RFM22B/23B
8.10. RSSI and Clear Channel Assessment
Received signal strength indicator (RSSI) is an estimate of the signal strength in the channel to which the receiver
is tuned. The RSSI value can be read from an 8-bit register with 0.5 dB resolution per bit. Figure 31 demonstrates
the relationship between input power level and RSSI value. The absolute value of the RSSI will change slightly
depending on the modem settings. The RSSI may be read at anytime, but an incorrect error may rarely occur. The
RSSI value may be incorrect if read during the update period. The update period is approximately 10 ns every
4 Tb. For 10 kbps, this would result in a 1 in 40,000 probability that the RSSI may be read incorrectly. This
probability is extremely low, but to avoid this, one of the following options is recommended: majority polling,
reading the RSSI value within 1 Tb of the RSSI interrupt, or using the RSSI threshold described in the next
paragraph for Clear Channel Assessment (CCA).
D7
D6
D5
D4
D3
D2
D1
D0
POR Def.
Add R/W
Function/Description
26
R
Received Signal Strength Indicator
rssi[7]
rssi[6]
rssi[5]
rssi[4]
rssi[3]
rssi[2]
rssi[1]
rssi[0]
—
27
R/W
RSSI Threshold for Clear Channel Indicator rssith[7] rssith[6] rssith[5] rssith[4] rssith[3] rssith[2] rssith[1] rssith[0]
00h
For CCA, threshold is programmed into rssith[7:0] in "Register 27h. RSSI Threshold for Clear Channel Indicator."
After the RSSI is evaluated in the preamble, a decision is made if the signal strength on this channel is above or
below the threshold. If the signal strength is above the programmed threshold then the RSSI status bit, irssi, in
"Register 04h. Interrupt/Status 2" will be set to 1. The RSSI status can also be routed to a GPIO line by configuring
the GPIO configuration register to GPIOx[3:0] = 1110.
RSSI vs Input Power
250
200
150
100
50
0
-120
-100
-80
-60
-40
-20
0
20
In Pow [dBm]
Figure 31. RSSI Value vs. Input Power
Tel: +86-755-82973805 Fax: +86-755-82973550 E-mail: sales@hoperf.com http://www.hoperf.com
57
HOPERF [ HOPERF ]
