Two years ago I took a deeper look into the APA102. Although it was more expensive than the common WS2812, and harder to come by, it had some intriguing properties. The main benefits are a timing-insensitive SPI interface, allowing easy interfacing to standard periphery, and a much higher PWM frequency of >19kHz, making the APA102 almost flicker free.
So much about that. Considering how things with LEDs from China go, it should not take too long for clones to appear? Indeed! Recently, several comments showed up on my blog, reporting about issues with APA102 LEDs they bought. It quickly turned out that these were SK9822, APA102 clones from the same company that already brought the SK6812 to us, a WS2812 clone.
One of these people was Mike. He developed the Weblight, a WebUSB controlled RGB LED. The prototype (shown below, red pcb) worked well, but when he commissioned a small production run (black pcb), the LED started to show odd update behavior. Mike was nice enough to share a couple of boards with me for further investigation.
Two years ago, I spent some time analyzing the algorithm used in a candle flicker LED as commonly found in cheap artificial candles. I reverse engineered the algorithm from the flickering pattern and recreated the algorithm in software. Turns out this is of interest for many people who are searching for artificial candle algorithms – there is a surge of traffic every year around December. However, I just reverse engineered one of the controller ICs – this does not mean that this is a good approximation of a real candle.
But how to get there? First, we need to understand how a real candle behaves. In a recent comment, Gary made the excellent suggestion to record a real candle on video and analyze the data. I noticed something similar could be done in a very quick-and-dirty way, by connecting a photodiode to a digital storage oscilloscope.
A couple of weeks ago I reported about a new type of RGB-LED with integrated controller, the APA102. One of the interesting new features of this device is a two-wire SPI interface instead of the proprietary one-wire protocol of the more common WS2812. Many microcontrollers have hardware SPI functions, which allow easy control of these LEDs, as opposed to timing critical bit banging. But it turned out this was not the end of the story. As pointed out by Bernd in a comment, there is some discrepancy between the datasheet and the actual behavior of the devices when it comes to the “end frame”. Reason enough to subject the APA102 to more scrutiny.
The diagram below summarizes the APA102 protocol as found in the data sheet.
I recently received an external USB battery as a promotional gift (see image below). While I always thought of these as a superfluous gimmick, I realized that these devices could be quite useful as mobile power source for various projects. After all, dealing with lithium ion batteries in your own projects can be dangerous and you need additional circuitry to ensure charging and voltage conversion.
External USB batteries can be charged with a normal micro-b USB charger and are supposed to output stabilized 5V at above 1A. And they come fully integrated at a price point where it is difficult to get even the battery alone. See Aliexpress for example and many others. Since there is little reason to trust hardware at this price point, I decided to tear the device down to see whether all the necessary parts are there.
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.
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?