Rainbow Electronics RF22 - Manual (Page 29)

RF22
Version: 0.1 Date: 12/23/2008
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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 RF22 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.
Add R/W Function/Description D7 D6 D5 D4 D3 D2 D1 D0 POR Def.
08 R/W Operating & Function
Control 2
antdiv[2] antdiv[1] antdiv[0] rxmpk autotx enldm ffclrrx ffclrtx 00h
7. RX Modem Configuration
7.1. Modem Settings for FSK and GFSK
The modem performs channel selection and demodulation in the digital domain. The channel filter bandwidth is
configurable from 620 to 2.6 kHz. The data-rate, modulation index, and bandwidth are set via registers 1C–25. The
modulation index is equal to 2 times the peak deviation divided by the data rate (Rb).
Table 16 gives the modem register settings for various common data-rates. Select the desired data-rate (Rb), and
Deviation (Fd) to determine the proper register settings. For data-rates and modulation types not listed in the table a
calculator tool within WDS can be used.
When Manchester coding is disabled, the required channel filter bandwidth is calculated as BW = 2 x (Fd + 0.25Rb)
where Fd is the frequency deviation and Rb is the data rate. For modulation indices below 1 the required channel filter
bandwidth is calculated as BW = Fd + Rb. The channel filter needs to be increased when the frequency offset between
transmitter and receiver is more than half the channel filter bandwidth. In this case it is recommended to enable the
AFC and choose the IF bandwidth equal to 2 x frequency offset.
Table16. RX Modem Configurations for FSK and GFSK
RX Modem setting examples for GFSK and FSK
Application parameters Register values (hex)
Rb Fd
mod
index
BW
-3dB
dwn3_bypass ndec_exp[2:0] filset[3:0] rxosr[10:0] ncoff[19:0] crgain[10:0]
kbps kHz kHz 1Ch 1Ch 1Ch 20,21h 21,22,23h 24,25h
2 5 5.00 11.5 0 3 3 0FA 08312 06B
2.4 4.8 4.00 11.5 0 3 3 0D0 09D49 0A0
2.4 36 30.00 75.2 0 0 1 683 013A9 005
4.8 4.8 2.00 12.1 0 3 4 068 13A93 278
4.8 45 18.75 95.3 0 0 4 341 02752 00A
9.6 4.8 1.00 18.9 0 2 1 068 13A93 4EE
9.6 45 9.38 95.3 0 0 4 1A1 04EA5 024
10 5 1.00 18.9 0 2 1 064 147AE 521