GTA03 LED controller: LP5521

Werner Almesberger werner at
Tue Jan 13 23:13:44 CET 2009

Joerg Reisenweber wrote:
> I don't think this direct CPU GPIO thing would be a better alternative
> to an emergency driver directly accessing I2C to force some few bytes
> into the LED controller.

A device can block the I2C bus indefinitely under some circumstances,
e.g., if you're unlucky enough to crash while in the middle of an I2C
transfer, SCL is low, and the device is sending a 0 or an ACK.

Interestingly, not even a start condition is a reliable way to clear
a stuck bus. (As I found out when experimenting with I2C from IDBG.)

We used to have I2C bus issues fairly often in the past, perhaps also
due to excessively large pull-ups causing badly deformed signals on
some earlier boards, but it seems that this subsystem is reasonably
under control now.

- Werner

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