The WS2812 LED driver IC has a one-wire interface which encodes bit
values as pulse widths.
The ICs themselves are basically shift registers. Roughly speaking, a
"short" pulse shifts in a zero bit, a "long" pulse shifts in a one
bit, and an inter-pulse gap exceeding a reset time threshold causes a
pixel to latch the shifted-in color values. Each chip has an output
pin for daisy chaining. Refer to the chip datsheets and comments in
Kconfig.ws2812 for more details.
To meet timing without hogging the core, this driver generates pulses
using SPI. To work, this requires the MOSI line to stay low between
SPI frames, and for inter-frame delays to be less than the reset pulse
time.
There are other ways do it (PWM + DMA on some SoCs, GPIO bit-banging
if no other tasks need the core), but this is a reasonably
general-purpose implementation.
Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>
LPD880x (e.g. LPD8803, LPD8806) devices are LED driver ICs which can
be controlled via a reduced SPI interface (clock and data only), and
support daisy chaining.
Add an led_strip driver for these devices.
Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>
This API covers drivers for strips, or strings, of individually
addressable LEDs. Both RGB and grayscale LED strip drivers can be
implemented within these APIs.
The API only provides for updating the entire strip, since not all
strips support updating individual LEDs without affecting the others.
Subsequent patches will add individual driver support.
Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>
Marti Bolivar