AX8052F151
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• It can perform Spectral Shaping. Spectral shaping
removes DC content of the bit stream, ensures
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
AX5051_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 microcontroller communicates with the framing unit
through a 4 level × 10 bit FIFO. The FIFO decouples
microcontroller 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 framing unit 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 microcontroller 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.
Interrupts are acknowledged by removing the cause for the
interrupt, i.e. by emptying or filling the FIFO.
To lower the interrupt load on the microcontroller, one of
the DMA channels may be instructed to transfer data
between the transceiver FIFO and the XRAM memory. This
way, much larger buffers can be realized in XRAM, and
interrupts need only be serviced if the larger XRAM buffers
fill or empty.
HDLC Mode
NOTE: HDLC mode follows High−Level Data Link
Control (HDLC, ISO 13239) protocol.
HDLC Mode is the main framing mode of the
AX8052F151. In this mode, the AX8052F151 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 20.
Flag Address Control Information FCS 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 AX8052F151 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 AX8052F151 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 AX8052F151 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
