Quad Update #3

When I started updating the quad I didn’t expect it to be an instant process, but I didn’t really expect it to take as long as it is taking me 🙁 The delays are due to a number of reasons, but the time it takes for some orders to arrive certainly is a large contributor.

Radio Control

I’ve been using a Spektrum DX8 for radio control and with the previous board The individual “servo” leads were connected directly from the AR8000 receiver. The satellite receiver, an SPM9645 was attached but both boxes were required and took up a reasonable amount of space.

Spektrum AR8000 & SPM9645

On the KroozSD board there is no place for the individual servo leads to attach and a UART connection is discussed. This didn’t make much sense initially, but after reading it became apparent that the SPM9645 was a fully fledged receiver and communicated via a serial interface, so the board would only need that one small module. The space savings and simplicity of a single wire are huge advantages, but once again I ran into the issue of how to attach it. I’m not sure what the connection is on the receiver (1.5mm JST?) but the UART connector is a 4 pin Molex PicoBlade, a connector I’m still unable to create. Inside the Spektrum package I did find a cable that allowed me to connect the receiver using the standard 3 pins, and as 2 of the pins are ground and power with a single cable for the communications I started looking at whether I could use that cable and one of the PWM connectors. The answer turned out to be yes, but there is a slight issue. The power supplied via those pins is 5V but the receiver is meant to be used with 3.3V, so while it may be able to handle the voltage it’s not worth the risk long term.
I did connect it for testing and to ensure that the code I was looking at worked, but the results never quite looked right and the values being set didn’t make sense to me. As I like to understand such things more investigation was required!
Longer term I have ordered a small voltage regulator that will allow me to use the pin connection safely, though it could be a while making it’s way here from China!

Can we read it?

There are plenty of articles on the web about the output from the satellite receiver, but many of them deal more with the timing than the values, so I decided to see if I could read the data and see what was going on. Knowing it was a serial stream and that it needed 3.3V I hooked up a Raspberry Pi I had and looked for the GPIO connections I needed to make.

Raspberry Pi GPIO pinouts

The pins therefore needed to be 3.3V (pin #1), GND (pin #9) and serial RX (pin #10). The cable does twist, so I was careful to ensure I connected them correctly 🙂

Raspberry Pi pinouts

Once connected I connected via SSH to the Pi and tried to read data, but nothing appeared and a number of messages appeared about problems with the serial port. Having read about the serial port being used for kernel messages I wasn’t surprised, but the posts I found to correct it didn’t seem to apply. After trying a few different suggestions I eventually came across a post suggesting the raspi-config command, which proved to be the solution. Switching off the option for kernel debug (under Advanced Settings) and restarting the Pi resulted in access to the serial port and data flowing from the receiver. I wrote a small python script and the excellent PySerial module (installed via pip).

Receiver connectedRaspberry Pi

Yes we can!

Now that data was flowing, the next step was to try and interpret it. Thankfully many had gone before me and so I knew what to expect!

The data flowed from the serial port in 16 byte blocks, each of which looked similar.

00 5A 0B E9 2E AA 13 E8 1B F3 31 56 FF FF FF FF
00 5A A1 56 01 62 3A 0C FF FF FF FF FF FF FF FF
00 5A 0B E9 2E AA 13 E8 1B F3 31 56 FF FF FF FF
00 5A A1 56 01 62 3A 0C FF FF FF FF FF FF FF FF
00 5A 0B E9 2E AA 13 E8 1B F3 31 56 FF FF FF FF
00 5A A1 56 01 62 3A 0C FF FF FF FF FF FF FF FF

This was what I had been expecting. 2 bytes of “header” followed by 7 sets of channel information. As the DX8 is an 8 channel transmitter and there are only 7 sets per frame, the repetition of the 2 frames made sense. Looking at details of the “header” I expected the first byte to the dropped frame count and the second byte to contain information about what was to follow, namely the number of bits per channel and the number of frames required.

+------------------------+---------+---+------+------+
| dropped frame count    |         |bit|      |frames|
+------------------------+---------+---+------+------+
16                                                   0

When I bound the receiver to the transmitter it helpfully told me that it had bound using DSMX and 22ms, which should (according to what I had read) give me 11 bit channel data, hence I expected to find that the transmitter data decoded as 2 frames and 11 bit data – which it did. The next step was to look at the channel data. Each channel is sent as 16 bits and as it was using 11 bits for the data should look like this.

+---+------------+-----------------------------------+
|   | channel ID | Channel value                     |
+---+------------+-----------------------------------+
16                                                   0

The channel ID’s all appeared to be within a range of 0 to 15, though those with an ID of 15 I chose to ignore as that appeared to suggest it wasn’t valid data. As I only have 8 channels this seemed a larger number than I was expecting so I decided to try and watch the values in real time using the small python script I had created. (I’m still refining the code but can put it on github if anyone is interested.)

It turns out that the main 4 channels – throttle, elevator, aileron and yaw – are split between 2 channel ID’s, with the left and right deflection being reported separately. In the case of the throttle this results in the value for channel 0 going up until you reach the midpoint where it drops to 0! Obviously only reading one channel would result in some interesting results so I need to look at combining the values appropriately.

GPS

The GPS and it’s cable have arrived and are just awaiting the addition of the connector to attach to the board.

DSCF3474DSCF3479

Unrelated…

Having played with the STM32 I’ve found myself contemplating using it for some other projects I have planned. Looking around for an easy way to start experimenting I came across the STM32 Stamp which looked perfect for my initial needs, but as my skills aren’t up to constructing one myself, does anyone know where I can buy something similar? I don’t really want a discovery board as I want to be able to use it on a breadboard while I experiment with connecting it.