I acquired a RasClock made by After Thought Software (http://afterthoughtsoftware.com/products/rasclock) . This is a real time clock using i2c. I wanted to play with its other features via i2c but when I run i2cdetect I get UU on its entry. Accessing it in python give "Device or resource busy".

How can I access it?

1 Answer 1


Both the UU and "Device or resource busy" are trying to tell you the same thing, that I2C address is already being used by some other bit of software.

Having had a quick look at the RasClock page, it asks you to add rtc-pcf2127a to /etc/modules during the setup. I imagine that rtc-pcf2127a is a driver for the clock, and is probably using that address on a regular interval.

If you want to play with the clock yourself I would suspect if you comment out the rtc-pcf2127a line (add a # to the start of the line) and reboot the Raspberry Pi then that address will be available to talk to.

  • I have managed to do that and I can access the rtc registers etc... Now it seems that it cannot be used as a RTC. Is there a way to still enable it to work as a RTC?
    – John Smith
    Jun 12, 2013 at 17:35
  • @JohnSmith: No, there isn't. Having two pieces of software talking to the same device would be trouble. However, the RTC functionality in linux really only needs to talk to it during startup and shutdown (when hwclock runs), so you can certainly unload the driver when hwclock isn't using it.
    – Ben Voigt
    Dec 10, 2013 at 23:23

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