NXP Semiconductors
FXTH87E
FXTH87E, Family of Tire Pressure Monitor Sensors
FXTH87ERM
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© NXP B.V. 2019. All rights reserved.
Reference manual
Rev. 5.0 — 4 February 2019
139 / 183
aaa-028034
D Q
R
D Q
R
D Q
R
D Q
R
D Q
R
D Q
R
D Q
S
S0
S1
S2
S3
S4
S5
S6
CLK
RFMRST
7-bit random number
Galois primitive polynomial = X7 + X3 + 1
Figure 41. LFSR implementation
A value of all zeros in the LFSR will remain unchanged with every clock input and cannot
be used as a starting "seed."
The resulting range of times for the initial and interframe pseudo-random time will be
as given in
Table 131
for both the design center and the variation resulting from the
tolerance of the MFO clock.
Table 131. Randomization interval times
Time Interval Including MFO Tolerance (ms)
Time
Interval
Randomization
Number
Ideal Time Interval (ms)
Minimum
Maximum
1
40
37.2
42.8
Initial
127
5080
4347.2
5434.6
1
1
0.93
1.07
Interframe
127
127
118.1
135.9
15.3.5 Frame number time
The frame number time, t
FN
, is only used between frames and is based on a selectable
time from 0 to 63 ms and the number of the frame that was just transmitted as given in
Table 132
. If the frame number time is not used, the value of the selected time should be
set to zero. The maximum number of frames is defined by the FNUM[3:0] control bits.
The range of the frame number time is a multiple of 0 to 63 ms using a clock generated
from the MFO divided by 125. The value of this time multiple can be changed by the
RFFT[5:0] bits in the RFCR6 register.
Table 132. Frame number interval times
Nominal frame number
time interval added (ms)
Value of
FNUM[3:0]
Number
of frames
Frame
interval where
time added
Minimum
Maximum
0
1
None
n/a
n/a
1
2
1 - 2
1
63
2
3
2 - 3
2
126
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