If I setup to do an RxRestart after every measurement of RSSI, and if I record the time interval between RSSI measurements, an interesting pattern emerges:
---------------------------------------------------------------------
Radio Configuration:
====================
Receiver-Mode(RX)
Packet Mode, FSK modulation scheme
[ActualBitrate] 4799.760bps = nominal bitrate of 4800bps
[RxBw] 10416Hz
[RxBwExp] B101 (Bits 0-2 of regRXBW) = 5
[theRxBwMant] B10 (Bits 3-4 of regRXBW) -> [RxBwMant] 24
---------------------------------------------------------------------
0.0. RSSI=-116
1.0. RSSI=-116, elapsed=108uSec
2.0. RSSI=-116, elapsed=104uSec
3.0. RSSI=-116, elapsed=116uSec
4.0. RSSI=-116, elapsed=104uSec
5.0. RSSI=-116, elapsed=108uSec
6.0. RSSI=-115, elapsed=1752uSec
7.0. RSSI=-115, elapsed=112uSec
8.0. RSSI=-115, elapsed=108uSec
9.0. RSSI=-115, elapsed=108uSec
10.0. RSSI=-115, elapsed=104uSec
11.0. RSSI=-115, elapsed=108uSec
12.0. RSSI=-116, elapsed=1760uSec
13.0. RSSI=-116, elapsed=104uSec
14.0. RSSI=-116, elapsed=108uSec
15.0. RSSI=-116, elapsed=108uSec
16.0. RSSI=-116, elapsed=104uSec
17.0. RSSI=-116, elapsed=108uSec
18.0. RSSI=-115, elapsed=1760uSec
19.0. RSSI=-115, elapsed=104uSec
20.0. RSSI=-115, elapsed=108uSec
21.0. RSSI=-115, elapsed=104uSec
22.0. RSSI=-115, elapsed=108uSec
23.0. RSSI=-115, elapsed=108uSec
24.0. RSSI=-113, elapsed=1756uSec
25.0. RSSI=-113, elapsed=108uSec
26.0. RSSI=-113, elapsed=108uSec
27.0. RSSI=-113, elapsed=112uSec
28.0. RSSI=-113, elapsed=108uSec
29.0. RSSI=-113, elapsed=104uSec
30.0. RSSI=-116, elapsed=1752uSec
31.0. RSSI=-116, elapsed=116uSec
32.0. RSSI=-116, elapsed=104uSec
33.0. RSSI=-116, elapsed=108uSec
34.0. RSSI=-116, elapsed=108uSec
35.0. RSSI=-116, elapsed=104uSec
36.0. RSSI=-119, elapsed=1736uSec
37.0. RSSI=-119, elapsed=104uSec
38.0. RSSI=-119, elapsed=108uSec
39.0. RSSI=-119, elapsed=104uSec
40.0. RSSI=-119, elapsed=108uSec
41.0. RSSI=-119, elapsed=108uSec
42.0. RSSI=-115, elapsed=1756uSec
43.0. RSSI=-115, elapsed=108uSec
44.0. RSSI=-115, elapsed=108uSec
45.0. RSSI=-115, elapsed=104uSec
46.0. RSSI=-115, elapsed=108uSec
47.0. RSSI=-115, elapsed=108uSec
48.0. RSSI=-114, elapsed=1756uSec
49.0. RSSI=-114, elapsed=108uSec
50.0. RSSI=-114, elapsed=108uSec
51.0. RSSI=-114, elapsed=104uSec
52.0. RSSI=-114, elapsed=112uSec
53.0. RSSI=-114, elapsed=108uSec
54.0. RSSI=-119, elapsed=1752uSec
55.0. RSSI=-119, elapsed=108uSec
56.0. RSSI=-119, elapsed=112uSec
57.0. RSSI=-119, elapsed=108uSec
58.0. RSSI=-119, elapsed=104uSec
59.0. RSSI=-119, elapsed=108uSec
60.0. RSSI=-117, elapsed=1732uSec
61.0. RSSI=-117, elapsed=108uSec
62.0. RSSI=-117, elapsed=108uSec
63.0. RSSI=-117, elapsed=104uSec
64.0. RSSI=-117, elapsed=108uSec
65.0. RSSI=-117, elapsed=108uSec
66.0. RSSI=-114, elapsed=1756uSec
67.0. RSSI=-114, elapsed=108uSec
68.0. RSSI=-114, elapsed=108uSec
69.0. RSSI=-114, elapsed=104uSec
70.0. RSSI=-114, elapsed=108uSec
71.0. RSSI=-114, elapsed=108uSec
72.0. RSSI=-115, elapsed=1756uSec
73.0. RSSI=-115, elapsed=108uSec
74.0. RSSI=-115, elapsed=108uSec
75.0. RSSI=-115, elapsed=104uSec
76.0. RSSI=-115, elapsed=108uSec
77.0. RSSI=-115, elapsed=112uSec
78.0. RSSI=-116, elapsed=1752uSec
79.0. RSSI=-116, elapsed=108uSec
80.0. RSSI=-116, elapsed=112uSec
81.0. RSSI=-116, elapsed=108uSec
82.0. RSSI=-116, elapsed=104uSec
83.0. RSSI=-116, elapsed=108uSec
84.0. RSSI=-115, elapsed=1760uSec
85.0. RSSI=-115, elapsed=104uSec
86.0. RSSI=-115, elapsed=108uSec
87.0. RSSI=-115, elapsed=108uSec
88.0. RSSI=-115, elapsed=104uSec
89.0. RSSI=-115, elapsed=108uSec
90.0. RSSI=-115, elapsed=1732uSec
91.0. RSSI=-115, elapsed=108uSec
92.0. RSSI=-115, elapsed=108uSec
93.0. RSSI=-115, elapsed=104uSec
94.0. RSSI=-115, elapsed=108uSec
95.0. RSSI=-115, elapsed=104uSec
96.0. RSSI=-116, elapsed=1760uSec
97.0. RSSI=-116, elapsed=108uSec
98.0. RSSI=-116, elapsed=104uSec
99.0. RSSI=-116, elapsed=108uSec
Very often when the RSSI does change, it coincides with a much longer time to take the RSSI measurement (an order of magnitude longer).
That is to say: at lower bitrates such as these, the period between fresh RSSI measurements is noticeably longer.