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Octoscroller Interface board


This code works with the PRU units on the Beagle Bone and can easily cause hard crashes. It is still being debugged and developed. Be careful hot-plugging things into the headers -- it is possible to damage the pin drivers and cause problems in the ARM, especially if there are +5V signals involved.


The WS281x LED chips are built like shift registers and make for very easy LED strip construction. The signals are duty-cycle modulated, with a 0 measuring 250 ns long and a 1 being 600 ns long, and 1250 ns between bits. Since this doesn't map to normal SPI hardware and requires an 800 KHz bit clock, it is typically handled with a dedicated microcontroller or DMA hardware on something like the Teensy 3.

However, the TI AM335x ARM Cortex-A8 in the BeagleBone Black has two programmable "microcontrollers" built into the CPU that can handle realtime tasks and also access the ARM's memory. This allows things that might have been delegated to external devices to be handled without any additional hardware, and without the overhead of clocking data out the USB port.

The frames are stored in memory as a series of 4-byte pixels in the order GRBA, packed in strip-major order. This means that it looks like this in RAM:

S0P0 S1P0 S2P0 ... S31P0 S0P1 S1P1 ... S31P1 S0P2 S1P2 ... S31P2

This way length of the strip can be variable, although the memory used will depend on the length of the longest strip. 4 * 32 * longest strip bytes are required per frame buffer. The maximum frame rate also depends on the length of th elongest strip.


<ledscape.h> defines the API. The sample rgb-test program pulses the first three LEDs in red, green and blue. The key components are:

ledscape_t * ledscape_init(unsigned num_pixels)
ledscape_frame_t * ledscape_frame(ledscape_t*, unsigned frame_num);
ledscape_draw(ledscape_t*, unsigned frame_num);
unsigned ledscape_wait(ledscape_t*)

You can double buffer like this:

const int num_pixels = 256;
ledscape_t * const leds = ledscape_init(num_pixels);

unsigned i = 0;
while (1)
	// Alternate frame buffers on each draw command
	const unsigned frame_num = i++ % 2;
	ledscape_frame_t * const frame
		= ledscape_frame(leds, frame_num);


	// wait for the previous frame to finish;
	ledscape_draw(leds, frame_num);


The 24-bit RGB data to be displayed is laid out with BRGA format, since that is how it will be translated during the clock out from the PRU. The frame buffer is stored as a "strip-major" array of pixels.

typedef struct {
	uint8_t b;
	uint8_t r;
	uint8_t g;
	uint8_t a;
} __attribute__((__packed__)) ledscape_pixel_t;

typedef struct {
	ledscape_pixel_t strip[32];
} __attribute__((__packed__)) ledscape_frame_t;

Low level API

If you want to poke at the PRU directly, there is a command structure shared in PRU DRAM that holds a pointer to the current frame buffer, the length in pixels, a command byte and a response byte. Once the PRU has cleared the command byte you are free to re-write the dma address or number of pixels.

typedef struct
	// in the DDR shared with the PRU
	const uintptr_t pixels_dma;

	// Length in pixels of the longest LED strip.
	unsigned num_pixels;

	// write 1 to start, 0xFF to abort. will be cleared when started
	volatile unsigned command;

	// will have a non-zero response written when done
	volatile unsigned response;
} __attribute__((__packed__)) ws281x_command_t;

LED Strips

Testing LEDscape


Beagle Bone Black cape and firmware for driving a large number of WS281x LED strips.






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