XE1202AI027TRLF Semtech, XE1202AI027TRLF Datasheet - Page 12

• Manufacturer Part Number: XE1202AI027TRLF
• Description: Narrow Band 433/868/915MHz RF Transceiver
• Manufacturer: Semtech

4.1.4.2
With the “RTParam_FEI” bit set to “1” and the RTParam_Fsel” bit set to “0”, the FEI function uses over sampling of
the signal at the output of the demodulator. When activated by the rising edge of DATAIN, this function provides an
8-bit word equivalent to the duty cycle of the demodulated preamble, stored in register “DataOut_FEI”.
Each sample is used to control an up/down counter, when the sample is "1" the content of the counter is
incremented, when the sample is "0" the content of the counter is decremented. As a consequence, the final 8-bit
value of the counter stored in “DataOut_FEI” gives an indication of the duty cycle of the demodulated signal. The
range of stored values is from -128 and +127. The further from 0 the value of DataOut_FEI, the higher the error on
the LO frequency. If the stored value in “DataOut_FEI” is typically zero, then the duty cycle of the preamble is about
50 %, and the LO frequency is nominally correct.
Since this FEI uses the signal before the bit synchronizer, its value can vary from one measurement to another,
due to the presence of jitter and glitches in the signal. If possible, it is advised to make 4-5 measurements and take
the average value.
The timing diagram for this FEI measurement is illustrated in Figure 5 below. The FEI function is activated at the
rising edge of the /EN signal when the RTParam_FEI bit is set to “1”. Then, the internal FEI counter is activated at
the rising edge of DATAIN. After a period TS_FE equal to the duration of 4 bits (see Electrical Specifications), the
counter is stopped and the contents are stored in the register DataOut_FEI. For a proper operation, the pulse
length on DATAIN has to be higher than 8µs.
The maximum delay between the rising edge of DATAIN and the first clock on the internal FEI counter is 1/(16*BR),
where BR is the bit rate.
© Semtech 2006
“RTParam_Fsel” = 0
RTParam_FEI
RTParam_FEI
DataOut_FEI
DataOut_FEI
fei_out
fei_out
datain
datain
Figure 4: FEI measurement timing diagram when “RTParam_Fsel” = 1
en
en
TS_FE
TS_FE
XX
XX
XX
XX
VAL1
VAL1
13
VAL2
VAL2
XE1202A TrueRF™
VAL3
VAL3
VAL2
VAL2
VAL4
VAL4
VAL4
VAL4
XX
XX
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