Clock configuration

A circuit and method for determining the frequency of a first oscillating reference signal generated by a first reference oscillator. The circuit comprises: a second reference oscillator arranged to generate a second oscillating reference signal having a known frequency, a boot memory storing boot code comprising clock configuration code, and a processor coupled to the boot memory and the second reference oscillator. The processor is arranged to execute the boot code from the boot memory upon booting, wherein when executed the clock configuration code operates the processor to determine the frequency of the first reference signal by reference to the second reference signal.