How Not to Write a Block

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1 How Not to Write a Block Tom Rondeau (tom@trondeau.com)

2 Introduction Review concepts of working through building a block. When errors happen, what to do about it. Explore some common problems. Tom Rondeau (tom@trondeau.com) Writing Blocks 2 / 44

3 Define a Task for our Block Looking for an event in data. Setup: Receive a digital signal (some form of QAM). We can demod it into a constellation in GNU Radio now. We need a trigger off a specific event when that even starts. We need to write a new block to handle this: What s the size of the event in samples? Look over the event size and determine if it s occurred. Emit a tag when it has on the item on which it began. Scale the imaginary part of the stream by 2. y = x r + j2x i Tom Rondeau (tom@trondeau.com) Writing Blocks 3 / 44

4 System Setup: All GNU Radio Blocks. Tom Rondeau Writing Blocks 4 / 44

5 Output doesn t look right... I should be seeing a real constellation Tom Rondeau (tom@trondeau.com) Writing Blocks 5 / 44

6 Let s add some instrumentation Time sinks added before and after the last block. Tom Rondeau (tom@trondeau.com) Writing Blocks 6 / 44

7 Input looks good, output doesn t So it s the pfb_clock_sync block that s the problem. Tom Rondeau (tom@trondeau.com) Writing Blocks 7 / 44

What to do about that block? Let s look at the manual page and hope it s complete: http://gnuradio.

8 What to do about that block? Let s look at the manual page and hope it s complete: cloc Lots of info here, including a sentence: taps: One of the most important parameters for this block... And continuing to read, we find we have to oversample the filter taps. Tom Rondeau (tom@trondeau.com) Writing Blocks 8 / 44

9 I don t understand what the description says In this case, there are a lot of examples that use this block: /usr/share/gnuradio/examples/digital/demod/pam_timing.grc How did I know this? grep to look in examples or source for this block. Tom Rondeau (tom@trondeau.com) Writing Blocks 9 / 44

10 Setting the right taps is much better Received constellation is easily identifiable now. Tom Rondeau Writing Blocks 10 / 44

11 Now to build a new block Do I know what I m doing? Maybe better check for some help. gnuradio.org has a set of tutorials. See How to write an out-of-tree (OOT) module Teaches us how to use gr_modtool and configure Show gr_modtool newmod Show gr_modtool add Tom Rondeau (tom@trondeau.com) Writing Blocks 11 / 44

12 First, create the module Unless you already have a module that this will fit in to. use gr_modtool newmod Tom Rondeau (tom@trondeau.com) Writing Blocks 12 / 44

13 Add the new block to it For help, gr_modtool help add use gr_modtool add Tom Rondeau Writing Blocks 13 / 44

14 Make sure it s working at this point No use spending time until we know we can build at all Do the minor amount to setup the class by filling in all the <+ ~ +> stuff Tom Rondeau (tom@trondeau.com) Writing Blocks 14 / 44

15 Build the project Run cmake and compile $ mkdir build $ cd build $ cmake -DCMAKE_INSTALL_PREFIX=<some dir>../ $ make $ make install Tom Rondeau (tom@trondeau.com) Writing Blocks 15 / 44

16 A word of advice... Tom Rondeau Writing Blocks 16 / 44

17 Let s understand the point of the block Derotate the input by the known frequency. See if a particular vector of samples exists on the input stream. Measure known vector sig against the output stream out Tom Rondeau (tom@trondeau.com) Writing Blocks 17 / 44

18 Let s do that signal processing Say we have defined a function that does the detection. call it do_some_signal_processing Call in the work function. Compile! Error? WAT? Tom Rondeau (tom@trondeau.com) Writing Blocks 18 / 44

19 Luke, you ve switched off your compiler output! What s wrong? Probably not useful to rely on the Force to find our bugs. Tom Rondeau (tom@trondeau.com) Writing Blocks 19 / 44

20 Compiler output Output can be cryptic and hard to parse. Read through it to understand what it s saying. Telling us we called the function with no parameters but it takes parameters. Tom Rondeau (tom@trondeau.com) Writing Blocks 20 / 44

21 Fix the error The function takes two parameters, the length of the detection signal and the input to test against. Recompile Tom Rondeau Writing Blocks 21 / 44

22 Negative. Negative. It didn t go in. It just impacted the surface. Tom Rondeau (tom@trondeau.com) Writing Blocks 22 / 44

23 What s wrong this time? can t convert from void* to gr_complex? Ohhh! That s what our cast was about! Use in instead of input_items. Tom Rondeau (tom@trondeau.com) Writing Blocks 23 / 44

24 You re all clear kid. Now let s blow this thing and go home. Tom Rondeau (tom@trondeau.com) Writing Blocks 24 / 44

25 Loop over all items in the derotated output Loop from 0 to noutput_items-d_len so we don t overrun the buffer Tom Rondeau (tom@trondeau.com) Writing Blocks 25 / 44

