periodic reset the module will cause the MCU to exit the power
saving mode as a POR. The PRF flag will be set to indicate periodic reset; write 1 to
PRFACK to clear this flag. The SRS register will have just the POR bit set.
In this STOP mode exit all registers will continue to hold their programmed values.
13.6.4 Active BDM/foreground commands
The PWU is frozen in ACTIVE BACKGROUND mode or executing foreground
commands, so PWU counters will also be stopped. Normal PWU operation will resume
as MCU exits BDM or foreground command is finished.
14 LF Receiver
The low-frequency receiver (LFR) is a very low-power, low-frequency, receiver system for
short-range communication in TPMS. The module allows an external coil to be connected
to two dedicated differential input pins. In TPMS systems a single coil may be oriented
for optimal coupling between the receiver in the tire or wheel and a transmitter coil on the
vehicle body or chassis.
This LFR system minimizes power consumption by allowing flexibility in choosing
the ratio of on to off times and by turning off power to blocks of circuitry until they are
needed during signal reception and protocol recognition. In addition, this LFR system
can autonomously listen for valid LF signals, check for protocol and ID information so the
main MCU can remain in a very low power standby mode until valid message data has
been received.
The LFR can be configured for various message protocols and telegrams to allow it to
be used in a broad range of applications. The message preamble must be a series of
Manchester coded bits at the nominal 3.906-kbps data rate. A synchronization pattern
is used to mark the boundary between the preamble and the beginning of Manchester
encoded information in the message body. The synchronization pattern is a non-
Manchester specific TPMS pattern. Messages can optionally include none, an 8-bit or
a 16- bit ID value. Messages may contain any number of data bytes with the end-of-
message indicated by detecting an illegal Manchester bit at a data byte boundary.
It is not intended that LFR may be actively receiving/decoding LF signals while physical
parameter measurements are being made; or during the time that the RFM may be
actively powered up and/or transmitting RF data. The resulting interactions will degrade
the accuracy of the LF detection.
FXTH87ERM
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Reference manual
Rev. 5.0 — 4 February 2019
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