
Semtech 2006
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14-22
XE8806A/XE8807A
The first example below (chapter 14.12.2.3) uses the XE1202A clock. In the other examples, the internal RC clock
is used.
14.12.2.3 Reception mode using NRZ coding and the XE1202A bit and message synchronizer
This RF link uses a wide band transmission in the 915MHz frequency band. The protocol of the messages
received is NRZ Mark. The start sequence of the message is a 4 byte pattern (10101010 10101010 10101010
11010100). The received messages have a data rate of 76.8 kbit/s with a frequency deviation of 100 kHz. The
following paragraphs will show how to set-up the XE1202A, how to set-up the RF interface and how to handle the
received data. The microcontroller derives its clock from the XE1202A.
14.12.2.3.1
XE1202A set-up
To set the XE1202A in reception mode, set the pins MODE(2:0) = 100.
To select the 915MHz band, the bits FSParam_band = 11. The bandwidth of the baseband filter is set to 200 kHz
by writing RTParam_BW = 11. The bit rate is set to 76.8 kbit/s by writing FSParam_BR = 100. The bit synchronizer
is enabled (RTParam_bits = 1). The pattern recognition is enabled (ADParam_Pattern =1) and the length of the
pattern is set to 32 bit (ADParam_Psize = 11). The pattern is written to the 4 pattern registers. The output clock is
enabled (RTParam_Clkout = 1) and its frequency is set to 2.44 MHz (ADParam_Clkfreq = 01).
Register
Register Address
7
6
5
4
3
2
1
0
RTParam1
00000
0
1
0
1
0
RTParam2
00001
0
1
0
1
FSParam1
00010
1
0
1
0
FSParam2
00011
0
FSParam3
00100
0
DataOut
00101
0
ADParam1
00110
1
0
1
0
ADParam2
00111
0
Pattern1
01000
1
0
1
0
1
0
1
0
Pattern2
01001
1
0
1
0
1
0
1
0
Pattern3
01010
1
0
1
0
1
0
1
0
Pattern4
01011
1
0
1
0
1
0
Table 14-27. XE1202 register set-up (see XE1202 datasheet for bit explanation)
14.12.2.3.2
RF interface set-up
We then set-up the RF interface of the microcontroller circuit as a receiver (RfifEnRx = 1 and RfifEnTx = 0).
The external clock frequency generated by the XE1202A is 2.44 MHz. The external clock source has to be
selected by writing SelExtClock = 1 in RegSysClock. The frequency used in the microcontroller is half of this (see
clock block documentation): 1.22 MHz. To select the correct baud rate of 76.8 kbit/s according to the equation in
chapter 14.10, fine*coarse=1.22e06/(16*76.8e3)=0.99. We can approximate this at 1 (see specification in Table
14-19). This can be done by setting RfifBRCoarse = 00 and RfifBRFine = 0000.
The external bit synchronization clock is switched off by setting the bit RfifRxClock = 0.
The decoder is enabled and set to NRZ Mark decoding by setting RfifEnCod = 1 and RfifPCM = 001.
The external start pattern detection is enabled by setting RfifEnStart = 10.
The start sequence detection interrupt is enabled by setting RfifRxIrqEn = 001.
The set-up of the interface is summarized in the Table 14-28.