NXP Semiconductors
FXTH87E
FXTH87E, Family of Tire Pressure Monitor Sensors
Table 111.ꢀLFCTL3 register field descriptions
Field
Description
LF Detector Output — This read-only bit follows the bit slicer output signal that goes high during the
presence of a carrier. It may change at any time. This bit is read only and unaffected by any reset.
7
0ꢀLF detector output low (no signal above threshold)
LFDO
1ꢀLF detector output high (received signal above threshold)
LFR Mode Toggle — This read/write bit enables the toggling of the CARMOD bit at each new LFON
sequence. Reset clears this bit.
6
0ꢀCARMOD bit does not change and determines detector mode.
TOGMOD
1ꢀCARMOD bit will be toggled every LFON detection sequence, starting by CARMOD selection.
Therefore, the reception chain will alternately look for a carrier frame or for a data frame.
LF SYNC Selection — Selects the type of SYNC pattern as described in Figure 34. Reset presets these
bits to the 01 (6T SYNC) option. Compatible with preamble consisting of minimum 2 ms Manchester data to
allow for proper averaging filter operation.
5:4
00ꢀFor factory test purposes, not intended for use in any application.
01ꢀ6T SYNC pattern
SYNC[1:0]
10ꢀ7.5T SYNC pattern
11ꢀ9T SYNC pattern
LF Carrier Detect Time — These read/write control bits select the length of time which the LFR input
detector must detect a carrier before validating it. In carrier mode (CARMOD = 1), if the carrier is active
for at least the time selected by the LFCDTM[3:0] bits and the LFCC counter value is reached, the LFCDF
flag in the LFS register will be set; and if the LFCDIE control bit is also set, the MCU will be interrupted
(wakeup).
In the data receive mode (CARMOD = 0) the LFCDTM[3:0] bits select the length of time which the LFR
input detector must detect a carrier before the effective receive chain is powered on. Once the carrier has
been validated the LFCDTM[3:0] bits ignored during the decode of the rest of the data.
Reset of the LFR results in LFCDTM[3:0] being reset to 0:0:0:0. The resulting carrier detect times are
defined by the following number of carrier periods needed to validate the carrier, with the corresponding
time for a carrier at 125 kHz in parenthesis:
0000ꢀCarrier detect = 8 (64 μsec) Data mode detect = 8 (64 μsec)
0001ꢀCarrier detect = 16 (128 μsec) Data mode detect = 8 (64 μsec)
0010ꢀCarrier detect = 32 (256 μsec) Data mode detect = 8 (64 μsec)
0011ꢀCarrier detect = 64 (512 μsec) Data mode detect = 8 (64 μsec)
0100ꢀCarrier detect = 128 (1024 μsec) Data mode detect = 8 (64 μsec)
0101ꢀCarrier detect = 256 (2048 μsec) Data mode detect = 8 (64 μsec)
0110ꢀCarrier detect = 512 (4096 μsec) Data mode detect = 8 (64 μsec)
0111ꢀCarrier detect = 1024 (8192 μsec) Data mode detect = 8 (64 μsec)
1000ꢀCarrier detect = 8 (64 μsec) Data mode detect = 8 (64 μsec)
3:0
LFCDTM
[3:0]
1001ꢀCarrier detect = 16 (128 μsec) Data mode detect = 16 (128 μsec)
1010ꢀCarrier detect = 32 (256 μsec) Data mode detect = 32 (256 μsec)
1011ꢀCarrier detect = 64 (512 μsec) Data mode detect = 64 (512 μsec)
1100ꢀCarrier detect = 128 (1024 μsec) Data mode detect = 128 (1024 μsec) (see note)
1101ꢀCarrier detect = 256 (2048 μsec) Data mode detect = 256 (2048 μsec) (see note)
1110ꢀCarrier detect = 512 (4096 μsec) Data mode detect = 512 (4096 μsec) (see note)
1111ꢀCarrier detect = 1024 (8192 μsec) Data mode detect = 1024 (8192 μsec) (see note)
Note: The auto-zero sequence needs to be performed every 1 ms. Therefore, LFR
detection times of 1024, 2048, 4096 and 8192 μsec the auto-zero sequence will be done
at each 1 ms interval. This auto-zero sequence lasts for 64 μsec. If the carrier is detected
again at the end of the auto-zero sequence it is assumed that the carrier was there for
the complete 64 μsec period of the auto-zero.
FXTH87ERM
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Reference manual
Rev. 5.0 — 4 February 2019
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