Driving the First WS2816 LED With AVR Assembly

WS2812/WS2816 LEDs are individually addressable RGB LEDs. Each LED is controlled via a single data line using a specific timing protocol.

This example shows how to light the first LED in full white using direct AVR assembly, without any library. It’s a great way to understand how the protocol works at the cycle-accurate level.

Supplies

Arduino Nano/Uno (ATmega328p)

1 WS2812 or WS2816 LED

Jumper wires

Optional: 470Ω resistor on the data line for signal stability

Understanding the WS2812/WS2816 Protocol

T0H = high time for a “0”
T1H = high time for a “1”
Reset = low > 50 µs to latch data into LED

⚠️ Timing is critical: if HIGH/LOW durations are off, the LED will misinterpret the data.

The Delay Loop Code

1️⃣ The delay loop code

ld_loop:

nop ; 1 CPU cycle (~62.5 ns at 16 MHz)

nop ; 1 CPU cycle (~62.5 ns)

dec r16 ; decrement r16 (~1 cycle)

brne ld_loop ; branch if r16 ≠ 0 (~2 cycles if taken, 1 if not)

ret ; return (~4 cycles)

CPU frequency: 16 MHz → 1 cycle = 62.5 ns
nop = 1 cycle
dec = 1 cycle
brne = 2 cycles when branch is taken, 1 cycle when not
ret = 4 cycles (only at the end of the loop)

How It Creates HIGH/LOW Time

Example for HIGH: r16 = 14

Each iteration of the loop while r16 ≠ 0 lasts approximately:

nop + nop + dec + brne (taken) = 1 + 1 + 1 + 2 = 5 cycles

Number of iterations = 14
Total time = 14 × 5 cycles × 62.5 ns ≈ 4.375 µs

⚠️ In practice, the exact timing is empirically tuned, so using 14 for HIGH and 12 for LOW gives ≈0.7 µs HIGH and ≈0.6 µs LOW at 16 MHz.

WS2816 Bit Timing Principle

WS2816 reads a bit based on the HIGH pulse duration:

~0.7 µs HIGH → bit “1”
~0.35 µs HIGH → bit “0”

The LOW period completes the bit and ensures a total bit period of ~1.25 µs, which is required by WS2816.

Example: Sending a “1” Bit

| -------- | --- | ----------- | ------------ |

| Set HIGH | PB0 | 14 | 0.7 µs |

| Set LOW | PB0 | 12 | 0.6 µs |

Total time per bit ≈ 1.3 µs, which is within WS2816’s spec (1.05 – 1.45 µs)

Repeating this 24 times sends one LED in full white.

The Avr Asm Code

Driving WS281x LEDs directly with AVR Assembly arduino uno addressing strip led

; first LED on ws2816 high white

.org 0

rjmp init ; reset vector -> jump to program start

.org 0x68 ; (arbitrary start address depending on flash map)

init:

sbi 4,0 ; set bit in I/O register: DDRB0 = 1 (set PB0 as output)

loop:

ldi r17,24 ; number of bits to send = 24 (1 LED x 24 bits)

sus:

; --- BIT "1" routine ---

; total time per bit = 1.25 µs ± 250 ns (valid range 1.05 – 1.45 µs)

sbi 5,0 ; set pin high = start of bit

ldi r16,14 ; delay for HIGH period (~700 ns, range 500–850 ns)

rcall ld_loop ; wait loop for HIGH

cbi 5,0 ; clear pin low = end of HIGH

ldi r16,12 ; delay for LOW period (~600 ns, range 450–1000 ns)

rcall ld_loop ; wait loop for LOW

dec r17 ; decrement bit counter

brne sus ; if not zero, send next bit

; --- Reset frame ---

; needed to latch data into WS2816 LEDs

rcall ld_loop ; here used as reset delay (>50 µs)

rjmp loop ; repeat the process forever

; ----------------------------------------------------

; Subroutine: ld_loop

; Provides a simple delay

; Each iteration ≈ 50 ns (at 16 MHz)

; Input: r16 = loop count

; ----------------------------------------------------

ld_loop:

nop ; no operation (~62.5 ns each at 16 MHz)

nop

dec r16

brne ld_loop ; repeat until r16 = 0

ret

Visit https://costycnc.github.io/avr-compiler-js, paste the existing code, then compile and upload it to the Arduino Nano. WebSerial is used, so no additional libraries or IDE are required.