R0010-02
Preamble
Sync word
Address Header
Payload
CRC
1–16 bytes
0–4 bytes0–127 bytes0–1 byte
16–32 bits
9–10 bits
Handled by modem
NOASEQ
LSBBit 1Bit 2Bit 3Bit 4Bit 5Bit 6Bit 7
Length
Bit 8
NOASEQ
LSBBit 1Bit 2Bit 3Bit 4Bit 5Bit 6Bit 7
Length
Bit 8Bit 9
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Packet Format
Figure 25-7. Air Interface Packet Format for Auto Mode
The preamble is a sequence of 1010 1010 or 0101 0101. It can be from 1 to 16 bytes. The type of
preamble and the number of bytes can be set up in the MDMCTRL2 register.
The Sync word field is a synchronization word that can have any length from 16 to 32 bits. The length is
programmed in the SW_CONF.SW_LEN register. The sync word itself is programmed in the SW0, SW1, SW2,
and SW3 registers for the primary sync word, and SW4, SW5, SW6, and SW7 for the secondary sync word.
The bit ordering of the sync word is set up with MDMCTRL2.SW_BIT_ORDER. If SW_BIT_ORDER is 0, the
LSB of SW0 (SW4) is transmitted first and the MSB of SW3 (SW7) is transmitted last. If SW_BIT_ORDER
is 1, the MSB of SW3 (SW7) is transmitted first and the LSB of SW0 (SW4) is transmitted last. The first bit
transmitted is always the same regardless of the sync word length; the unused bits for sync word length of
less than 32 bits are the ones that would have been transmitted last.
The optional length byte in basic mode (see Figure 25-6) is present if PRF_TASK_CONF.MODE = 01. It
indicates the number of address and payload bytes following the length byte. If the length field is not
present, the length is fixed as described in Section 25.9.2.
The optional address is 1 byte if present; the length is configured with the PRF_PKT_CONF.ADDR_LEN
register. In the transmitter, the address can be used for identification or to direct the message to a
particular receiver, and in the receiver, the address can be used to filter out messages from unknown or
unwanted transmitters and to distinguish between messages from different transmitters. See
Section 25.9.2 for details on how the address is used. Note that for the packet format in Figure 25-7 or if a
length field is not used, the address field immediately follows the sync word, and can thus be seen as an
extension of it.
The 9-bit or 10-bit header shown in Figure 25-7 is shown in more detail in Figure 25-8 and Figure 25-9.
This field consists of a 6-bit or 7-bit length followed by a 2-bit sequence number and a flag called
NO_ACK (NOA in Figure 25-8 and Figure 25-9) to inform that acknowledgment of the packet is not
expected. If the configuration is to use a fixed length, the value of the length field is ignored in the
receiver. It can be configured always to set the length field to 110011 in the transmitter for fixed-length
packets.
Figure 25-8. Bits of 9-Bit Header
Figure 25-9. Bits of 10-Bit Header
The payload can be from zero to 255 bytes in basic mode, but the sum of the number of address and
payload bytes must not exceed 255. In auto mode, the payload can be from 0 to 63 bytes with a 9-bit
header or 0 to 127 bytes with a 10-bit header. The maximum packet length can be limited, see
Section 25.9.2.3.1 and Section 25.9.2.3.2
311
SWRU191C–April 2009–Revised January 2012 CC2541 Proprietary Mode Radio
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