NXP Semiconductors
FXTH87E
FXTH87E, Family of Tire Pressure Monitor Sensors
FXTH87ERM
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Reference manual
Rev. 5.0 — 4 February 2019
125 / 183
Table 111. LFCTL3 register field descriptions
Field
Description
7
LFDO
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.
0 LF detector output low (no signal above threshold)
1 LF detector output high (received signal above threshold)
6
TOGMOD
LFR Mode Toggle — This read/write bit enables the toggling of the CARMOD bit at each new LFON
sequence. Reset clears this bit.
0 CARMOD bit does not change and determines detector mode.
1 CARMOD 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.
5:4
SYNC[1:0]
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.
00 For factory test purposes, not intended for use in any application.
01 6T SYNC pattern
10 7.5T SYNC pattern
11 9T SYNC pattern
3:0
LFCDTM
[3:0]
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:
0000 Carrier detect = 8 (64 μsec) Data mode detect = 8 (64 μsec)
0001 Carrier detect = 16 (128 μsec) Data mode detect = 8 (64 μsec)
0010 Carrier detect = 32 (256 μsec) Data mode detect = 8 (64 μsec)
0011 Carrier detect = 64 (512 μsec) Data mode detect = 8 (64 μsec)
0100 Carrier detect = 128 (1024 μsec) Data mode detect = 8 (64 μsec)
0101 Carrier detect = 256 (2048 μsec) Data mode detect = 8 (64 μsec)
0110 Carrier detect = 512 (4096 μsec) Data mode detect = 8 (64 μsec)
0111 Carrier detect = 1024 (8192 μsec) Data mode detect = 8 (64 μsec)
1000 Carrier detect = 8 (64 μsec) Data mode detect = 8 (64 μsec)
1001 Carrier detect = 16 (128 μsec) Data mode detect = 16 (128 μsec)
1010 Carrier detect = 32 (256 μsec) Data mode detect = 32 (256 μsec)
1011 Carrier detect = 64 (512 μsec) Data mode detect = 64 (512 μsec)
1100 Carrier detect = 128 (1024 μsec) Data mode detect = 128 (1024 μsec) (see note)
1101 Carrier detect = 256 (2048 μsec) Data mode detect = 256 (2048 μsec) (see note)
1110 Carrier detect = 512 (4096 μsec) Data mode detect = 512 (4096 μsec) (see note)
1111 Carrier 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.
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