RFM69 OOK-mode RSSI behaviour

With the RFM69 in continuous FSK mode, polling the RSSI register at sufficient rate can be used to acquire OOK signals like on oscilloscope. However when doing so with receiver mode set to OOK, it becomes clear that the RSSI register is used internally for some other purposes:

The stepped behaviour is related to the programmed bit-rate, 2500bps in this case, resulting in bit durations of 400us. The register is used for some filtering and possibly thresholding the signal to generate the signal on the DATA pin. Strangly the RSSI signal trails the DATA signal (I acquire the RSSI first, and the sample the DATA pin!).

However, in OOK mode, the maximum RSSI is still a good indication for strength of the OOK signal, and the behaviour at the noise floor is not impacted compared to FSK reception.

2 thoughts to “RFM69 OOK-mode RSSI behaviour”

  1. “The DCF code is transmitted five times with 48 second intervals between 3-6 minutes past a new hour. The sensor data transmission stops in the 59th minute. Then there are no transmissions for three minutes, apparently to be noise free to acquire the DCF77 signal. On similar OOK weather stations the DCF77 signal is only transmitted every two hours.”

    I found this REALLY useful in trying to sort out my old broken weather station (Watson W-8681). Not understanding the two hourly time transmission protocol had really impeded my knowing whether my new DFC77 receiving antenna was working or not. I had been expecting the time to update as soon as I switched on the base unit and of course it didn’t. I’m still not sure my new antenna is sensitive enough to get the DCF77 signal reliably, but at least I know not to pull it all apart if my indoor unit doesn’t update right away. Thanks for your reverse engineering work on the transmission protocol, and for sharing it.

Leave a Reply

Your email address will not be published. Required fields are marked *