AN2707 Freescale Semiconductor / Motorola, AN2707 Datasheet - Page 4

• Manufacturer Part Number: AN2707
• Description: Software Drivers for Tango3 RF Transmitter and Romeo2 RF Receiver ICs
• Manufacturer: Freescale Semiconductor / Motorola

AN2707
Message Formats
4
NOTE:
NOTE:
Software Drivers for Tango3 RF Transmitter and Romeo2 RF Receiver ICs
The drivers support the messaging formats defined in the Romeo2 datasheet.
Communications using ID or tone signalling, with or without the header field are
supported.
The Romeo2 driver uses Romeo2's on-board data manager hardware.
The drivers extend the message formats shown in the datasheet, by defining
length, data and checksum fields for each message.
A message contains the following fields.
Preamble — The Preamble is a fixed format field that allows Romeo2 to
determine the timing of bits on the RF link. A Preamble field is required before
each ID and Header field. See Romeo2 datasheet for more detailed
information.
ID — Each Romeo2 device can be assigned an 8-bit ID number. It will only
receive messages with this particular ID. This allows each Romeo2 device in
an RF network to have a unique ID. A Tango3 transmitter can send messages
with any ID. The ID field can also be used to implement Tone signalling, a
simplified message format where each receiver uses the same fixed ID. See
Tone Signalling on page 7
The ID word must not contain the bit sequences '0110' or '1001'. These bit
sequences are used as the header word field. See the Romeo2 datasheet for
more detailed information.
Header — The Header field is a 4-bit fixed format field. It notifies Romeo2 that
message data is next. The header field is fixed to '0110' in this driver
implementation. When Romeo2 receives the Header byte, it expects to receive
the Length and Data fields next. It is possible to send messages with or without
this field.
Length — The Length field is a byte containing the length of the Data field.
Data — The Data field comprises 0–127 data bytes.
Checksum — The Checksum field is a byte containing a checksum of the ID
and data fields. The checksum value is calculated by adding all bytes in the ID
and Data fields, using modulo 256 addition. (In MCU assembly language, this
equates to adding the bytes using the 'Add with Carry' instruction.)
EOM (End of Message) — The EOM field is a fixed format field that indicates
the end of a message.
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