NXP Semiconductors
FXTH87E
FXTH87E, Family of Tire Pressure Monitor Sensors
time not to scale
1 to 16 data frames
with identical data
t0
SPACE
DATA FRAME SPACE DATA FRAME
SPACE DATA FRAME
Initial interval
Interframe intervals
Start of time
interval for
datagram
t
DATA
Space intervals have no carrier frequency output, equivalent to data = 0 and may have the
RFM crystal oscillator, VCO and PLL turned off by the IFPD bit.
aaa-028032
Figure 39.ꢀDatagram overview
The generation of the initial and interframe time intervals can be done with a combination
of a programmable counter, a pseudo- random interval generator and a frame counter as
shown in Figure 40. The initial time interval can be done by adjusting the start time using
the MCU or using this interval timing generator.
time not to scale
t0
Initial interval
Interframe interval 1
Interframe interval 2
t
40 × t
RAND
t
t
t
t
t
t
FN
BASE
BASE
RAND
FN
BASE
RAND
Start of time
interval for
datagram
t
t
, t
BASE RAND
and t may be all zero in the initial interval.
FN
, t
and t may be all zero in an interframe interval.
BASE RAND
FN
All interframe intervals may have different t
, t
BASE RAND
and t times.
FN
aaa-028033
Figure 40.ꢀInitial and interframe timing
Note: If tBASE and tFN are both set to non-zero, and tRAND is set to 0, the system
will decrement both tBASE and tFN simultaneously rather than serially, such that the
effective Interframe Interval will be equal to the larger of tBASE or tFN settings.
15.3.1 Initial time interval
When generating an initial time interval the MCU loads the RFM interval generator
variables and then goes into the STOP1 mode. When the initial time interval ends the
data in the RFM data buffer is automatically sent and the MCU will wake at the end of the
transmission. The initial time interval is made up of two components:
(11)
where:
• tINIT = Total time interval before first frame is transmitted in ms
• tBASE = Base time in ms; ≤ 5 ms not recommended
• tRAND = Pseudo-random time in ms based on a Galois 7-bit LFSR
The components of this time are described in the following sections.
FXTH87ERM
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Reference manual
Rev. 5.0 — 4 February 2019
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