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Frequency and Channel Programming
25.5 Frequency and Channel Programming
For normal transmit and receive tasks, the carrier frequency is set by using the
PRF_CHAN.FREQ
register.
The carrier frequency is 2379 + n MHz, where n is the value of this register, and n can be from 0 to 116.
This gives a frequency range from 2379 MHz to 2495 MHz. Note that this frequency range extends
beyond the ISM band.
If
PRF_CHAN.FREQ
is set to 127, and for the Rx and Tx test commands, the frequency must be
programmed directly in hardware registers. In this case, the synthesizer frequency is set by programming
the 7-bit frequency word located in
FREQCTRL.FREQ[6:0]
. The synthesizer frequency is given by
2379 + n MHz, where n is the value of
FREQCTRL.FREQ[6:0]
, and is programmable in 1-MHz steps.
The device supports synthesizer frequencies in the range from 2379 MHz to 2495 MHz. The usable
settings for
FREQ[6:0]
is consequently 0 to 116.
In Rx, the system operates on a low intermediate frequency (IF) of 1 MHz for data rates up to 1 Mbps, and
on a zero IF for 2 Mbps. In Tx, the system supports operating on low IF or zero IF. The IF to be used for
Tx can be programmed in the register
MDMTEST1.TX_TONE
. The receiver may operate on a positive or
negative IF when the data rate is 1 Mbps and lower; this is controlled with
MDMTEST1.RX_IF
.
When the symbol rate is 1 Mbps or lower and the LLE programs the frequency, it uses a
±
1 MHz IF on Tx.
For both Rx and Tx, a negative IF is used when
PRF_CHAN.FREQ
<
62, and a positive IF is used when
PRF_CHAN.FREQ
≥
62
When the symbol rate is 2 Mbps and the LLE programs the frequency, it uses an IF on Tx as specified in
PRF_RADIO_CONF.TXIF
. This IF may be zero, or
±
1 MHz,
±
2 MHz, or
±
3 MHz. The recommended
setting is
±
1 MHz. A negative IF is used when
PRF_CHAN.FREQ
<
62, and a positive IF is used when
PRF_CHAN.FREQ
≥
62.
For all data rates, the setting of
MDMCTRL1.PHASE_INVERT
is taken into account by the LLE when finding
the setting for
MDMTEST1.TX_TONE
. The
FREQCTRL
register is programmed corresponding to the
programmed IF in order to operate on the channel specified by
PRF_CHAN.FREQ
.
25.6 Modulation Formats
The CC2541 supports GFSK and MSK modulation formats. For GFSK modulation, the deviation can be
set to 160 kHz or 250 kHz (320 kHz or 500 kHz for 2 Mbps). The data rate can be set to 250 kbps, 500
kbps, 1 Mbps, or 2 Mbps. The desired modulation scheme is set in the
MDMCTRL0.MODULATION
register.
Not all combinations of modulation format, data rate and deviation are supported.
gives an
overview of supported combinations.
Table 25-10. Supported Modulation Formats, Data Rates, and Deviations
Modulation Format
Data Rate
Deviation
MDMCTRL0.MODULATION
GFSK
2 Mbps
500 kHz
0011
GFSK
2 Mbps
320 kHz
0111
GFSK
1 Mbps
250 kHz
0010
GFSK
1 Mbps
160 kHz
0110
GFSK
250 kbps
160 kHz
0100
MSK
500 kbps
–
1001
MSK
250 kbps
–
1000
25.7 Receiver
When the receiver is started, it searches for the preamble and the sync word. These are used for
frequency offset compensation and bit and byte synchronization. The sync word can be programmed to be
from 16 to 32 bits.
Checking the sync word is done in a two-stage process. First, a correlation value is calculated. If this
309
SWRU191C
–
April 2009
–
Revised January 2012
CC2541 Proprietary Mode Radio
Copyright
©
2009
–
2012, Texas Instruments Incorporated