So I'm actually surprised that you could sporadically get 256us to work at 55kbit.
I'm currently rereading the rfm69cw spec because I'm implementing a manual listen style mode for a board with the AM1805 RTC. Please check page 14 (Receiver Specification). You can see there how the time from PLL locked state to RX ready depends on both the data rate and the RxBW:
TS_RE_AGC Receiver wake-up time, from PLL locked state, AGC enabled
RxBw = 10 kHz, BR = 4.8 kb/s, 3.0ms
RxBw = 200 kHz, BR = 100 kb/s, 163us
Add to that the 80us it takes to reach PLLlock and the 40us 2 bits of RX for RSSI detection take at 50khz and you can see pretty clearly that with 55khz 256us just shouldn't work
There are also a couple of other nuggets in there. Observe for example that:
Receiver wake-up time, from PLL
locked state to RxReady
RxBw = 10 kHz, BR = 4.8 kb/s, 1.7ms
RxBw = 200 kHz, BR = 100 kb/s, 96us
In other words RxReady is reached much quicker if automatic gain control is off.
Given these data points I'm going to try the following:
Listen mode nodes listen for wakeup signals using 300kbit and 500khz RxBw without AGC, resulting in total time in Rx to RSSI detection of less than 198us - potentially significantly less if receiver wake-up time reduces similarly from 200khz RxBW to 500khz RxBW as it does from lower RxBWs.
The expectation here is not to actually try to receive packets at these super aggressive settings (potentially underground). It's just to detect the gateways willingness to send packets. Once a node knows the gateway wants to send a packet it switches to rx using more conservative settings.
This is particularly attractive for RTC nodes which won't need a regular wakeup burst to stay in sync. Going just by the RX power consumption and assuming a 3s idle window we come out at:
0.000198s / 3s * 16mA = 1uA which would be pretty phenomenal for a node that can be reached in 1.5s on average.
Joe