Andres Vahter

embedded systems hacker

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Convert subprocess stdout stream into non-blocking iterator in Rust

In one of my programs I had to interact with another subprocess. This subprocess took data from stdin and wrote result to stdout. It wasn’t just simple reading and writing - it took constant data stream from stdin and somewhere in the middle writes something to stdout. I had to capture this something as soon as possible and take actions according this response.

The main would look like this:

    fn main() {
        let process = Process::new();

        loop {
            let data = get_some_data();

            for response in proc.responses() {
                // do something based on response

 Blocking iterator

Firstly I just tried to read lines from process stdout. However I discovered that this BufReader is blocking. Code for the blocking stdout iterator is below. Because it is blocking we cannot put it into loop

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Examples of creating Rust bindings

Couple of months ago I developed two SDR tools called doppler and demod. These tools were using unix pipe to transfer IQ data stream from one process to another and they were written in C that time.

I have read a lot of blog posts about Rust from the beginning of the year 2015 and I decided to give it a try. The best way to learn a new language is to use it, therefore I decided to rewrite those tools in Rust. This post mostly shows how to use Foreign Function interface - FFI to create Rust bindings.


Firstly libgpredict was improved to use cmake for building and installing proper library. It is much easier to deal with Rust FFI if library is used instead of just bunch of C files. For example C version of doppler statically linked needed files from git submodule, which was fine for C version.

I could not use rust-bindgen for generating Rust functions from C because it did not

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Doppler correction tool for SDR

Doppler is the last tool that is missing for receiving data from satellites using software defined radio (SDR) in a UNIX fashion:

Write programs that do one thing and do it well. Write programs to work together. Write programs to handle text streams, because that is a universal interface.

– Doug McIlroy

As I mentioned in my earlier post there exists many GUI based SDR tools for doing doppler correction and demodulation. However there aren’t tools available for doing this:

rtl_sdr | doppler | demod | multimon-ng

Some readers might already know that rtl_sdr is used for getting IQ data out of simple SDR dongles and multimon-ng is a tool that decodes actual packets from demodulated audio. Demod is a new tool that I wrote for demodulating IQ stream. Currently it supports only FM modulation, however it is quite easy to add more demodulators that are supported by liquid-dsp library.

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Liquid-dsp based command line FM demodulator

In my last post I wrote about how I hacked Google radioreceiver into command line based FM demodulator that can be used to pipe demodulated audio into multimon-ng for AX.25 packet decoding. However it had some significant flaws. For example as far as I know it only worked reliably if input sample rate was 1024 kHz and output 48 kHz. I did not look too much into problem because I wanted to rewrite it anyway to make it more universal for supporting more modulations beside FM and AM.

I am follower of and recently there was a post about new SDR software called CubicSDR. What makes it interesting for me is that it uses library called liquid-dsp for doing signal processing. I tried to find decent signal processing library that is intended for SDR, but I ended up forking Google radioreceiver out of frustation. I did not have any luck finding such library until I stumbled upon

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Pipe SDR IQ data through FM demodulator for FSK9600 AX25 reception


I thought that RTL-SDR and its command line tools are so common in these days that software for decoding everything and especially simple FSK9600 definitely exists. I was kind of right…except there are some corner cases. I looked for command line tools because decoding should work on BeagleBoneBlack which is not the most powerful computer.

There is rtl_fm that uses RTL-SDR dongle to receive actual signal from air and it also demodulates signal on the fly. Demodulated audio signal can then be piped to multimon-ng that decodes FSK9600 AX25 packets.

rtl_fm -f 437.505M -M fm -s 1024000 -r 22050 | multimon-ng -t raw -a FSK9600 /dev/stdin

Actually I have not tested this command in real situation, but it should work with strong signals. Narrow band FM uses +- 2.5 kHz deviation, but some nano satellites are using deviation around +- 3.5 kHz, therefore rtl_fm’s demodulation process

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Using GNU Radio to show I/Q wav file on waterfall plot

