FXTH870xD
Sensors
Freescale Semiconductor, Inc.
99
Figure 74. Telegram Format (Carrier Preamble)
12.14
Error Detection and Handling
When the DECEN bit is set, LFR messages are monitored for data rate or SYNC errors, incorrect message ID, and Manchester
coding errors. When an error is detected the LFR goes back to sniff mode until the end of ON time completion, if ONMODE is
set; or turns off until the start of the next scheduled sampling interval, if ONMODE is cleared. Because the MCU uses more power
than the LFR module, it is desirable to keep the MCU in low power standby modes as much as possible. Therefore the handling
of these errors will be performed by the LFR and not require additional software processing by the MCU.
When the DECEN bit is clear, there is no monitoring on data. The MCU needs to poll the state of the LFDO bit and create its own
decoding scheme within software on the detected signal. To be able to start the polling only when data are received, the carrier
detection flag is enabled in data mode when DECEN = 0. During data reception, the auto-zero sequence is performed at each
LFO period. The MCU needs also to determine the end of the telegram and turn off the LFR (LFEN = 0) during two LFO cycles
before any other operations.
12.15
Continuous ON Mode
In the Continuously ON mode, the LFR module will remain on continuously while the LFEN bit is set. The Continuously ON mode
is controlled by setting the LFSTM[3:0] bits.
In the Continuously ON mode, if a signal is successfully processed by the digital, the LFR module will stop and restart
automatically. The gap is 2-3 LFO periods. Also if TOGMOD bit is set, the LFR module will stop after the ON time cycle and re-
start automatically, after having changed the CARMOD bit.
12.16
Initialization Information
When power is applied to the MCU, the LFR must be initialized and configured before it can begin to receive LF messages.
Several systems in the LFR require factory trimming to ensure operation within specified limits. After these trim values are written,
they remain constant until the next MCU reset.
The application program must set up control bits and registers to configure the LFR to determine the structure of the message
telegram, the input sensitivity, and other LFR options. It is good practice to clear the flags in the LFS register before enabling
interrupt sources in order to avoid any immediate interrupt requests.
PREAMBLE
SYNC
HIGH ID
DATA
DATA
6, 7.5 or 9T
0, 8T or 16T
8T
t
LFPRE
IDSEL[1:0]
Repeat for 0-n bytes
LOW ID
Содержание FXTH870 D Series
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