RFM22
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.
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.
6.9. TX Retransmission and Auto TX
The RFM22 is capable of automatically retransmitting the last packet in the FIFO if no additional packets were
loaded into the TX FIFO. Automatic Retransmission is achieved by entering the TX state with the txon bit set. This
feature is useful for Beacon transmission or when retransmission is required due to the absence of a valid
acknowledgement. Only packets that fit completely in the TX FIFO are valid for retransmit. When it is necessary to
transmit longer packets, the TX FIFO uses the circular read/write capability.
An Automatic Transmission is also available. When autotx = 1 the transceiver will enter automatically TX State
when the TX FIFO is almost full. When the TX FIFO is empty the transceiver will automatically return to the IDLE
State.
POR
Def.
Function/Descri
ption
D7
D6
D5
D4
D3
D2
D1
D0
Add R/W
Operating &Function
Control 2
antdiv[2] antdiv[1] antdiv[0] rxmpk autotx enldm ffclrrx ffclrtx 00h
R/W
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