Multiplexed LED matrix

Row/column LED matrix without a driver chip — the BBC micro:bit display, a bare 8×8 matrix. The MCU drives the row lines and the column lines from GPIO and lights one row at a time.

Status at a glance

Aspect	Status
Device descriptor	`configs/devices/led_matrix_mux.yaml` (primitive `led_matrix`)
Buses	GPIO: any number of row pads and column pads, on any ports
Catalog type id	`led-matrix-mux`
Example systems	`configs/systems/microbit-v2.yaml` (5×5, columns driven by GPIOTE), `configs/systems/microbit-v1.yaml` (3 × 9 lines arranged as 5×5)
Tier	modeled

Pins / attachment

Config key	What
`row_pins`	list of pads, one per row line, electrical order
`col_pins`	list of pads, one per column line, electrical order
`row_active_high`, `col_active_high`	active level of the lines (default `true`; the micro:bit's columns are active low)
`layout`	one list per row line, one cell per column line: `"x,y"` (the picture pixel of that LED) or `"-"` (no LED). Default: the electrical matrix
`persistence_us`	persistence window (20000)

external_devices:
  - id: "led_matrix"
    type: "led-matrix-mux"
    connection: "gpio0"
    config:
      row_pins: ["P0.21", "P0.22", "P0.15", "P0.24", "P0.19"]
      col_pins: ["P0.28", "P0.11", "P0.31", "P1.05", "P0.30"]
      col_active_high: false

Support matrix

Behavior	Status	Notes
LED lit only when its row AND its column line are active	✅	active levels from config
Persistence of vision	✅	lit time integrated exactly per GPIO store / pad drive; picture per 20 ms window
Brightness	✅	on-time per window; 255 = lit for a whole row slot (window / row lines)
Blank when not refreshed	✅
Electrical-to-picture layout	✅	`layout` (micro:bit V1 3×9 → 5×5)
Pads owned by a peripheral	✅	reads the pad level, so an nRF GPIOTE Task-mode column counts
Readback	✅	log `frames` (`peripheral_log`, 0–9 per pixel), `framebuffer` artifact format `gray8`
Current sharing, LED colour	❌	a lit LED is a duty
Light sensing through the matrix	❌	column lines are never read back

How to run

assertions:
  - peripheral_log: {peripheral: led_matrix, log: frames, contains: "09090 99999 99999 09990 00900"}

The frames log has one line per change of the picture, one digit group per row, top first, for example 09090 99999 99999 09990 00900 at cycle 3200000 (the micro:bit heart).