RFM22B/23B
period of time. If a sync is not recognized in this period, a timeout will occur, and the search for preamble will be re-
initiated. The timeout period after preamble detections is defined as the value programmed into the sync word
length plus four additional bits.
6.10. Receive Header Check
The header check is designed to support 1–4 bytes and broadcast headers. The header length needs to be set in
register 33h, hdlen[2:0]. The headers to be checked need to be set in register 32h, hdch[3:0]. For instance, there
can be four bytes of header in the packet structure but only one byte of the header is set to be checked (i.e.,
header 3). For the headers that are set to be checked, the expected value of the header should be programmed in
chhd[31:0] in Registers 3F–42. The individual bits within the selected bytes to be checked can be enabled or
disabled with the header enables, hden[31:0] in Registers 43–46. For example, if you want to check all bits in
header 3 then hden[31:24] should be set to FF but if only the last 4 bits are desired to be checked then it should be
set to 00001111 (0F). Broadcast headers can also be programmed by setting bcen[3:0] in Register 32h. For
broadcast header check the value may be either “FFh” or the value stored in the Check Header register. A logic
equivalent of the header check for Header 3 is shown in Figure 25. A similar logic check will be done for Header 2,
Header 1, and Header 0 if enabled.
Example for Header 3
rxhd[31:24]
BIT
Equivalence
comparison
WISE
hden[31:24]
=
BIT
WISE
chhd[31:24]
header3_ok
bcen[3]
Equivalence
comparison
FFh
=
hdch[3]
rxhd[31:24]
Figure 25. Header
6.11. TX Retransmission and Auto TX
The RFM22B/23B is capable of automatically retransmitting the last packet loaded in the TX FIFO. Automatic
retransmission is set by entering the TX state with the txon bit without reloading the TX FIFO. 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 can be automatically retransmitted.
An automatic transmission function is available, allowing the radio to automatically start or stop a transmission
depending on the amount of data in the TX FIFO.
When autotx is set in “Register 08. Operating & Function Control 2", the transceiver will automatically enter the TX
state when the TX FIFO almost full threshold is exceeded. Packets will be transmitted according to the configured
packet length. To stop transmitting, clear the packet sent or TX FIFO almost empty interrupts must be cleared by
reading register.
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