WS2812B-Driver

A WS2812B LED driver for the STM32F102 (48 MHz). Drives an arbitrary-length strip over a single wire using hardware PWM + DMA — no blocking loops, no CPU intervention during transmission.

---

How It Works

Protocol

The WS2812B protocol encodes each bit as a fixed-width 1.25 µs pulse with a variable duty cycle:

Bit	High	Low
1	0.6 µs	0.65 µs
0	0.3 µs	0.95 µs
Reset	—	> 50 µs

Timer config

f = 1/T = 1/(1.25 * 10^-6) = 800000 = 800 kHz PWM Frequency

f(pwm) = F(clk)/(ARR+1)(PSC+1)

f = 800 kHz  →  (ARR+1)(PSC+1) = 60
PSC = 0,  ARR = 59

CCR values are precomputed as:

#define WS2812B_HI_VAL  30   // 0.6/1.25 * 60 ≈ 29
#define WS2812B_LO_VAL  15   // 0.3/1.25 * 60 ≈ 14

The reset pulse is appended directly to the DMA buffer (64 zero-valued slots ≈ 80 µs), so no manual delay is needed after a frame.

---

DMA Buffer Layout

#define WS2812B_RST_CYCLES   64   // 80 µs reset
#define BITS_PER_LED         24   // GRB, MSB first
#define DMA_BUF_LEN          ((NUM_LEDS * BITS_PER_LED) + WS2812B_RST_CYCLES)

Each entry in WS2812B_DMA_BUF[] is a uint32_t CCR value (HI_VAL or LO_VAL). The DMA is configured Memory → Peripheral, Word (32-bit) to match the width of TIMx_CCRx.

---

LED Color Layout

Colors are stored in a union that mirrors the GRB wire order (MSB first):

typedef union {
    struct { uint8_t b; uint8_t r; uint8_t g; } color;
    uint32_t data;
} WS2812B_LED_ATTR;

The struct ordering (B, R, G in memory) places G in the most-significant byte of data, so iterating bits 23 → 0 naturally produces the G7…G0, R7…R0, B7…B0 sequence required by the protocol.

---

Driver API

// Set a single LED's color (buffered)
void WS2812B_SetColor(uint16_t index, uint8_t r, uint8_t g, uint8_t b);

// Flush buffer to LEDs via DMA (non-blocking, returns HAL_BUSY if previous transfer is in flight)
HAL_StatusTypeDef WS2812B_Update(void);

// Call from HAL_TIM_PWM_PulseFinishedCallback
void WS2812B_Callback(void);

WS2812B_Update() starts a DMA transfer and uses a volatile flag to prevent overlapping transmissions, which is cleared in the DMA completion callback.

Verification of Signals using an Oscilloscope

The 24-bit data can be observed in the above image.

To further validate bit-level correctness, a test pattern was transmitted by setting the blue channel to 10 (binary: 00001010) while keeping other channels constant.

---

Demo Effect

The included main loop drives a sine-wave brightness envelope over a rainbow color wheel, creating a traveling wave across the strip:

// phase_step controls spatial spread; angle controls wave position
float local_angle = angle + (i * phase_step);
uint8_t brightness = (uint8_t)((sinf(local_angle) + 1.0f) * 10.0f);
Wheel(hue + i, &r, &g, &b);  // rainbow color per LED

HAL_Delay(10) sets frame rate; angle += 0.05f controls wave speed; hue++ drifts the rainbow.

Demo Video

---

Toolchain

• STM32CubeMX — clock peripheral config
• arm-none-eabi-gcc + make — build
• OpenOCD — flash and debug

make -j16 DEBUG=1 -f STM32Make.make
openocd -f openocd.cfg -c "program build/debug/ws2812.elf verify reset exit"

---

Notes

Timing is tuned for a 48 MHz HCLK. Changing the system clock requires recalculating ARR and CCR values.