Here’s the build log for week six of development for the open source function generator I’m trying to prototype and build in time for for OSHWA 2020. At this point, it’s clear I won’t be delivering this design to OSHWA Summit 2020. However, that’s just as much a factor of OSHWA Summit was moved to an all-digital event as it was my level of readiness for it.

Each day’s entry represents an hour’s work per day - the hour before I leave for my job every weekday morning.

This is my log for the week ending March 13, 2020. Here’s links to the prior weeks’ logs:

The design doc for this project is available on my blog if you’re into design documents/philosophy.

All of my hardware and software source files are on GitHub. Check ‘em out!

Mar 13 2020

  • I took a PTO day for the OSHWA Summit. The summit ended up getting canceled due to coronavirus. (Like most events in the USA these days!)
  • I ended up taking the day to do a bunch of research on the analog output stage of what I’m already referring to as “bFunc Rev 2”.
  • Looking at three main design improvements for the next rev:
    • Eliminating the DC offset of the DDS IC
    • Controlling DC offset programmatically
    • Controlling amplitude programmatically
  • Eliminating the DC offset in the DDS chip
    • The AD9837 uses an onboard 200 ohm resistor and an onboard current source to generate the DDS voltage output
    • This has advantages for simplicity, but disadvantages for function generators
    • In particular: it’s got a DC offset that naturally futzes with function generated output.
    • ADI seems like they have the best means of elminating this annoying offset:
      • Switching to a complementary output DDS chip (e.g. AD9834)
      • Transformer-coupling the output to a 50 ohm or 200 ohm load
  • Adding controllable DC offset
    • This seems to be typically achieved with a DAC, a unipolar-to-bipolar conversion stage, and a summing amplifier.
    • This circuit, from TI’s Analog Engineering Cookbook, is particularly interesting.
    • Lots of great references out there on unipolar-to-bipolar conversion from both Texas Instruments and Maxim Integrated.
  • Amplitude control
    • A VGA seems like one way to do this, but they do seem fairly expensive, and somewhat finicky.
    • Another option - current mode opamp with a digital potentiometer for a gain-setting resistor.
      • This may risk capacitively overloading the feedback terminal of a current mode opamp.
      • Digital pots have excess capacitance between their resistor taps - it’s a reasonably well known shortcoming.
    • This bears more research!

Mar 12 2020

  • Posted my USB woes on reddit, got some great feedback from /r/embedded.
  • The obvious answer, provided by /u/lordlod, is to set usb_packet_flag in the CDC_Receive_FS() callback - not in some random other USB interrupt!
    • This did just the trick - now my CDC_Receive_FS() function is happily appending text to my command buffer.
  • Now, for a bit of learning about gdb, and for how to properly send newlines and text prompts.
    • gdb has been a huge asset at this stage. I don’t know much about it but these few commands have done me a world of good:
      • break main.c:243 - sets a breakpoint at line 243 of main.c
      • continue or just c if you’re into the whole “brevity” thing - let the function run until it hits a breakpoint
      • next or just n - run the current line of code, and go to the next line
      • disable 2 - disables breakpoint 2. (Each breakpoint is assigned a number when you run the break command.)
    • I really need to print out this handy gdb cheat sheet and hang it near my bench.
    • In any case - I’m setting a breakpoint in my superloop at the point where it processes a command after receiving a return keypress. I’m not 100% sure whether the correct return keypress is a newline (\n), a carriage return (\r), or some combination of those two. (Wikipedia seems to think it could be any of those, based on what kind of system I’m running.)
  • Starting to tailor the SetWaveformMode() function to stop the DDS output if it’s running, as well as start it if it’s not.
    • This will require a little DDS state variable - DDS_Output_Enable for example.
  • Matter of idle consideration: how do I check how much flash and RAM I’m using with my project? I should figure that out at some point.
    • arm-none-eabi-size seems like it has something to do with that, but I’m not 100% sure how to interpret all off that. Guessing I’ll have to go into the STM32F072 memory map.
    • arm-none-eabi-size --format=SysV -x build/bfunc_rev1.elf puts output in a format I can read - now, how do I map this onto the STM32F072’s available code space?

Mar 11 2020

  • Managed to get declarations right for usb_packet_flag correct, but there’s still junk data getting loaded in and out of the buffer.
    • I think the IRQ is getting called for packets coming in, as well as packets going out.
    • Need a way to check the direction of the USB transaction.
    • There is a register field that will allow you to do this: USB_ISTR
    • Trouble is, I don’t know how to access it through the PCD_HandleTypeDef

Mar 10 2020

  • Stuck on progress today because I left my got-danged boards at work!!
  • More of an R&D morning, then.
    • First - how would we signal to the superloop that we have new characters incoming from the USB CDC class?
    • First guess: try USB_IRQHandler() in the Src/stm32f0xx_it.c file
    • Declare a volatile extern uint8_t that lives in main.c; use this variable as a flag between IRQ and superloop
    • Set this value to 1 every time the USB IRQ fires
    • It may end up doubling the number of chars stuck into the buffer - I don’t completely understand the reference material but I believe that the USB interrupt may fire both on the INPUT packets and the OUTPUT packets. If that’s the case, I’ll have to add a bit of logic to check which direction the data is headed in when the interrupt fires.
  • Desktop Software Addons
    • Added some extra buttons to set Freq1/Phase1 instead of just Freq0/Phase0

Mar 9 2020

  • An idea for a STM32-side command line parser (from the ancient archives of AVRFreaks)
    • Used the code sample in Graynomad’s post on this page as an inspiration/starting point
    • Starting out by implementing a loop like in function command_handling_loop(); goal is just to buffer something in MCU memory then print back when ENTER key is pressed
    • Two issues here:
      • Something about implementing that loop is locking things out in the STM32 - implementing the switch() statement line by line is helping to sort out where that lockout is happening
      • There’s no way to intelligently inform the while() loop that new CDC chars are arriving. It’s assuming that the character in UserRxBufferFS[0] always needs to be appended to the write buffer
        • Need some sort of interrupt or flag telling that loop that the character in the buffer is fresh
    • Also - probably about time to remove the HAL_Delay() functions
  • Also need to finish reworking all of my existing boards to hand off to people at OSHWA Summit. (Yeek!)
  • They moved the conference to all-digital. GERD DANGIT.
    • As far as I’m concerned that’s basically a cancelation.
  • I did manage to do a mass-rework of all of my faulty boards with a little help from my good friend The Dremel Tool. Lots of nicely segmented pins now - and with minimal damage to the actual pin plating! (I got pretty good at it by the tenth one.)


Mar 8 2020

  • I know I said “no weekend work” at the outset of this project, but I _really_ want to have something semi-complete to hand off at OSHWA summit later this week. (Semi-complete == someone can run git clone and, with the proper hardware, feasibly begin using a bFunc board.) Thus, I started work on a very simple Python GUI that will allow a user to select waveform types and frequency settings from a handy GUI interface.
  • (Found some more time later in the day, so I added another label/text box/button to do phase settings as well.)
  • Next steps are to add in some serial terminal functionality (I’ll finally have to confront my longstanding fear of the pyserial module)
  • Also need to finish the command line parser on the board side.
  • Also need to finish reworking all of my existing boards to hand off to people at OSHWA Summit. (Yeek!)
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