RFM31 Datasheet, PDF (34/137 Pages) –

RFM31
Table 14. Minimum Receiver Settling Time
Mode
Approximate
Recommended preamble Recommended preamble
receiver settling time
length with 8-bit
length with 20-bit
detection threshold
detection threshold
(G)FSK AFC Disabled
1 byte
20 bits
32 bits
(G)FSK AFC Enabled
2 byte
28bits
40 bits
(G)FSK AFC Disabled +Antenna
1 byte
—
Diversity Enabled
64 bits
(G)FSK AFC Enabled +Antenna
2 byte
—
Diversity Enabled
8 byte
OOK
2 byte
3 byte
4 byte
OOK + Antenna Diversity
8 byte
—
Enabled
8 byte
Note: The recommended preamble length and the preamble detection threshold may be shortened when occasional packet
errors are tolerable.
6.7. Invalid Preamble Detector
When scanning channels in a Frequency Hopping System, it is desirable to determine if a channel is valid in the
minimum amount of time. The preamble detector can output an invalid preamble detect signal. When an error is
detected in the preamble, the Invalid Preamble Detect signal (nPQD) is asserted, indicating an invalid channel. The
signal can be used to qualify the channel without requiring the full preamble to be received. The Preamble Detect
and Invalid Preamble Detect signals are available in "Register 03h. Interrupt/Status 1" and ―Register 04h.
Interrupt/Status 2,‖.
The Invalid Preamble Detector issues an interrupt when no valid preamble signal is found. After the receiver is
enabled, the Invalid Preamble Detector will be held low for 16 Tb (Tb is the time of the bit duration) to allow the
receiver to settle. The 16 Tb is a fixed time which will work with a 4-byte Preamble (or longer) when AFC is
enabled, or a 3-byte preamble (or longer) when AFC is disabled. The invalid preamble detect interrupt can be
useful to save power and speed-up search in receive mode.
It is advised to disable the invalid preamble interrupt when Antenna Diversity is enabled. The Invalid Preamble
Detect interrupt may be triggered during the Antenna Diversity algorithm if one of the antennas is weak but the
other is capable of still receiving the signal if the Antenna Diversity algorithm is allowed to complete.
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