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3.11.6 Packet Handling in Firmware
When implementing a packet oriented radio protocol in firmware, the MCU needs to know when a packet
has been received/transmitted. Additionally, for packets longer than 64 bytes the RX FIFO needs to be
read while in RX and the TX FIFO needs to be refilled while in TX. This means that the MCU needs to
know the number of bytes that can be read from or written to the RX FIFO and TX FIFO respectively.
There are two possible solutions to get the necessary status information:
Interrupt Driven Solution
In both RX and TX one can use one of the GDO pins to give an interrupt when a sync word has been
received/transmitted
(IOCFGx.GDOx_CFG = 0x06). In addition, there are two configurations for the IOCFGx.GDOx_CFG
register
that
are
IOCFGx.GDOx_CFG = 0x01) and two that are associated with the TX FIFO (IOCFGx.GDOx_CFG = 0x02
and IOCFGx.GDOx_CFG = 0x03) that can be used as interrupt sources to provide information on how
many bytes are in the RX FIFO and TX FIFO respectively (see
SPI Polling
The PKTSTATUS register can be polled at a given rate to get information about the current GDO2 and
GDO0 values respectively. The RXBYTES and TXBYTES registers can be polled at a given rate to get
information about the number of bytes in the RX FIFO and TX FIFO respectively. Alternatively, the number
of bytes in the RX FIFO and TX FIFO can be read from the chip status byte returned on the MISO line
each time a header byte, data byte, or command strobe is sent on the SPI bus.
An interrupt-driven solution should be used, as high-rate SPI polling reduces the RX sensitivity.
Furthermore, as explained in
but finite, probability that a single read from registers PKTSTATUS , RXBYTES, and TXBYTES is corrupt.
The same is the case when reading the chip status byte.
See the TI web site for software examples.
3.12 Modulation Formats
CC11x1-Q1 supports amplitude, frequency, and phase shift modulation formats. The desired modulation
format is set in the MDMCFG2.MOD_FORMAT register.
Optionally, the data stream can be Manchester coded by the modulator and decoded by the demodulator.
This option is enabled by setting MDMCFG2.MANCHESTER_EN = 1. Manchester encoding is not
supported at the same time as using the FEC/Interleaver option.
3.12.1 Frequency Shift Keying
CC11x1-Q1 can use Gaussian shaped 2-FSK (GFSK). The 2-FSK signal is then shaped by a Gaussian
filter with BT = 1, producing a GFSK modulated signal. This spectrum-shaping feature improves adjacent
channel power (ACP) and occupied bandwidth.
In 'true' 2-FSK systems with abrupt frequency shifting, the spectrum is inherently broad. By making the
frequency shift 'softer', the spectrum can be made significantly narrower. Thus, higher data rates can be
transmitted in the same bandwidth using GFSK.
When FSK/GFSK modulation is used, the DEVIATN register specifies the expected frequency deviation of
incoming signals in RX and should be the same as the TX deviation for demodulation to be performed
reliably and robustly.
The frequency deviation is programmed with the DEVIATION_M and DEVIATION_E values in the
DEVIATN register. The value has an exponent/mantissa form, and the resultant deviation is given by:
f
xosc
f
(8
DEVIATION_
=
´
+
dev
17
2
Copyright © 2009–2010, Texas Instruments Incorporated
and/or
when
a
complete
associated
with
the
Section 3.6.3
and the errata notes, when using SPI polling, there is a small,
DEVIATION_
E
M)
2
´
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SWRS076B – 11-07-22-013 - APRIL 2009 – REVISED APRIL 2010
packet
has
RX
FIFO
(IOCFGx.GDOx_CFG
Table
3-17).
CC11x1-Q1
been
received/transmitted
=
0x00
Detailed Description
and
(4)
37
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