charlieplex-led-matrix

Description

Monochrome / grayscale LED matrix driven by charlieplexed GPIOs.

N GPIO lines drive up to N*(N-1) LEDs. Each pixel is lit by driving one of
the GPIO lines high and one low while all the other lines are held at high
impedance (input / disconnected). A timer (counter device) ISR scans the
pixels one at a time, fast enough that persistence of vision makes the whole
matrix appear lit. Grayscale is produced by modulating, per pixel, the
fraction of scan cycles in which that pixel is driven.

The framebuffer is laid out row-major (pixel index = y * width + x); the
pixel-pairs property maps each framebuffer pixel to the pair of GPIO lines
that light it.

Examples

/* 2x2 matrix on 3 charlieplexed lines */
/ {
    matrix: charlieplex-led-matrix {
        compatible = "charlieplex-led-matrix";
        gpios = <&gpioa 0 0>, <&gpioa 1 0>, <&gpioa 2 0>;
        counter = <&timer0>;
        refresh-frequency = <100>;
        width = <2>;
        height = <2>;
        grayscale-bits = <1>;
        /* (high << 8) | low, framebuffer order */
        /* pixel-pairs[0] = 0x0001 -> (0,0) lit by gpios[0] high, gpios[1] low */
        pixel-pairs = <0x0001 0x0100 0x0002 0x0200>;
    };
};

Properties

Properties not inherited from the base binding file.

Name

Type

Details

gpios

phandle-array

 
The bank of charlieplexed GPIO lines that pixel-pairs indexes into. All
lines must be on GPIO controllers that support configuring a pin as
disconnected/high-impedance (the default for any standard Zephyr GPIO
controller).

The per-line GPIO polarity flag (e.g. GPIO_ACTIVE_LOW) is ignored: the
driver drives each line at physical levels, and every line is driven both
high and low across a scan depending on which pixel is lit, so polarity
has no meaning here. Use flags 0 (GPIO_ACTIVE_HIGH); a stray polarity
flag is harmless.

This property is required.

counter

phandle

 
Timer instance used as the scan time source.

This property is required.

refresh-frequency

int

 
Target whole-matrix refresh rate in Hz (e.g. 100).

This property is required.

pixel-pairs

array

 
One entry per pixel, in framebuffer (row-major) order; length must equal
width * height. Each entry encodes the pair of GPIO lines that light that
pixel as (high_index << 8) | low_index, where the indices are positions
in the gpios list. The pixel is lit by driving gpios[high_index] high and
gpios[low_index] low.

This property is required.

grayscale-bits

int

 
Number of brightness bits per pixel. 1 = monochrome (on/off). Higher
values trade refresh headroom for brightness levels (2^bits levels).

This property is required.

Legal values: 1, 2, 3, 4

height

int

 
Height of the panel driven by the controller, with the units in pixels.

This property is required.

width

int

 
Width of the panel driven by the controller, with the units in pixels.

This property is required.