LEDCtl (WS2812b LED Controller)

LED strips and panels based on some form of the WS2812 RGB LEDs have gained an enormous popularity the past couple of years with the price of the individual LEDs dropping well below 10 cent each. These LEDS contain 3 individual LEDs: one red, one green and one blue and the intensity of each of these can be set at 256 different levels.

Overall Design Parameters

The "device" should be able to drive 2-4 strings of up to 1000 leds on each string and the length and type of each string should be configurable.

The "device" will offer multiple ways of controlling LEDS:

• USB/CDC (emulated Serial over USB)
• Serial rx/tx lines
• I2C
• SPI

Picking the CPU

It might be possible to find a better CPU in ST's enormous line of processors, but since we've used the STM32F103 on our own Green Pill board, that processor will be the initial choice.

The "limiting" factor of this choice is the available memory. The STM32F103 has got 20 kB of RAM. Our "device" will need a "buffer" holding the RGB values of each LED. So with 1 string of 1000 LEDS we will need around 3k of RAM for this purpose alone. With 2 strings that value doubles to 6k (which might still be doable) but if we want to support 1000 LEDs on 4 strings each that will require more than half the RAM, most likely leaving too little for the software to run.

Hayes Like Serial Interface

To control the attached LEDs a simple serial interface is used.

AT Commands

Command	Parameters	Example	Response	Description
&F		AT&F	OK	Factory reset
ATLsnnn=rrrgggbbb	s=string nnn=led rrr=3 digit value	ATL0000=010010010	Set a specific led on a specific string to a specific value
Sx=y	y = value	ATS0=64	OK	Set register to a certain value. Example will set register 0 to a value of 64
Z		ATZ	OK	Reset device

S Registers

Register	Valid values	Description
0	0-999	LEDs on string 0
1	0-999	LEDs on string 1
2	0-99	"Width" of a zero (percentage)
3	0-99	"Width of a one (percentage)