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The green activity LED blinks on both read and write activity on the SD card. On newer models it is connected to a GPIO expander , which is connected via I²C to the SoC.

The green activity LED blinks on both read and write activity on the SD card. On newer models it is connected to a GPIO expander , which is connected via I²C to the SoC.

When using Linux, the activity LED can be assigned to other "triggers" instead. See the discussion below.

Blinking the activity LED is a good first exercise for bare-metal programmers, that is people writing their own Operating Systems/drivers or people seeking microcontroller-like control over their RPi.

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