Using Raspberry Pi to drive WS2812B LEDs

Those leds use a serial protocol to communicate their "color" with the host computer.

The serial protocol uses a bit-time of 1.25 microseconds, or 800kbps. For each led you need to send 24 bits.

If you would write a program that does the GPIO up-and-down things in the right cadence, then the Linux system will mess that up because there are interrupts and maybe other programs that want their slice of CPU-time.

The only option I can think of is to write a kernel driver.

HO STOP! HACKTIME!

Connect the serial in of the WS2812 to the MOSI pin. Configure the SPI interface for 2.4MHz. Then send an SPI packet: a bunch of zeroes (*) (reset)  and then you encode:  0x1 0y1 0z1 .... the xyz are the bits you try to send, and the 0.1 (*) are the "protocol bits" that the WS2812 wants for synchronization.

Anyway, then the raspberry pi will DMA the data to the SPI port and the signal will be nice and clean without interruptions.

(*) I don't remember the polarity of the reset signal and the data bits. The way I've written it here, it can't work, so one or the other needs to be inverted.

Also needs a PI Health Warning here

The world is full of demolished PIs where the PI has tried and failed to drive WS28xx LEDs, in frustration the PI launched itself from the highest possible location into the abyss.

Seriously though, If you Google this you will find there have been many attempts to drive these LEDs with a PI and all so far have failed with anything but a few LEDs or completely.

A kernel driver may as stated by Roger may be your only hope but Linux being Linux, it still may suffer interference or other timing issues with longer strings

what I have seen successful is using say an ATMEGA328 to drive the LEDs but a PI to perform the complicated Math of other routines to calculate the patterns, this way the critical timing is un affected by the multitasking operating system of the PI. Communications can easily be set up over the SPI or I2C bus

If you are successful in writing and running a kernel mode driver, the world will be knocking at your door for a copy

Good luck

Thank you both for your responses.

HAH.. me write a kernel? I would not know where to even begin beginning..

So with you mentioning the ATMEGA328, could it therefore be a better solution to use the Arduino to actually drive the LEDs but use the Pi to send the commands? Is this what you are suggesting? I am guessing that if I was to go through the Arduino route then I could use the Adafruit NeoPixel libraries?

Thanks again,

Nathan

Yes correct

The Arduino and the Adafruit library work very well, I use this combo myself with great success, including IR remote control or over the serial and Ethernet

You can run several hundred LEDs if needed but if you use too many you may have to go for a MEGA with more memory and or speed

Peter

Great,

I already own an Uno R3 and a Mega however I was thinking about expanding my microcontroller collection with either an Arduino Mini or Nano V3.0 Would either of these be sufficient to drive these LEDs or would the Uno be required as a bare minimum?

I am wanting to design a controller box to contain the Raspberry Pi, a twin relay board, an external power supply for the LEDs and obviously the Arduino attached to the Pi, so would ideally like to use the smallest Arduino possible.. I antisipate requiring no more than 20 of these LED's to illuminate the bed.

Nathan

Driving the LEDs will work with almost any of the micro controllers as long as it is running at least 8Mhz clock, the issue you will run into is when you start adding other capabilities like Ethernet, IR, RF etc and you start eating up memory, even programming a bunch of patterns will eat away at the memory slowly the more you add.

If you are using the PI to send out the patterns or selecting from a range of pre-programed then that should be ok. You can always add an SD card then you could have a very large number of patterns to select from

Guys, consider me misguided: The raspberry pi SPI implementation skips a clock every byte. There is a gap between each byte. For a synchronous transfer that doesn't matter, but for a timed protocol as the WS2812 it ruins the whole thing.

one of the reasons I am suggesting the use of an ATMega micro instead of the PI for the actual control of the LEDs, PI can be used as a Master sending commands to the ATMega to change light pattern etc

Hi again,

Thank you to all who have been replying to this thread, I feel like I am about to make a breakthrough.

So, progress update:

I have installed nanpy on my Raspberry Pi and I have it hooked up with my Arduino Uno (For now, I have ordered a Nano) and I have successfully been able to light a regular common anode RGB LED on pin 13 and gnd of the Uno.

Now. My next question is how would I write a python script to control the WS2812B LEDs? I am guessing PWM signal would do it?

Thanks again,

Nathan

The WS2812B LEDs do not take PWM or even direct LED power, they have a chip built in that you talk to through a data line, serial in and a serial out to be precise

each chip will take 24Bits of the serial data to represent RGB (8 bits each), once it has its 24bits, any more data is passed onto the next LED in the sequence. All this happens at a precise 800Khz

so the code you write on the PI will simply have to decide what color and intensity you want for each LED and instruct the Uno to do that

if you follow the Adafruit library examples your see what I am referring to, nice thing about these chips is you don't need power drivers for them, just give them 5V and then tell them what color you want. DONE.

Peter