![](http://datasheet.mmic.net.cn/170000/MC13214_datasheet_9174623/MC13214_150.png)
Modem Modes of Operation
MC1321x Reference Manual, Rev. 1.0
7-14
Freescale Semiconductor
7.3.5
Clear Channel Assessment (CCA) Modes (including Link Quality
Indication)
A special case of receive function called Clear Channel Assessment (CCA) modes is available to measure
received energy from the selected channel. The CCA function exists as two algorithms:
1. Clear channel assessment - measuring channel energy and comparing to a preset threshold.
2. Energy detect (ED) - measuring channel energy and giving an indication of measured strength.
The energy detect algorithm is also used for Link Quality Indication (LQI) during a normal RX operation.
The LQI is reported as part of the RX operation.
The CCA modes are associated with the following register fields:
1. cca_type[1:0], Control_A Register 06, Bits 5 - 4, determines channel energy assessment algorithm
where value 0x1 selects CCA and value 0x2 selects energy detect.
2. cca_vt[7:0], CCA_Thresh Register 04, Bits 15 - 8, sets the threshold level for the CCA function.
3. The average power of the signal is displayed in field cca_final[7:0], RX_Pkt_Latch Register 2D,
Bits 15 - 8. This field is used for CCA, ED, and as LQI during an RX operation.
4. power_comp[7:0], CCA_Thresh Register 04, Bits 7-0, provides an offset that is added to the
measured value of the average energy from a CCA/ED function or LQI value from an RX function.
5. Status bit cca, IRQ_Status Register 24, Bit 1, is used only for the CCA algorithm and is set to “1”
when a busy channel is detected.
6. Status bit cca_irq_status, IRQ_Status Register 24, Bit 5, is set to “1” when the power measurement
is complete for CCA or ED. If cca_mask, Control_A Register 06, Bit 10, is set an interrupt request
will be also generated when the cca_irq_status is set.
7. For CCA Mode, it is preferred that tx_strm, rx_strm and use_strm_mode control bits should be
cleared to zero.
7.3.5.1
Clear Channel Assessment Function (use_strm is zero)
The CCA function measures the average energy of the channel and compares it to a preset threshold as
required by the 802.15.4 Standard. In CCA Mode, the receiver first warms-up from Idle in 144 s. The
average value of the signal power, as measured over the next 128 s (8 symbol periods), is calculated and
stored in cca_final[7:0].
To determine the decimal equivalent of the value stored in cca_final[7:0], one must convert the hex value
to its decimal value, divide by two, and change the sign; where this calculated value is equivalent to the
received signal strength in dBm:
Signal strength in dBm = - (dec (cca_final[7:0] / 2))
The value stored in cca_final[7:0] is also affected by an offset stored in power_comp[7:0], CCA_Thresh
Register 04, Bits 7-0. The default value of power_comp[7:0] = 0x8D and is added to the measured value
during the CCA/ED/LQI function to give the stored value in cca_final[7:0].
The compensation number added to the internal measured value is power_comp[7:0] / 2 and the resulting