#355 Snow Flake

The Snow Flake is an Atmel SAM D ARM Cortex-M0-controlled LED ornament produced by @LuckResistor and shared with the Boldport Club community as a special project.

Here’s a quick demo of my first Snow Flake running in solo mode…

Notes

When @LuckyResistor announced the Snow Flake to the Boldport Club community, I knew I had to join in! It was fascinating watching the design evolve and the production process get underway. And then one day my batch of 5 snow flake kits arrived!

I’ve just completed building the first, and it went without a hitch. Once I have built some more I will be able to experiment with co-ordinated operation, but even a single snowflake makes an impressive display.

Parts

The Snow Flake is a custom 2-sided board with SMD components. The heart of the board is an Atmel SAM D ARM Cortex M0, which drives an array of 19 white LEDs.

Circuit Schematic

Power Board

The kit came with a bonus small white board that is an optional small power converter. It can be used to build a simple power adapter to convert USB 5V down to 3.3V for the snow flakes. I haven’t used this yet.

There’s a BOM for the Snow Flake Power Converter on octopart.

Construction

I followed the basic build procedure recommended by @LuckResistor:

• Solder the 0402 LEDs onto the board.
• Solder all of the other components the other side of the board.
• Before you program the chip, power the board with 3.3V and test the LEDs. Use a wire to ground and touch the correct pins of the MCU with ground and check if all LEDs light up. If not, search for problematic solder joints.
• Connect the wires to the snow-flake and chain them.
• Program all snow-flakes with the firmware.

I picked up some construction hints along the way:

• The polarisation of the LEDs: They always point inwards, and the pads have a very small marking visible in the solder mask.
• The LEDs are very small! I used a procedure based on a technique described by @prof:
  • unwind 19 LEDs from the tape onto a white plate or saucer
  • organise the LEDs so they are all face up (I use a very fine paintbrush and toothpick for this)
  • use tweezers to transfer the LEDs to the board near their final position
  • apply solder paste to the LED pads
  • use multimeter diode tester to get the right orientation
  • push/move the LED onto the pasted pads with the probe leads or tweezers

@LuckResistor produced some videos to demonstrate construction:

Snow Flake Panel Assembly - LED Side

Snow Flake Panel Assembly - Components

Assembly

I don’t have a stencil but I decided to try hand-applied paste and hot air. This is fiddly, but worked fine.

The LED side is most challenging, as those suckers are mighty small!

Component-side was more straight-forward, but it was important to test all component connections ( I had a few that were open after the first pass with hot-air).

My final 4 Snow Flake’s coming off the line:

Programming

An SWD programmer is required to program the micro controller. Options inclde:

• Atmel ICE
• Black Magic Probe
• Segger J-Link
• STlinkV2 - or a clone from aliexpress;-)
• or a DevBoard with built-in programmer

I have both a Black Magic Probe and STlinkV2 on hand, but only had the correct cable for the BMP, so I went with that.

I haven’t compiled the sources from scratch yet. My first tests were with the binaries from threebytesfull. Note: @luckyresistor subsequently added FirmwareBinaries to the GitHub project.

The snow flakes have to be programmed individually using a SWD programmer. Perhaps someone will take up @luckyresistor’s challenge to write a boot loader to automatically write the firmware from the first snow flake over the data connection to all other snow flakes;-)

Flash with the Black Magic Probe

I knew the BMP should work fine, since threebytesfull had already documented the same.

With the ARM gcc toolchain installed, all that is required is:

• find the port of the BMP - I’m on MacOSX, so it appeared for me as /dev/cu.usbmodem7BB19AA1
• the compiled ELF binary

Plug in a Snow Flake and flash the chip:

$ arm-none-eabi-gdb -nx --batch \
  -ex 'target extended-remote /dev/cu.usbmodem7BB19AA1' \
  -ex 'set confirm off' \
  -ex 'monitor version' \
  -ex 'monitor tpwr enable' \
  -ex 'shell sleep 0.1' \
  -ex 'monitor swdp_scan' \
  -ex 'attach 1' \
  -ex 'load' \
  -ex 'compare-sections' \
  -ex 'kill' \
  assets/SnowFlake.elf

Black Magic Probe (Firmware v1.6.1) (Hardware Version 3)
Copyright (C) 2015  Black Sphere Technologies Ltd.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

Target voltage: 3.3V
Available Targets:
No. Att Driver
 1      Atmel SAMD20E17A (rev E)
0xfffffffe in ?? ()
Loading section .text, size 0x33f4 lma 0x0
Loading section .relocate, size 0x68 lma 0x33f4
Start address 0x0, load size 13404
Transfer rate: 19 KB/sec, 893 bytes/write.
Section .text, range 0x0 -- 0x33f4: matched.
Section .relocate, range 0x33f4 -- 0x345c: matched.

Power and Wiring

For now with one Snow Flake, I’ve simply wired a 2xAAA battery pack to the VCC and GND with some “invivible” 30AWG. When I have a few more Snow Flakes I’ll have to figure out a better arrangement for power and data communications.

The QRP Challenge!

How much power does a Snow Flake use? @LuckResistor estimates around 18mA, so approximately 100 hours running time with 2 x 900mAh.

My first Snow Flake was deployed to test the theory. Powered by 2 “almost new” Panasonic Evolta LR03EG AAA batteries, it ran continuously for 125 hours. The batteries were providing 2.35V (in circuit) at this point.

This Snow Flake just won’t quit! I finally pulled the plug before it died completely, as it was getting ridicuously dim. Here’s what it looked like:

A Four-Flake Mobile

I used four Snow Flake to make a little decoration for my Fretboard CI build status indicator. Just a little hack to give it some xmas cheer;-)

I mounted the four snowflakes from an improvised bit of wire art. The copper wire carries 3.3V and additional wire (just 30AWG) is ground. I didn’t wire up the data lines because I thought it would make the wiring too prominent, and synchronisation is not important.

The Fretboard is supplied with 5V/2A, so I tapped 3.3V with an AMS1117-3.3 in an Altoids tin to power the flakes.

Here’s the first bench test:

I haven’t really been able to capture the full effect in a photo. IRL it is much more impressive and glittery:

Credits and References

• Snow Flake - original order form (currently inactive)
• SnowFlakeProject - GitHub
• Snow Flake Power Converter BOM - octopart
• Black Magic Probe
• Atmel SAM D ARM Cortex M0 info
• LED ornaments - interesting article in Elektor issue 12/1983 p30: various old-school oscillator and LED-driver ideas
• ..as mentioned on my blog