Blink Some LEDs

SK6812 RGB LEDs

Pin	Description
VDD	5 volt power supply
DIN	Data Input
DOUT	Data Output to next LED
GND	Ground

SK6812 RGB LEDs are commonly found in long strips of color changing lights. 3 LED dies (red, blue, green) and an SK6812 controller chip are bonded to a bit of metal called a leadframe. The leadframe is put inside a casing and covered in epoxy. Then the whole thing is baked, like a cake.

The colors are set with a single wire, time-based protocol. Multiple LEDs can be chained together, with the DOUT of one LED connecting to the DIN of the next.

16 of these RGB LEDs are on the Bus Pirate 5 PCB, all wired up and ready to go. We're just going to control one of them because all 16 LEDs at full power would be too much current for some USB ports.

info

Each LED die in the SK6812 (red, blue, green) uses 20mA at full power. 60mA is required to light all three dies at maximum brightness.

Choose LED Mode

Bus Pirate [/dev/ttyS0]

HiZ>m

Mode selection
1. HiZ
2. UART
3. I2C
4. SPI
5. LED
6. DUMMY1
x. Exit
Mode > 5

First, enter the Bus Pirate LED control mode.

• Type m then press enter for the mode menu.
• Type 5 then press enter to choose the LED mode.

Configuration

Bus Pirate [/dev/ttyS0]

LED type
1. WS2812/SK6812/'NeoPixel' (single wire interface)*
2. APA102/SK9822 (clock and data interface)
3. Onboard LEDs (16 SK6812s)
x. Exit
Type (1) > 3

Mode: LED

LED-()>

Now configure the LED mode to use the onboard SK6812 LEDs. Many prompts have a default option in ( ), in this case 1. For this tutorial we want option 3, the onboard LEDs.

• Type 3 then press enter.

Light it up

Data Structure

Byte #	bit 7	bit 6	bit 5	bit 4	bit 3	bit 2	bit 1	bit 0
1	G7	G6	G5	G4	G3	G2	G1	G0
2	R7	R6	R5	R4	R3	R2	R1	R0
3	B7	B6	B5	B4	B3	B2	B1	B0

Each LED is controlled by 24 bits of color information: 8 bits of green, 8 bits of red and 8 bits of blue. That means each LED has 256 brightness levels from 0 to 255.

info

Usually we talk about RGB (red, green, blue) LEDs, but note that the data order for the SK6812 is GRB (green, red, blue).

Green

Bus Pirate [/dev/ttyS0]

LED-()> [0b111111110000000000000000.24
RESET
TX: 0b111111110000000000000000.24
LED-()>

Let's start by lighting the green die in the LED. The Bus Pirate understands binary, hexadecimal and decimal formatted numbers. We'll use binary entry first so it's clear exactly how the bits control the LED.

• The data structure table above shows the format for controlling each color in the LED. The first eight bits control green, the next 8 control red and the final 8 control blue.
• Enter the WS2812 bus reset command [. This command has two purposes. First, it tells the Bus Pirate to process this line as bus syntax. Second, it generates a 50us RESET delay so the LEDs are ready for new data.
• Type 0b for binary entry followed by eight 1s to set the green die to full brightness: 0b11111111.
• Add 00000000 for red and 00000000 for blue to turn those dies off.
• Finally, add .24 to tell the Bus Pirate we want to send 24 bits in one transfer.
• The final command is kind of unwieldy: 0b111111110000000000000000.24
• Press enter to send the command to the LED. The green die should light at full brightness.

Now the pattern for working with the SK6812 starts to become clear. We can enter 0b000000001111111100000000.24 to light the red die, or 0b000000000000000011111111.24 to light the blue die.

Bus Pirate [/dev/ttyS0]

LED-()> = 0b11111111
=0xFF =255 =0b11111111
LED-()> = 0b00000000
=0x00 =0 =0b00000000
LED-()>

Binary entry is handy because you see exactly what each bit is doing, but that's a lot of 1s and 0s to type every time. Let's have a look at hexadecimal numbers - much shorter and less prone to mistyping.

• The = command converts between number formats.
• Type = 0b11111111 followed by enter.
• The Bus Pirate displays the value in hexadecimal (0xff), decimal (255) and binary (0b11111111).
• Do the same for our red and blue bits: = 0b00000000 followed by enter.
• The Bus Pirate displays the value in hexadecimal (0x00), decimal (0) and binary (0b00000000).

Bus Pirate [/dev/ttyS0]

LED-()> [0xff0000.24
RESET
TX: 0xFF0000.24
LED-()>

Now let's put it all together and send the same command in hexadecimal.

• Begin with [ to tell the Bus Pirate to expect bus syntax and send a WS2812 RESET command.
• Type 0x to tell the Bus Pirate we're writing a hexadecimal number.
• Type the value ff0000.24 followed by enter.
• The Bus Pirate sends the command.
• Output display mode is set to auto by default, so the Bus Pirate shows the number in the same format it was entered. You can use the o command to change the output display mode.
• The LED stays green because we sent the same command in a different number format.

Red

Bus Pirate [/dev/ttyS0]

LED-()> [0x00ff00.24
RESET
TX: 0x00FF00.24
LED-()>

Red is controlled by the second eight bits of data.

• Type [0x00ff00.24 followed by enter to light the red die.

Blue

Bus Pirate [/dev/ttyS0]

LED-()> [0x0000ff.24
RESET
TX: 0x0000FF.24
LED-()>

Blue is controlled by the last eight bits of data.

• Type [0x0000ff.24 followed by enter to light the blue die.

Purple

Bus Pirate [/dev/ttyS0]

LED-()> [0x00ffff.24
RESET
TX: 0x00FFFF.24
LED-()>

At this point you may wonder if you can mix colors. Why, yes, you can! Let's try red plus blue.

• Type [0x00ffff.24 followed by enter.
• The red and blue die light to full brightness.
• The color changes to purple.

White

Bus Pirate [/dev/ttyS0]

LED-()> [0xffffff.24
RESET
TX: 0xFFFFFF.24
LED-()>

Through the magic of additive color we can make a whole rainbow of tones and shades. We can also make something approximating white light using all three dies.

• Type [0xffffff.24 followed by enter.
• All three LED dies light to full brightness.
• The color is a sort of white at the cool (blue) end of the spectrum.

Brightness

Bus Pirate [/dev/ttyS0]

LED-()> = 128
=0x80 =128 =0b10000000
LED-()>

Up to this point we've used all the LED dies at full brightness (0xff/255/0b11111111), but we can also control the power level by sending smaller values. Let's reduce the power by half.

• Half of 255 is approximately 128.
• Type = 128 followed by enter to convert 128 to an easier-to-use hexadecimal number.
• 0x80 hexadecimal is the same as 128 in decimal notation.

Bus Pirate [/dev/ttyS0]

LED-()> [0x808080.24
RESET
TX: 0x808080.24
LED-()>

Finally, send a command to set all three dies at half power.

• Type [0x808080.24 followed by enter.
• All three LED dies are lit at half power and the brightness is reduced.
• The SK6812 controller chip in the LED uses pulse-width modulation to blink the dies on 50% of the time and off 50% of the time.

Even though the LED dies are only lit 50% of the time it doesn't appear half as bright. This is because human perception of brightness is non-liner. To make it really dim try sending a smaller value like 0x10: 0x101010.24 followed by enter.