Getting the RSSI value to show in your Betaflight’s OSD (FrSky based)

I had a question pop up a few days ago in the form of a YouTube comment.  It was how did I get the RSSI value to show in my OSD on one of my Mini-Quads.  In this particular case, I was using the MicroMinimOSD with the MWOSD firmware, but even with the newer flight controllers with OSD’s built in, the technique is still the same… and in some cases, it’s easier.

Here’s a quick guide on how to set things up using FrSky receivers and Taranis radios (The X9D or Q7, they will both work the same).  Let’s start by using a telemetry based receiver, such as the ever popular X4R-SB.

It’s been a recurring issue in RSSI signals that often the OSD wants an analogue (0-3.3v) RSSI signal, but the receiver either has tricky pads to solder onto, or a dedicated servo output in PWM… which would mean building in a PWM-to-voltage filter which is extra stuff, extra hassle and not necessarily that accurate.  But with a telemetry receiver, the radio already knows what the RSSI value is, so the more sensible, more accurate, and easier thing to do is for the radio to send the RSSI value as it’s own RC channel to the flight controller, and have that flight controller pull it out of the SBUS stream.  There’s a bit of faff in setting it up for the first time, but it’s a five minute job at most.

First off, add yourself an input (on the INPUTS page) on a spare channel.  I usually use channel 7.  The input source should be RSSI, and I name the input RSSI as well so there’s no confusion what it is.

But what happens if there’s no RSSI available as the input source?  If that’s the case head on over to the telemetry page, which will probably look something like this –

You will notice we’ve got no values under the sensors.  I don’t know quite why in some cases the sensors (such as RSSI) are automatically discovered or added at bind time, and in other cases, they are not.  But if you end up like this, then scroll down to “Discover new sensors” and (while your receiver is powered up and you are connected to it, click enter.  You should now see that one or more sensors are found, leaving you with a screen similar to this –

You can now go back to the inputs page and RSSI should be available as a source.

Next, make sure this input has a scale and weight of 100

I’ve done this on more models than I can count, but right out of the blue, one of them started putting a unit of measure of dB on the scale.  If this is the case, then set it to a scale of 100dB

Next, go to the MIXER page and add a mix on channel 7.  The source should be RSSI, and now the weird part.  We need to set it with an offset of -100 and a weight of 200.  Why is this ?  Well, without it, the RSSI value would go from 0 to 100, which is good as far as an RSSI value goes, but represents only half the PWM range, so effectively this would only give you and RSSI value to 50 to 100.  By adding the weight and offset values, and RSSI value of 0 is -100 (in Taranis terms), RSSI of 50 is 0, and RSSI of 100 is still 100, so this covers the whole PWM range, although I should note that the simplified values of -100 to 100 have very little to do with the actual PWM value (measured in usecs).

There’s a step to do on your flight controller as well, on the receiver tab, there’s a drop down that lets you set your RSSI RC channel.  In this case show in the screen shot below, this is set to channel 8 (as it’s for the firmware on an XM+) but to go with what I’ve set on the Taranis in the example, this would be set to channel 7.  All you need to do is then drag the RSSI value somewhere on the screen (if you are using the built in Betaflight OSD) or enable the “Use FC RSSI” option if you are using MWOSD.

Since writing this, Betaflight has made the slight change of using the AUX channel number, instead of the specific RC channel.  Basically Aux 1 is channel 5, so can just subtract 4 from your actual channel you use in order to get the correct AUX channel.  in the example below, the RSSI channel of 8 would become AUX 4.

But wait, what about none telemetry receivers, such as the popular and tiny XM+  As it comes out of the box, there are some pads to solder from to give you RSSI, but a far easier solution is to install the RSSI enabled firmware which will set either channel 8 or 16 (depending on the version) as the RSSI channel.  In this way, you don’t have to do anything at all on your radio – instead just setup the RSSI RC channel in the receiver tab, and you are good to go !

If you are worried about flashing new firmware on the XM+, as luck would have it I already made a video to guide you through the process.  You can check it out right here.

The firmware download page for the XM+ is here, In my recent Martian III build, I used the version with RSSI on channel 8.