I previously reported on reverse engineering a candle flicker LED. My approach was to extract the “flicker” pattern from the input current variation and to deduce the algorithm from statistical analysis.
Reverse engineering the controller chip
Of course there is another, more involved, approach. And that is to reverse engineer the circuit directly from the die. Andrew Zonenberg from Siliconpr0n decapsulated and imaged the controller chip from one of my LEDs. You can find his report here.
He managed to obtain very high-resolution optical microscopy images of the top-level metal. It turns out that the controller chip is manufactured in a relatively coarse CMOS process with one metal layer and 1-2 µm resolution. This is 1980ies technology. But of course, that is all that is needed for a circuit as simple as a flicker-LED.
There is a new addition to the popular WS2812 family of RGB LEDs with integrated controller: A 8mm through hole version. Right now they seem to be in pilot production stage. The only place that has them is Soldering Sunday where they are called PixelBits. My understanding is that they will also be available at the usual sources later this year. I got a couple of them to test for compatibility with my light_ws2812 library.
What’s pretty cool about these LEDs is that they are diffuse – no more blinding unidirectional light. This might be very useful for indicator lights. Furthermore, you can easily wire them freeform without a pcb. I see a lot of RGB LED cubes coming up…
After investigating the timing of the WS2812 protocol in the previous part, the question is now how to use this knowledge for an optimized software implementation of a controller. An obvious approach would be to use an inner loop that uses a switch statement to branch into separate functions to emit either a “0” symbol or a “1” symbol. But as it is often, there is another solution that is both more elegant and more simple. Continue reading
WS2812 LEDs are amazing devices – they combine a programmable constant current controller chip with a RGB LED in a single package. Each LED has one data input and one data output pin. By connecting the data output pin to the data input pin of the next device, it is possible to daisy chain the LEDs to theoretically arbitrary length.
Unfortunately, the single-line serial protocol is not supported by standard microcontroller periphery. It has to be emulated by re-purposing suitable hardware or by software timed I/O toggling, also known as bit-banging. Bit-banging is the preferred approach on 8 bit microcontrollers. However, this is especially challenging with low clock rates due to the relatively high data rate of the protocol. In addition, there are many different revisions of data sheets with conflicting information about the protocol timing. My contribution to this was the light_ws2812 library V1.0 for AVR and Cortex-M0, which was published a while ago. A V2.0 rewrite of the lib was in order due to various reasons. And, to do it right, I decided to reverse engineer and understand the WS2812 LED protocol to make sure the lib works on all devices.
Let’s reverse-engineer a LED, pedantic mode.
Lately, cheap electronic candles seem to be everywhere. I never paid much attention to them until recently it came to my attention that they actually use a special type of light emitting diode with integrated “candleflicker” controller. Now this is something different – who doesn’t like obscure LEDs? Half an hour later I had managed to score a bag of candleflicker-LEDs from the chinese manufacturer.
Very nice, you can not do that with real candles. But the interesting part is of course: How do they work? Considering that they literally sell for a few cents a piece, there can not be very expensive electronics involved. This raises another question: Are these cheap LEDs really worse than all the self-made microcontroller based LED-candles around the web?
The NXP LPC800 series is an interesting entry in the Cortex M0+ microcontroller market, aiming to replace 8 bit microcontrollers at the low end. The largest member of the family, the LPC812 is a device with 16kb flash and 4kb SRAM. It is available in a TSSOP20 package which is really small, but still easily solderable. Since I prefer to work with microcontrollers with as little added clutter as possible, I designed a small break-out board for this device.
The board includes push buttons for reset and activating the serial bootloader. It has an integrated 500mA 3.3V LDO to supply both the MCU and connected devices from the USB port. The four pins on the top side (5V,RX,TX,GND) can be used to connect a cheap USB to serial adapter (<$2 on ebay) and program the device via the internal bootloader using FlashMagic.
The entire board with LPC812 is smaller than a DIP LPC1114. Due to it’s narrow design, it is even useful on very small 170 pin bread-boards.
The Github repository has all design data.