contiki-os/contiki — pull request #634 by hexluthor (2014-04-11 16:53:35+00:00)
Title: CC2538: Set RTIMER_ARCH_SECOND based on XOSC frequencies and SYS_CTRL(_CONF)_OSC32K_USE_XTAL.

The rtimer frequency depends on the clock configuration, specifically whether the low-frequency crystal or internal RC oscillator is selected. This patch computes the proper frequency in both cases.