Here’s what the first week of development looks like to build an open source function generator in time for for OSHWA 2020. Each day’s entry represents an hour’s work per day - the hour before I leave for my job every morning, to be precise.

Feb 7 2020

  • Initial placement underway!
    • Like the cross-association between layout and schematics in KiCAD.
    • Highlight a component in EESchema, and it will be highlighted/selected in PCBNew
    • Kind of hard to do on my Macbook. I wish I had a second monitor.
    • Wish there was a way to select a cmpt, then hit a button that was like “Jump to Layout Editor” - I always seem to select a component in EESchema, then click into PCBNew, and find it’s been de-highlighted.
  • Deleted the secondary LDO
    • Not really using any analog functions of my MCU
    • Could use it as aux power to the DDS, but that’d mean a long power route somewhere that I don’t really want to futz with.
  • Starting to think about test point placement

Feb 6 2020

  • Make LFCSP-10 footprint for AD9837
    • No need! I was searching for the wrong footprint in the default library. Looking for LFCSP-WD package found it. It was already there! And a check in the footprint editor showed the dimensions matched.
  • Finish assigning footprints
    • This is more nerve-racking than I expected. Constant thought: “What if I fuck one up?” Instant empathy for team of PCB designers at work.
    • SeikoEpson - your recommended footprint pattern for SG5032 oscillators calls out the wrong part number! WHY!!!
  • Find the right footprint for the crystal (not the oscillator)

Feb 5 2020

  • Replaced LDOs with MIC5504 (much cheaper!)
  • Added missing power net for 3.3V rail to DDS oscillator
  • Assign RefDes’es
    • Done! Easy!
  • Make layout floorplan!!
  • Assign footprints
    • Wow, this takes a long time, and is anxiety ridden.
  • Added breadboard headers for mounting (that’s the whole point of this fucking thing!!)
    • Would make a whole lot more sense if they connected to something though

Feb 4 2020

  • Broke SCH into hierarchical components. Never done a design this way before - we’ll see how this goes!
  • Implemented STM32F072C8T7 Reference Design STM32F072 Reference Schematic
  • Add TVS for USB
    • I didn’t do this, and may end up regretting it. Note to self - buy a ground strap.
  • Add JTAG connector for MCU!
    • Done - went with SWD connection. Compatible with ST-Link, or that Segger I have kicking around in my box of random debugger crap.

Feb 3 2020

  • Not a whole lot of sch work, but a lot of questioning my Arch choices.
  • Might STM32 be a better choice?
    • STM32F072C8T7
      • Datasheet
    • Supports Mac toolchain (AVR does not)
    • Familiar debug protocol (SWD)
    • Just about identical pricing ($2.75 for 10)
    • STM32 HW Quickstart Guide (Includes HW Reference Design) -

Jan 31 2020

  • Make symbol for AD9837
    • This is easy, and pretty intuitive
    • The symbols are the reverse of what I’m used to (orcad) - bubble is the connection point for the wire, not for the symbol box
    • I like that you can automagically include the datasheet link and part information
    • Also really enjoying the hot keying setup - just hit a button and you bring up a new dialog
    • Also - how do you make the interior of the SCH symbol yellow like everything else?
  • Start reference design of AD9837
    • Issue number 1: what’s the primary power source of this thing?
    • Lots of jumper options for selecting power into the device
    • Do I gather from the reference design that the AD9837 is supposed to be powered by its own POL, and everything else has a separate POL? If so, I’ll definitely be finding a cheaper LDO.
    • Packaged oscillators. Convenience = yay! Price = woof. $1.10 per piece.
  • Hierarchical design!
    • That’s very cool, and a really different design paradigm than I’m used to.
    • See a lot of advantages to it - easily replicate circuit “chunks”
    • Is it as easy to do in layout as it is in SCH? Do you get layout rooms by default as well?

Jan 30 2020

  • Start SCH capture of ATSAMD21 MCU
    • Use Datasheet Section 42 (SCH Checklist) as guideline for SCH capture
      • Note to MFGs: this is a great way to get HW engineers to use your parts
      • It’s also a shitload cheaper and more scalable than hiring a bunch of FAEs - paper scales, people don’t!
      • Have a step-by-step guide saying “This is how to implement this thing in HW”
      • We’re typically some combination of time-crunched, and lazy
      • Brilliant!
    • Atmel's Power Supply Reference design for the SAMD21
  • Need to Find 5.0V -> 3.3V LDO
    • Added NCV8114ASN330T1G from On Semi (was in KiCAD’s design library)
    • Could probably find something cheaper though

Jan 29 2020

Still mostly parts research at this point.

  • ATSAMD21
    • Only capable of 3.3V IO voltages, so that kind of dictates what everything else must run at.
  • AD9837
    • Voltage out only - no need for 200 ohm resistor to terminate current driver.
    • No comparator output - square wave achieved by driving MSB of phase accumulator at the VOUT pin.
⤧  Next post Start Sooner ⤧  Previous post Introducing bFunc - a Design Document for an Open Source Function Generator