Cananka, the Raspberry Pi HAT: Requirements

[This post is part two in the series.]

Signalling-wise you can see CAN bus as 5V based but its automobile roots make 12V supply voltage quite common and that is what I am actually using at home. Annoying thing when using 12V is that every CAN board has to drop voltage to 5V needed for logic. Good thing is that you are carefree with longer cable runs. When using 5V, even drop of 10% is a problem and 12V can give you much more breathing room.

Considering I will connect this device to the potentially harsh world, it would be splendid to have its CAN bus portion completely electrically isolated from the Raspberry Pi board. Therefore it will need a major screwup on the input interface to take Raspberry to its death. As CAN bus can span quite a lot of distance and you don’t know all components will be on the same power circuit, this will also help to deal with ground loops and all those different potentials.

Having CAN driver isolated also means we have to power it from somewhere. While my “slave” devices obtain power from the 12V line, for HAT I’ve decided to go with on-board DC-to-DC converter. This makes my device essentially compatible with any CAN bus, regardless of its voltage. Removing external power from consideration also makes it less likely to have a outside high-currents flowing around.

Of course, it would be nice if we could power Raspberry Pi from our CAN power rail. Since isolation is name of the game, we need DC-to-DC converter capable of at least 1.3A (preferably 2A) at 5V. As these modules are usually expensive, this has to be optional part. And yes, back-powering device should be safe even if user forgets to unplug it from USB.

On mechanical side, in addition to HAT basic requirements, possibility of having the HAT on while Raspberry Pi is in the case would be desired. Official case is preferred but other cases should be taken into consideration. This will greatly limit component placement and it could even impact size as, on the first glance, the official HAT dimensions might be a smidge too big for the official case.

On software side it would be ideal to make a board compatible with existing CAN device driver already present in Linux. Default settings for device should be as close as possible to defaults used by that driver. While creating your own driver is possible and not that complicated considering simplicity of a CAN bus, it makes a little sense to create something you will have to compile every time when new OS is installed if you can go with something already present in the kernel.

Onto the component selection.

Cananka, the Raspberry Pi HAT

[This the first post in the series.]

As I got myself a Raspberry Pi, I started thinking about connecting some electronics to it. And in Raspberry world you can either just connect some wires to connector or go about making a HAT.

Raspberry Pi HAT specification defines both mechanical properties and a minimum of electronics while still allowing you to do whatever you please with available IO. Pretty much the only strict requirements here are board size (65x56 mm), hole placement, and presence of a special EEPROM with board identification data. A bit more complicated than bare wiring but simple enough.

Armed with knowledge of what’s needed, I had to decide what I’ll actually make. As I played a lot with CAN bus lately, it seemed as a reasonable choice. Not only I go with something familiar on a first try but I get to connect it to the rest of my humble CAN bus world.

Early into the project one has to decide on a name. In this case the name was obvious - Canberry. Unfortunately it was obvious to someone else too as there is already a project with that name. Lesser man would give up here as not only the name is taken but it is taken by a really similar project. However, project is not exactly the same and differences will make exercise worth-while.

With the preferred name taken, choice fell onto CanPi. Guess what - there is also project named CANPi out there. One guess at what it is about. :)

For a while I’ve settled onto Can314 and actually used it for a few days in my notes until, unrelated to electronics, I was playing with Red Hat. Suddenly its younger sister Fedora came to mind. As fedora is type of hat, what else to call Raspberry Pi HAT than Candora. Name seemed good for a while but I wasn’t completely happy.

However, now I had an idea. With the help of Wikipedia hat page I’ve came upon a few other names inspired by hats: Canbeen, Canullo, Canetto, Canouch… just to name a few.

But at the end Cananka, a name based upon Russian ushanka hat, won. Not only it is easy to pronounce but Google only finds 7100 matching pages.

With the name selected, onto the requirements.

Visual Studio in Small

With Visual Studio 15 release getting close, a lot of new features are seeing the light of a day. One feature that doesn’t get deserved attention is the new installer shown to the world on Build 2016 conference.

It is a simple and tiny (in Visual Studio terms) installer. Executable is under 1 MB and it gets all other components from the Internet. On its own it is not a news, but some optimization magic has been happening there so your downloads are reasonably small (again, in Visual Studio terms). I am sure as release day grows closer some things might gain some “fat” but idea is there. I just hope they will still allow you to install it from media too - having Visual Studio download itself multiple times for multiple computers kinda nullifies smaller downloads.

You can give it a test run yourself and see whether it improves your workflow.

