Following on from my experiments with the radio control receiver I still found myself unable to simulate the binding process. Everything I’d read suggested that a series of pulses needed to be sent just after power up, but the code in paparazzi that did this didn’t work for me.

Capturing the pulses

The starting point was using the existing AR8000 as when this is powered up with a bind plug inserted and the satellite receiver attached both go into bind mode (rapid flashing of their orange lights). It was obvious that the AR8000 sent the pulses to the satellite to trigger this, so by watching the signal line I should be able to see them. The next question was how.
Bring out the RaspberryPi 🙂

Both receivers are happy at 3.3V and the GPIO pins on the RaspberryPi are also 3.3V, so that part was easy. Using the +3.3V and GND pins supplied power and after some surgery on a cable I ended up with this (apologies for my poor drawing skills).

Bind signal setup

The next step was to watch for the signals.

pigpiod and piscope

While looking for a way to capture the changes in signal I came across a post detailing using a daemon and client to do that exact thing. The daemon is pigpiod which needs to be installed and running on the RaspberryPi (obviously). Downloading, building and installing were very easy. Running the daemon proved to be as underwhelming as I had hoped – it just sat and did it’s thing 🙂 The daemon allows the output of GPIO pins to be monitored, but the data can be sent remotely, making it very flexible.

I decided to run piscope on my laptop, so grabbed the code, built and then ran it. Here I did hit a minor surprise when I was greeted with a segfault. Trying the usual command line options for help resulted in the same response. Looking at the instructions on the site provided the answer.

export PIGPIO_ADDR= ./piscope

Now I was greeted with a the application running.

piscope running

I opted to only have the GPIO pin I was using selected (Misc > GPIOs) shown and made sure that the Live feed was running.

GPIO selection. A Select All/Select None option would be nice :-)


Powering on the AR8000 with the bind plug inserted resulted in a series of pulses. Finding the correct scale and position to look at these in the piscope app took a few attempts but with some experimentation I had a screen showing what I wanted. There were 9 pulses sent, each was 120us long and the gap between them was 120us. This agreed with the code I had seen which meant I was a little confused why the binding wasn’t working when attached to the autopilot board. Once the binding was complete, the usual flow of information was visible and the 11ms separation of frames was very evident.

Having never thought about using the RaspberryPi this way before, I’ve been pleasantly surprised by how straightforward it was.

pullup not pulling up?

Paparazzi allows you to have a “bind pin” which functions much like the bind plug for the AR8000 – it starts the bind process. I had this setup but looking at the code revealed that the bind code was always being executed, even if the bind pins weren’t activated. Hmmm. More investigation revealed that the function call that was meant to activate the pin and leave it “pulled up” wasn’t doing that so the pin was always being sensed as clear. Easy enough to find and fix. Once fixed the code was only being executed when the bind pin was activated though I still didn’t get the binding process started.

Single file pull request?

The next step would have been to issue a pull request to the main paparazzi branch, but there’s a previous change that I didn’t want to include in the pull request. I’m afraid my github fu is too weak to know how to do such a thing – answers on a postcard! I tried using the cheery-pick option, but that didn’t work and so now I’m left with my local repo in an odd state of affairs! D’oh.

This is not the GPS you want…

Having made the cable to connect my GPS to the board I was surprised when it failed to work. Further investigation has revealed that I ended up buying the wrong version! So, if you want a GPS from Navilock, the one you want should end with TTL not ERS. All is not lost as I can add a serial interface to the module and will therefore be able to use it in another project. I have found another supplier for a GPS module and it should be here soon.

Molex picoblade

The 1mm connectors really are as fiddly and annoying to use as their size suggests. I’ve managed to make the cables I need but the process hasn’t been without a few failures. The crimpers seem good and are very well made. Arriving wrapped in japanese newspaper was a nice touch 🙂