Modem Modes of Operation
MC1321x Reference Manual, Rev. 1.6
7-18
Freescale Semiconductor
7.3.5.3
CCA / ED While in Stream Mode (use_strm is one)
It is recommended that a CCA or ED operation be performed while use_strm control bit is not active.
However, for software timing considerations, there are times that it may be desirable to leave use_stream
active and a CCA/ED can be done through an alternate procedure.
The CCA / ED operation is done in a manner similar to that described in the previous two Sections.
However, before asserting RXTXEN (starting the sequence), the MCU should write Register 38 to the
value 0x01FF. This register value should only be used for the CCA or ED operation and not for an RX
operation. The default value of 0x0008 should be written to Register 38 be initiating an RX operation.
7.3.5.4
Link Quality Indication
Link Quality Indication is a measure of the signal quality during an actual receive operation. Its value is
stored in field cca_final[7:0], RX_Pkt_Latch Register 2D, Bits 15 - 8. In Stream Mode, note that LQI is
valid and can be fetched with the same read as when fetching rx_pkt_latch[6:0].
The format for the LQI is the same as CCA:
Signal strength in dBm = - (dec (cca_final[7:0] / 2))
NOTE
, the required 802.15.4 Standard accuracy and range limits are
shown. A 3.5 dBm offset has been programmed into the LQI reporting level
to center the level over temperature. Typical values of LQI returned from an
RX operation are from about -95 dBm to about -18 dBm giving a
cca_final[7:0] range of decimal values 190 (0xBE) to 36 (0x24). When used
in an 802.15.4 application, the LQI must be scaled and limited to have a
range of decimal 0 to 255. Values of LQI above approximately -30 to -25
dBm can be non-linear and should not be used as an indication of absolute
received power.
7.4
Frequency of Operation
The MC1321x is designed to operate in the 2.4 GHz band, covering 16 channels and using 5 MHz of
spacing between each channel. The MC1321x uses two local oscillators (LO). The first LO synthesizer is
the main LO for the receiver and the carrier generator for the transmitter. This block is comprised of a
Fractional-N (Frac-N) PLL frequency synthesizer.
The fractional and integer components of the Frac-N must be programmed properly to perform a
transceiver operation on a particular channel. The integer setting is LO1_Int_Div Register 0F, Bits 7-0
(lo1_idiv[7:0]) and the fractional setting is LO1_Num Register 10 (lo1_num[15:0]). For programming a
desired frequency
f
channel
:
Where ([N+1].frac)
dec
is the RFrac-N divider (in decimal) consisting of integer “N” and
fraction “frac” -
f
channel
= ([N+1].frac)
dec
* 16 MHz
After finding ([N+1].frac)
dec
the register settings become:
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