No Delays.h

After not touching project for a while, using MPLAB X 3.26 with a newer XC8 compiler (1.37) brought me error: (141) can't open include file "delays.h": No such file or directory. For some reason delays.h got cut from the environment. Fortunately, the only function I’ve used from that header was Delay10KTCYx - one you can directly replace with a _delay macro.

For example

Delay10KTCYx(6);

can be directly substituted for:

_delay(60000);

But that works only up to 19 - values 20 and above will give you error: (1355) in-line delay argument too large.

For those we can always create a more direct substitute:

void Delay10KTCYx(unsigned char count) {
    do {
        _delay(10000);
    } while(--count != 0);
}

SDR on Raspberry Pi 3 (Software)

[This is a post 2 in series, for hardware setup go here]

The first piece of software we need to get Raspberry Pi running is of course its operating system. If you have a monitor and keyboard attached, you can proceed with NOOBS install without any trouble. If you don’t want to deal with imaging SD card, this is the best as you can even buy it pre-programmed.

However, I like to complicate and thus decided to go with the headless install of Raspbian Jessie Lite. The most noticeable difference between normal Raspbian and Raspbian Lite is that you get a graphical interface only in one of them. Guess which is which.

After creating image and booting machine up with the prepared SD card, one challenge is how to find it. I personally use Import function of MagiWOL to find a new IP around, but you can also log into your router and figure IP from there. Chickens can even attach monitor, I won’t judge. With IP in hand just connect to machine via SSH client (e.g. PuTTY). User name and password are usual pi/raspberry.

First action after fresh install would be to update to the latest packages:

sudo apt-get update

Next you proceed with installing all the goodies (steps taken from Ham Radio Science):

sudo apt-get install -y git cmake libusb-1.0-0.dev build-essential
git clone git://git.osmocom.org/rtl-sdr.git
cd rtl-sdr/
mkdir build
cd build
cmake ../ -DINSTALL_UDEV_RULES=ON -DDETACH_KERNEL_DRIVER=ON
make
sudo make install
sudo ldconfig

Now we want to create two scripts. Yes, you can run stuff manually but these scripts are going to make it easier if you want to do it automatically on startup. Notice each is just a single cat command with a lot of data:

cat > /home/pi/rtl-sdr/build/rtl_tcp.sh <<- EOF
#!/bin/bash
INTERFACE=eth0
IP=`ip addr show $INTERFACE | grep inet | grep -v inet6 | awk '{print $2}' | cut -d'/' -f1`
if [[ $IP != '' ]] ; then
    /usr/local/bin/rtl_tcp -a $IP
else
    echo "Cannot find IPv4 address on $INTERFACE interface." >&2
    exit 1
fi
EOF

# cat > /home/pi/rtl-sdr/build/rtl_tcp.service <<- EOF
[Unit]
After=network-online.target

[Service]
Type=simple
ExecStart=/home/pi/rtl-sdr/build/rtl_tcp.sh
ExecStop=killall rtl_tcp
Restart=on-failure
RestartSec=10
User=pi

[Install]
WantedBy=multi-user.target
EOF

Finally we have a chutzpah of steps for the remaining details (including disabling TV driver for the SDR device):

chmod +x /home/pi/rtl-sdr/build/rtl_tcp.sh
sudo cp /home/pi/rtl-sdr/rtl-sdr.rules /etc/udev/rules.d
sudo bash -c 'echo "blacklist dvb_usb_rtl28xxu" > /etc/modprobe.d/dvb_usb_rtl28xxu.conf'

If all steps are completed, we can now run our newly created script:

# /home/pi/rtl-sdr/build/rtl_tcp.sh
 Found 1 device(s):
   0:  Realtek, RTL2838UHIDIR, SN: 00000001
 Using device 0: Generic RTL2832U OEM``
 Found Rafael Micro R820T tuner``
 [R82XX] PLL not locked!
 Tuned to 100000000 Hz.
 listening...
 Use the device argument 'rtl_tcp=192.168.200.47:1234' in OsmoSDR (gr-osmosdr) source
 to receive samples in GRC and control rtl_tcp parameters (frequency, gain, ...).

If you wish program to start automatically, you can also make use of the newly created service file:

sudo systemctl enable /home/pi/rtl-sdr/build/rtl_tcp.service
sudo init 6

After machine is booted back up (it should take less than a minute), you can use your favorite SDR program with a support for RTL-SDR over TCP to connect on port 1234. I personally find SDR# well behaved and problem-free but any other will do too.

PS: All this was done on a default partition that has under 200 MB free. To increase partition size run sudo raspi-config and choose Expand Filesystem.

[2017-01-16: Adjusted file creation commands to work better with shell copy/paste] [2020-11-11: Added extra cmake parameters.]