Drive a small number of WS2811/WS2812/WS2812b/SK6812 LEDs with on-the-fly temporal dithering and gamma correction using an AVR (like the attiny85) running at 8MHz or 16MHz, perfect for tiny wearables.
A timer triggered interrupt handler bitbangs frames while keeping track of temporal dithering remainders. The time in between interrupts can be spent generating new frames.
More than 9 x number_of_leds bytes of RAM are needed due to double buffers and temporal dithering bookkeeping.
The code (without the animation generation code) takes up about 1K of flash memory including the gamma correction table.
extern uint8_t *frame; /* buffer to paint on */ /* Initializes buffers and timer */ void init(void); /* Endless loop, calling next_frame() & suspending to save energy */ void run(void); /* to be implemented by animation creator */ void next_frame(void);
Generate the gamma correction map used by the bitbang code.
python gamma_map.py 0x3f00 0x18 2.5
This generates a gamma map with a max brightness of 0x3f (63), no temporal dithering with a period < 0x18/0x100 to prevent flickering, and a gamma of 2.5.
MEASURE_VCC / inv_vcc
The code also periodically measures the bandgap voltage compared to the supply voltage. This may be used to shut down the power supply to the LEDs when a LiPo/LiIon battery is drained. However, this bandgap voltage is not calibrated out-of-factory and can vary a lot between individual chips.
#ifdef MEASURE_VCC extern volatile uint16_t inv_vcc; /* uncalibrated inverted bandgap voltage measurement inv_vcc = 1024*bandgap/vcc */ #endif