NXP Semiconductors
FXTH87E
FXTH87E, Family of Tire Pressure Monitor Sensors
15.3.2 Interframe time intervals
When generating an interframe time interval the MCU loads the RFM interval generator
variables and then goes to the STOP1 mode. When the interframe 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 interframe time interval is made up of three components:
(12)
where:
• tIFRM = Total time interval between each transmitted frame in ms
• tBASE = Base time in ms; ≤ 5 ms not recommended
• tFN = Time adder in ms for frame number
• tRAND = Pseudo-random time in ms based on a Galois 7-bit LFSR
The components of this time are described in the following sections.
15.3.3 Base time interval
The base time interval, tBASE, is used in the initial time interval and in datagram
transmissions with two or more frames. The programmable frame space interval is based
on a simple 8-bit, count-down timer as described by the RFBT[7:0] control bits in the
RFCR4 register. This time interval is forced to zero when the RFBT[7:0] are all clear.
The range of the base time must be set to 0 or between 5 and 255 ms using a clock
generated from the MFO divided by 125.
15.3.4 Pseudo-random time interval
The pseudo-random time interval, tRAND, is used both in the initial and the interframe
time intervals if the LFSR[6;0] bits are set to something other than all zeros. When the
ISPC bit is set the pseudo-random initial time interval before the first data frame will be
40 times the value of tRAND
.
When the LFSR[6:0] bits are used the tRAND time will vary based on a pseudo-
random generated binary number using a Galois linear feedback shift register (LFSR)
implemented using the primitive polynomial for a 7-stage register as shown in Figure 41.
This LFSR creates a sequence of 127 binary numbers including $01 through $3F which
are each repeated only once in each sequence of 127 clocks of the shift register. The
LFSR is initialized to $40 during power up of the device. When a random interval is
to be determined the contents of the LFSR are sampled as the "random number" for
calculating the required interval time. Following the use of the random interval the LFSR
is clocked once to advance it to the next pseudo-random number.
Note: The LFSR bits in RFCR5 are the seed and not the current LFSR random number,
which is not accessible.
The range of the pseudo-random time is 1 to 127 ms using a clock generated from the
MFO divided by 125. The current value of the LFSR can be changed and/or read by the
LFSR[6:0] bits in the RFCR5 register.
FXTH87ERM
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Reference manual
Rev. 5.0 — 4 February 2019
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