26 That compiles. Now to signal the detection with a tag. do_some_signal_processing returns a value of 1 if the event occurred or 0 if not. When a 1 occurs, we d like to send a tag. Don t be too hasty. That is my motto. Treebeard Let s see if this happens before we try to work with the tag system. Print debug info to stdout Use the GNU Radio Logger: Tom Rondeau (tom@trondeau.com) Writing Blocks 26 / 44

27 Debugging There are many right ways to debug. Maybe some faster than others. Do what works for you, but have some trick you use. Here, we use the logger to emit a DEBUG message when the detector returns 1. Tom Rondeau (tom@trondeau.com) Writing Blocks 27 / 44

28 Runs and prints the output info every few seconds. Tom Rondeau Writing Blocks 28 / 44

Now build the tag and send it Learn about the tag stream system: http://gnuradio.

29 Now build the tag and send it Learn about the tag stream system: Create a gr::tag_t object use add_item_tag(0, tag); Tom Rondeau (tom@trondeau.com) Writing Blocks 29 / 44

30 And add our adjustment of the data stream Rescaling the imaginary part by 2. Tom Rondeau Writing Blocks 30 / 44

31 Ready to test. Rework GUI tools to see better. Bottom constellation plot is the input to our detector and runs freely. Top constellation plot is output and is set to trigger off our detected tag. Tom Rondeau Writing Blocks 31 / 44

32 Ready to test. Rework GUI tools to see better. Logger says we are detecting and sending the tag. It s also missing too many detections. The tag trigger is never firing, so the block isn t seeing the tag. Tom Rondeau (tom@trondeau.com) Writing Blocks 32 / 44

33 This could be difficult to debug Tom Rondeau Writing Blocks 33 / 44

34 Debugging tags? Use the Tag Debug block Prints all tags to stdout. Won t help here we still don t see any output. Tom Rondeau (tom@trondeau.com) Writing Blocks 34 / 44

35 Misunderstood the tag offset value Ah, the absolute offset of the item, not just the current offset, i, in the work function. Tom Rondeau Writing Blocks 35 / 44

36 What about that other problem? Signal of interest happening periodically, but the detection is worse than expected. Wait... why am I iterating over one value and returning another? Don t we want to process all items? Tom Rondeau (tom@trondeau.com) Writing Blocks 36 / 44

37 But we can t process beyond the input stream! We re only looking at noutput_items-d_len items. But that doesn t process all of our samples. And what if noutput_items < d_len? We can actually tell the runtime to give us those extra samples. Tom Rondeau (tom@trondeau.com) Writing Blocks 37 / 44

Setup the block to work with the scheduler We want to scan over a length of symbols. Use a GNU Radio scheduler feature called set_history. Yeah, awesome, how was I supposed to know that?

38 Setup the block to work with the scheduler We want to scan over a length of symbols. Use a GNU Radio scheduler feature called set_history. Yeah, awesome, how was I supposed to know that? Fair question. The Scheduler isn t well represented in the manual. Makes sure we look over the entire length of our detection requirement. call set_history(len) in constructor. Compile Tom Rondeau (tom@trondeau.com) Writing Blocks 38 / 44

39 Final, working block Tom Rondeau Writing Blocks 39 / 44

40 Working! Wrangling cats! Tom Rondeau Writing Blocks 40 / 44

41 Other things to think about Segmentation faults and core dumps: Tutorial page on gnuradio.org: TutorialsDebugging Operating system tools to get to know: grep or git grep git cmake Shell commands and environmental variables Tom Rondeau Writing Blocks 41 / 44

42 Still don t know? Google Stack Overflow can solve a lot of questions Find us on the mailing list gnuradio.org/redmine/projects/gnuradio/wiki/mailinglists Find us on IRC (freenode server) #gnuradio Tom Rondeau (tom@trondeau.com) Writing Blocks 42 / 44

43 Challenge problem: Understand this block and its interactions with the scheduler: Tom Rondeau Writing Blocks 43 / 44

set_relative_rate check_topology forecast get_tags_in_range

44 Challenge problem: Understand this block and its interactions with the scheduler: gr::block consume_each enable_update_rate set_relative_rate check_topology forecast get_tags_in_range Optional outputs Tom Rondeau Writing Blocks 44 / 44

New GNU Radio Tools: Performance Counters, Logging, Thread Controls, QTGUI

New GNU Radio Tools: Performance Counters, Logging, Thread Controls, QTGUI Tom Rondeau trondeau.com 2013-10-03 Tom Rondeau (trondeau.com) New GNU Radio Tools: Performance Counters, Logging, Thread 2013-10-03