NXP Semiconductors
FXTH87E
FXTH87E, Family of Tire Pressure Monitor Sensors
bytes into a buffer where they remain until a second byte is read ($1808 or $1809). This
allows coherent 16-bit reads in either order. The coherency mechanism is automatically
restarted by an MCU reset, exit of STOP1, or a write to the PMCT register at address
$180B bits 7 = 1, or bit 5 = 0.
Once enabled, the FRC continues to increment (and subsequently rolls over) in RUN,
STOP4, STOP3, and STOP1 modes, unless halted as defined below.
13 Periodic Wakeup Timer
The periodic wakeup timer (PWU) generates a periodic interrupt to wakeup the MCU
from any of the STOP modes. It also has an optional periodic reset to restart the MCU.
It is driven by the LFO oscillator in the RTI module which generates a clock at a nominal
one millisecond interval. The LFO and the wakeup timer are always active and cannot
be powered off by any software control. The control bits are set so that there is either a
periodic wakeup, a periodic reset, or both a wakeup interrupt and a periodic reset. No
combination of control bits will disable both the wakeup interrupt and the periodic reset.
In addition, there is no hardware control that can mask a wakeup interrupt once it is
generated by the PWU.
13.1 Block diagram
The block diagram of the wakeup timer is shown in Figure 25. This consists of a
programmable prescaler with 64 steps that can be used to adjust for variations in the
value of the LFO period. Finally there are two cascaded programmable 6-bit dividers to
set wakeup and/or reset time intervals.
6-BIT
PERIODIC
RESET
6-BIT
WAKEUP
DIVIDER
WCLK
RCLK
PROGRAMMBLE
PRESCALER
LFO
DIVIDER
TRE
PRST
PRF
TRO
PRFAK
WUFAK
CONTROL
LOGIC
WUKI
WUF
WDIV[5:0]
WUT[5:0]
PRST[5:0]
aaa-028018
Figure 25.ꢀWakeup timer block diagram
The wakeup divider (PWUDIV) register selects a division of the incoming 1 ms clock
to generate a wakeup clock, WCLK. The WCLK frequency can be calibrated against
the more precise external oscillator using the TPMS_LFOCOL firmware subroutine as
described in Section 16 "Firmware". This subroutine turns on the RFM crystal oscillator
and feeds a 500 kHz clock to the TPM1 for one cycle of the LFO. The measured time is
used to calculate the correct value for the WDIV[5:0] bits for a WCLK period of 1 second.
The TPMS_LFOCOL subroutine cannot be used while the RFM is transmitting or the
TPM1 is being used for another task.
The wakeup time register (PWUSC0) selects the number of WCLK pulses that are
needed to generate a wakeup interrupt to the MCU. The periodic reset register
(PWUSC1) selects the number of wakeup pulses that are needed to generate a periodic
reset of the MCU. Both the wakeup time counter and the periodic reset timer are
incrementing counters that generate their interrupt or reset when the desired count is
FXTH87ERM
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Reference manual
Rev. 5.0 — 4 February 2019
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