and when output is converted to an analog control signal in d a converter 130 controlling lpo 110 which outputs an operating frequency f lpo the d a converter is indicated here explicitly but can be part of the tank circuit of the lpo that is the tank of the relaxation type lpo is implemented as a capacitor bank in which capacitors can be switched in or out directly by the binary word to control the frequency of the lpo during the sleep period x osc and calibration unit 120 are powered down the lpo keeps running using the control signal as stored in register 140 a more detailed example of the calibration logic 120 is depicted in fig2 a down counter 230 is pre loaded with a value representing the number of lpo cycles in one wake up i e monitor period at wake up the counter 230 is enabled by a wake up signal wk and starts counting down at the lpo rate the up counter 280 is enabled as long as the wk is high and the content of the down counter 230 is not zero once enabled the up counter 280 counts at the x osc rate the output a of the up counter 280 is fed to a comparator 250 where it is compared with a reference value n ref at the end of the monitoring window when the down counter 230 reaches zero the comparison between the content of the up counter 280 and the reference value n ref is used to determine whether the control value in control register.