Modem Modes of Operation
MC1321x Reference Manual, Rev. 1.6
Freescale Semiconductor
7-15
The resulting formula for dBm means that the power_comp[7:0] value must be incremented or
decremented by 4 to cause a 1 dBm change, which equates to a 1/4 dBm resolution for the power_comp
correction.
The power_comp[7:0] default value is 0x8D and the value must be incremented to set a lower final CCA
value. As an example, if one were putting in -30 dBm signal and the cca_final[7:0] was reporting -26 dBm,
then the correction value in power_comp[7:0] should be increased by (4 * delta) or (4 * 4) in this case. The
new compensated value of power_comp[7:0] = 0x8D + 0x10 = 0x9D.
NOTE
As stated above, the default value for power_comp[7:0] = 0x8D. For the
MC1321x device an offset of 3.5 dBm is needed to center the reported
CCA/LQI value over temperature. As a result, it is suggested that a value of
0x9B be programmed into power_comp[7:0] for best accuracy.
Since the AGC is set to a fixed gain during the CCA procedure, input signals above -65 dBm will not be
reflected correctly due to saturation. This is not a problem since the purpose of this measurement is to
detect a signal above a threshold that is not to exceed -75 dBm. See the 802.15.4 Standard for details.
Figure 7-6. CCA Reported Power Level vs. Input Power
The value contained in cca_final[7:0], is compared to the preset threshold of cca_vt[7:0], CCA_Thresh
Register 04, Bits 15 - 8. If cca_final[7:0] is equal to or less than the threshold, then cca is set to 1, indicating
a busy channel. If cca_final[7:0] is greater than the threshold, cca remains at 0. Once the CCA operation
is complete, cca_irq is asserted.
The value of cca_vt[7:0] is calculated:
Threshold value = hex (| (Threshold Power in dBm) * 2 |)
A suggested threshold is -82 dBm or 0xA4 = cca_vt[7:0].
The following is a typical sequence for CCA operation (not using a timer-based start):
-100
-90
-80
-70
-60
-50
-90
-80
-70
-60
-50
Input Pow er (dBm)
R
eport
ed Pow
e
r Level (dBm)
802.15.4 Accuracy
and range Requirements
Содержание freescale semiconductor MC13211
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