The chip has a CPU (12), memories (13, 14) for programs and data, peripheral units (18, 19) for interacting with the outside world, and an internal RC oscillator (17) for providing clock signals. One of the peripheral units (18) has a timer counter incremented at a frequency derived from the RC oscillator. The method does not try to change the frequency of the RC oscillator. Instead, an external calibration source (21) is connected to a capture input of the timer unit to provide a signal having a reference frequency, e.g. the mains frequency. The counter is sampled on active edges of that signal, and the sampled values are processed to derive a calibration ratio. After these calibration steps, a software correction is applied to parameters handled by programs stored in memory based on the calibration ratio to compensate for frequency variations of the RC oscillator.