User's Manual

APPN-TT551 Databook B&B Electronics, Inc.
17
6.3 SPI Protocol
A SPI message is composed of a 4 byte header followed by 0 or more bytes of
data. The header data is full-duplex. That is, the TX message header is sent to
the Airborne Device Server module by the host at the same time that the RX
message header is sent to the host from the Airborne Device Server.
The TX message header consists of a Command (CMD) byte, followed by three
Parameter (PARM) bytes. They are described in the SPI Commands section 0
below.
The RX message header is shifted out as the first four bytes of an SPI message
regardless of the contents of the TX message header. The RX message header
consists of a RX Data Available field and a TX Buffer Available field. The RX
Data Available field indicates the number of data bytes the Device Server has
available for the host. The data can be received by the RXDATA command. The
TX Buffer Available field indicates how many data bytes the Device Server is
able to accept from the host. This data is to be shifted in by the host using the
TXDATA (Table 16) command. Both fields are 16 bit values and are stored in
little-endian format (Least significant byte first).
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The /SPI_SEL signal must be de-asserted between successive SPI messages. The
messages will not be processed correctly if /SPI_SEL is held asserted across multiple
messages.
Table 13 - TX Message Header
0 1 2 3
CMD PARM1 PARM2
Table 14 - RX Message Header
0 1 2 3
RX Data Available TX Buffer Available
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SPI data is transferred most significant bit first (msb).