RFM23
The Preamble Detector output may be programmed onto one of the GPIOs or read in the Interrupt Status registers.
6.7. Preamble Length
The required preamble length threshold will depend on when the receive mode is entered in relation to the
transmitted packet. When the receiver is enabled long before the arrival of the packet, then a short preamble
detection threshold might result in false detects on the received noise before the actual preamble arrives. In this
case, it is recommended to program a 20 bit preamble detection threshold. A shorter Preamble Detection
Threshold might be chosen when occasional false detects are tolerable. When antenna diversity is enabled, it is
advised to use a 20 bit preamble detection threshold. When the receiver is synchronously enabled just before the
start of the packet, then a shorter preamble detection threshold might be chosen (e.g., 8 bit).
The required preamble length is determined from the sum of the receiver settling time and the preamble detection
threshold. The receiver settling time is listed in Table 15.
Table 15. Minimum Receiver Settling Time
Approximate
Recommended preamble
length with 8-bit
detection threshold
20 bits
Recommended preamble
length with 20-bit
detection threshold
32 bits
Mode
receiver settling time
(G)FSK AFC Disabled
(G)FSK AFC Enabled
(G)FSK AFC Disabled +Antenna
Diversity Enabled
1 byte
2 byte
28bits
40 bits
1 byte
—
64 bits
(G)FSK AFC Enabled +Antenna
Diversity Enabled
2 byte
2 byte
8 byte
—
3 byte
—
8 byte
4 byte
8 byte
OOK
OOK + Antenna Diversity
Enabled
Note: The recommended preamble length and the preamble detection threshold may be shortened when occasional packet
errors are tolerable.
6.8. 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.
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