AX5051
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16
transitions for the demodulator bit timing recovery, and
makes sure that the transmitted spectrum does not have
discrete lines even if the transmitted data is cyclic. It
does so without adding additional bits, i.e. without
changing the data rate. Spectral Shaping uses a
self−synchronizing feedback shift register.
The encoder is programmed using the register
ENCODING, details and recommendations on usage are
given in the AX5051 Programming Manual.
Framing and FIFO
Most radio systems today group data into packets. The
framing unit is responsible for converting these packets into
a bit−stream suitable for the modulator, and to extract
packets from the continuous bit−stream arriving from the
demodulator.
The Framing unit supports four different modes:
• HDLC
• Raw
• Raw with Preamble Match
• 802.15.4 Compliant
The micro−controller communicates with the framing
unit through a 4 level × 10 bit FIFO. The FIFO decouples
micro−controller timing from the radio (modulator and
demodulator) timing. The bottom 8 bits of the FIFO contain
transmit or receive data. The top 2 bit are used to convey
meta information in HDLC and 802.15.4 modes. They are
unused in Raw and Raw with Preamble Match modes. The
meta information consists of packet begin / end information
and the result of CRC checks.
The AX5051 contains one FIFO. Its direction is switched
depending on whether transmit or receive mode is selected.
The FIFO can be operated in polled or interrupt driven
modes. In polled mode, the micro−controller must
periodically read the FIFO status register or the FIFO count
register to determine whether the FIFO needs servicing.
In interrupt mode EMPTY, NOT EMPTY, FULL, NOT
FULL and programmable level interrupts are provided. The
AX5051 signals interrupts by asserting (driving high) its
IRQ line. The interrupt line is level triggered, active high.
Interrupts are acknowledged by removing the cause for the
interrupt, i.e. by emptying or filling the FIFO.
Basic FIFO status (EMPTY, FULL, Overrun, Under−run,
and the top two bits of the top FIFO word) are also provided
during each SPI access on MISO while the micro−controller
shifts out the register address on MOSI. See the SPI interface
section for details. This feature significantly reduces the
number of SPI accesses necessary during transmit and
receive.
HDLC Mode
NOTE: HDLC mode follows High−Level Data Link
Control (HDLC, ISO 13239) protocol.
HDLC Mode is the main framing mode of the AX5051. In
this mode, the AX5051 performs automatic packet
delimiting, and optional packet correctness check by
inserting and checking a cyclic redundancy check (CRC)
field.
The packet structure is given in the following table.
Table 14.
Flag Address Control Information FCS (Optional Flag)
8 bit 8 bit 8 or 16 bit Variable length, 0 or more bits in multiples of 8 16 / 32 bit 8 bit
HDLC packets are delimited with flag sequences of
content 0x7E.
In AX5051 the meaning of address and control is user
defined. The Frame Check Sequence (FCS) can be
programmed to be CRC−CCITT, CRC−16 or CRC−32.
The receiver checks the CRC, the result can be retrieved
from the FIFO, the CRC is appended to the received data.
For details on implementing a HDLC communication see
the AX5051 Programming Manual.
Raw Mode
In Raw mode, the AX5051 does not perform any packet
delimiting or byte synchronization. It simply serializes
transmit bytes and de−serializes the received bit−stream and
groups it into bytes.
This mode is ideal for implementing legacy protocols in
software.
Raw Mode with Preamble Match
Raw mode with preamble match is similar to raw mode.
In this mode, however, the receiver does not receive
anything until it detects a user programmable bit pattern
(called the preamble) in the receive bit−stream. When it
detects the preamble, it aligns the de−serialization to it.
The preamble can be between 4 and 32 bits long.
802.15.4 (ZigBee) DSSS
802.15.4 uses binary phase shift keying (PSK) with
300 kbit/s (868 MHz band) or 600 kbit/s (915 MHz band) on
the radio. The usable bit rate is only a 15
th
of the radio bit
rate, however. A spreading function in the transmitter
expands the user bit rate by a factor of 15, to make the
transmission more robust. The despreader function of the
receiver undoes that.
In 802.15.4 mode, the AX5051 framing unit performs the
spreading and despreading function according to the
802.15.4 specification. In receive mode, the framing unit
will also automatically search for the 802.15.4 preamble,
meaning that no interrupts will have to be serviced by the
micro−controller until a packet start is detected.
