Disk activity LED

A blinking “the storage card is busy” light. The same idea as the little activity LED on a USB stick or an old hard drive. It flickers while data is being written to the card and stays dark when the card is idle.

During a flight the board streams sensor data to the SD card. This module drives a small GPIO LED so a human standing next to the rocket can see that logging is actually happening.

It is kept completely separate from the PWM status LED used by the rest of the notification library: that LED shows the flight state, this one shows disk activity, and they sit on different pins so they can never overwrite each other.

How it works

Whatever code writes to the SD card (the flight data logger) calls disk_led_activity() once per write. Each call does two things:

1. turns the LED on, and

2. arms a countdown (the “hold timer”) to turn it off again a few milliseconds later (CONFIG_AURORA_NOTIFY_DISK_LED_HOLD_MS, default 40 ms).

The trick is that every new write restarts the countdown. So the timer only ever fires once the card has been quiet for a whole hold window:

What the card is doing	What you see on the LED
a single quick write	one short blink
a rapid burst of writes	a fast flicker
continuous writing	looks steadily on
idle	off

The “turn off later” step runs on a Zephyr delayable work item rather than by sleeping, so disk_led_activity() returns immediately and never stalls the logger thread that called it.

How to wire it into a board

A board opts in with two things:

1. a devicetree chosen entry auxspace,disk-led pointing at a gpio-leds child node that says which pin the LED is on, for example:

   / {
       status_leds: status-leds {
           compatible = "gpio-leds";
   
           sd_activity_led: led_sd {
               gpios = <&gpio1 7 GPIO_ACTIVE_HIGH>;
               label = "SD Activity LED";
           };
       };
   
       chosen {
           auxspace,disk-led = &sd_activity_led;
       };
   };
   

2. CONFIG_AURORA_NOTIFY_DISK_LED=y in the board’s application config.

If either is missing, disk_led_activity() compiles down to a do-nothing call, so shared code (such as the data logger) can invoke it unconditionally without #ifdefs.

Configuration

Kconfig	Default	Purpose
CONFIG_AURORA_NOTIFY_DISK_LED	n	Enable the SD activity LED backend.
CONFIG_AURORA_NOTIFY_DISK_LED_HOLD_MS	40	How long the LED stays lit after the most recent disk access.

API Reference

static inline void disk_led_activity(void)

Report an SD-card access; blinks the activity LED.

Call once per SD write. Lights the LED and re-arms a CONFIG_AURORA_NOTIFY_DISK_LED_HOLD_MS off-timer, so frequent calls keep it lit. Returns immediately; safe to call from any thread.