 Installing GNU Radio

Finally I managed to get GNU Radio working on Mac OS X. Firstly I used Homebrew repository from here to install GNU Radio and HackRF tools. However it did not went well. Gnuradio-companion always crashed with the following error:

  Warning: Block with key "xmlrpc_server" already exists.
  Ignoring: /usr/local/Cellar/gnuradio/
  Fatal Python error: PyThreadState_Get: no current thread
  Abort trap: 6

It turned out that some of the libraries were still using Mac OS X system’s Python instead of Python installed using Homebrew. I got it to work after renaming system’s Python and rebuilding boost libraries.

sudo mv /System/Library/Frameworks/Python.framework/ /System/Library/Frameworks/Backup.Python.framework
brew reinstall boost

Moreover robotastic fork uses old GNU

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Turning Losi 1/24 short course truck into autonomos folk racer [part 1]

This year’s Robotex is not far away. It takes place from 29 - 30 november. This time it will be special because I decided to participate. Actually I decided it last December after Robotex 2013 when I saw Folkrace competition. It is entry level competition where 5 autonomos cars race against each other. Robot that is fastest in completing 3 laps is declared as winner. Check a video from last year’s competition. They are rather slow, we can do much better!


My teammate Elmo suggested that we should use ready made RC car with brushless motor. Building mechanincs without proper tools is nightmare, therefore RC car platform is very reasonable choice for getting started as quickly as possible. We decided to use Losi 1/24 short course truck. It fits within 15x20 cm dimensions and it has brushless motor. We did not know at this time what else to look for.
First thing was to replace NIMH

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Fixing TinyOS rfxlink layer ACK handling

In my last post I wrote about a strange situation where packet with a length of 248 bytes was received by application layer.
Today I propose a possible fix or rather workaround to that situation.


This issue happens because error detection in rfxlink layers is not very effective. The problem I was experiencing was caused by incorrect ACK packet. Correct ACK packet must have following fields: fcf and dsn. First bit In fcf field must be set and dsn field must contain the same dsn number that original packet had.
ACKs are handled in in the following way:

tasklet_async event message_t* SubReceive.receive(message_t* msg)
    RADIO_ASSERT( state == STATE_ACK_WAIT || state == STATE_READY );

    if( call Config.isAckPacket(msg) )
        if( state == STATE_ACK_WAIT && call Config.verifyAckPacket(txMsg, msg) )
            call RadioAlarm.cancel();

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Debugging data corruption issue in a distributed system written for TinyOS

TinyOS is operating system that is used for developing applications for distributed wireless sensor networks. That is what academics used it for. However in these days it is more popular to call it operating system for Internet of things (IoT). TinyOS abstractions are quite nice to use if everything works as expected, however if you have to check under the hood hell breaks loose.

 The problem

The problem with my application is that occasionally it receives a packet where payload length is claimed to be 248 bytes long. However application TOSH_DATA_LENGTH is just 86 bytes and Atmega128RFA1 radio buffer is 127 bytes long. Therefore it is impossible that this kind of packet can be received from air.
Fortunately there were asserts in the module where problematic packet reception was caught.

event message_t* Radio.receive[uint8_t id](message_t* msg, void * payload, uint8_t len) {

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How to add Energia support to UTFT library

For some reason I like MSP430 microcontrollers and I always put them into my new designs. This time I decided to use MSP430FR5739 as painting chamber thermostat controller. This board has 2.2" TFT screen with touch controller, 2 NTC thermistors and 4 power MOSFET outputs for controlling actuators like relays and pneumatic solenoids.


Eagle schematic and layout can be found here. I have not tested yet if this fancy Linear LT1161 high-side MOSFET driver works properly. I am more interested in getting TFT to work. The display I am using uses HX8340 display driver.

At glance there seems to be 2 ways to get it to display something rather quickly:

  • UTFT Arduino library
  • 43oh Universal Color LCD Graphics Library

There are pros and cons with both solutions. UTFT is meant for Arduino which does not know nothing about MSP430, however recently Energia project has matured quite a bit

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